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Biology (A-Z)

Thursday, March 22, 2007

American Concise Encyclopedia

Abiotic factors Physical and chemical conditions that affect the ability of agiven species to live and reproduce in a particular place. Included in theabiotic factors are temperature, light, water, oxygen, pH (acid-base balance)of soil, type of substrate, and the availability of minerals. Certain kinds ofplants and animals will flourish in a natural community if the conditions arepresent that permit their survival. Species interact to influence the survivalof one another. One important principle of ecology is that no living organismis independent of other organisms or of the physical environment, if they sharethe same community.
Abscisic acid A growth-inhibiting plant hormone produced in the bud that helpsto prepare the plant for winter by directing the leaf primoridia to form scalesand by inhibiting cell division in the vascular cambium.
Abscission layer A layer along which a leaf or fruit naturally separates fromthe stem.
Absorption The passage of dissolved materials through the cell membrane, intoand out of the cell.
Acetylcholine A neurotransmitter secreted by motor neurons. When an impulsearrives at the knobs in the axon branches of a motor neuron, a neurotransmitteris released from the synaptic vesicles. Neurotransmitters are chemicalsubstances that diffuse across the synaptic gap and initiate a second impulsewhen chemoreceptors on the dendrites are stimulated. Cholinergic cells secretethe neurotransmitter, acetylcholine. Adrenergic cells secrete theneurotransmitter, epinephrine.
Acid rain This is rainfall that can be as acid as vinegar. It is formed whengases of nitrogen oxide and sulfur dioxide are given off into the atmosphere asby-products of fuel combustion by automobiles, homes, factories, and powerplants. As the fumes are carried into the air by wind currents, they combinewith water vapor molecules and are transformed into microscopic drops of nitricacid and sulfuric acid. When it rains or snows, the precipitation returns theacids to earth, sometimes thousands of miles from their origin. Lakes andstreams have become so acidified that the populations of trout, salmon, andother fish are being destroyed. There is also concern about the possibleeffects of acid rain on soil minerals and nitrogen-fixing bacteria.
Acquired characteristics Variations that cannot be passed from parent tooffspring. These variations are not in the individual's gene pool. Lamarck(1809) advanced the hypothesis that acquired variations are transmitted to theoffspring. According to him, structures that are in use are better developedand preserved, whereas unused organs eventually disappear. Characteristics thusacquired through use could be inherited, he thought, by the next generation.Facts do not seem to support his conclusions.
Acromegaly A condition of overgrowth of the bones in the hands, feet, and jawbrought on by an oversecretion of somatotropin by the pituitary gland.
Acrosome The forward tip of an animal sperm that contains the enzymes thathelp the sperm penetrate the egg. See Sperm.
ACTH (adrenocolticotropic hormone) Stimulates the cortex of the adrenal glandto produce a number of hormones, including cortisone. In severe cases ofrheumatoid arthritis, it has brought about dramatic relief from disability andpain.
Actin One of the major proteins of muscle. The other muscle protein is myosin.Actin makes up the Z band of a sarcomere.
Action potential The graph of an action potential demonstrates the sequence ofevents involved in the transmission of a nerve impulse. A threshold value(Ecrit.) must be attained before depolarization occurs. Once the thresholdvalue is reached, an impulse is generated throughout the nerve fiber. A nervecell will transmit the impulse either totally or not at all. There are nograded responses. This maximal firing condition is referred to as an "all ornone response."
Activation energy The smallest amount of energy that must be available from anoutside source that enables a chemical reaction to start.
Active site The specific place on the surface of an enzyme where a substrateattaches by weak chemical bonds and where catalysis occurs.
Active transport The movement of a substance across the cell membrane againsta concentration gradient, that is, from a region of low concentration to aregion of high concentration. Active transport involves the expenditure ofenergy.
Adaptation A trait that aids the survival of an individual or a species in agiven environment. An adaptation may be a structural characteristic such as thehump of a camel, a behavioral characteristic such as the mating call of a bullfrog, or a physiological characteristic controlling some inner workings oftissue cells. Adaptations permit the survival of species in environments thatsometimes seem forbidding. For example, some bacteria are able to live in hotsprings that have temperatures up to 80 degrees C (175 degrees F). They haveadaptations that permit the carrying out of metabolic functions at extremelyhigh temperatures.
Adaptive radiation The evolutionary division of a single species into severalspecies adapted to divergent forms of life. For example: Darwin's finches.
Adenine A nitrogenous base contained in DNA.
ADH (antidiuretic hormone) Also called vasopressin. A hormone secreted by thehypothalamus that prevents the excretion of urine by stimulating the kidneynephrons to reabsorb water.
Adrenals Compound glands located at the top of the kidneys. They have an outerzone known as cortex and an inner one known as medulla. The cortex secretes acomplex of substances appearing to be closely related chemically. Theircombined action controls sodium, potassium, and chloride metabolism, whichaffects water balance, causes the change of glycogen to glucose, and influencessex. Deficiency results in a syndrome known as Addison's disease. The patientsuffering from it experiences a general decline in muscular strength and sexualactivity, a lowering of blood pressure, disturbance of digestion, and abronzing of the skin. The best-known cortical secretion is cortisone, which hasbeen used in treating arthritis and allergies. The medulla secretes a hormonecalled epinephrine (adrenaline), which is produced in accelerated quantitieswhen one is stimulated by anger or fear. Apparently it is the only endocrinegland directly responding to nervous stimuli. The influence of epinephrine onthe conversion of glycogen to glucose, stimulation of heart muscles, enrichmentof blood supply to muscles, and acceleration of blood coagulation are withoutdoubt useful to provide more and quick energy or to stop bleeding in times ofstress or injury.
Afferent or sensory neurons Nerve cells that transport impulses from sensereceptors to the central nervous system.
AIDS AIDS is a baffling disease that has recently attracted considerableattention. The name stands for "acquired immune deficiency syndrome." It refersto a severe breakdown in the body's immune system. A person with AIDS isvulnerable to a variety of infections and tumors that would normally beattacked by the body's white blood cells. As a result, the person becomes weakand dies. The cause of AIDS is a virus. It was identified in 1984 by Dr. LucMontagnier of the Pasteur Institute in Paris, who called it LAV- I, and alsonamed by Dr. Robert C. Gallo of the National Cancer Institute, HTLV-III. It isnow known as HIV, or human immunodeficiency virus. It is believed that the AIDSvirus kills a specialized type of white blood cell, called the helper T4 cell,which normally protects the body from infection by destroying foreignsubstances that enter it. The disease is believed to be passed through bloodand semen, but not passed by casual contact, such as sneezing or using the sameutensils. In the United States, certain groups of people have the greatest riskof getting the disease: male homosexuals, intravenous drug users who sharecontaminated needles, and babies born to infected mothers. Before 1987,recipients of contaminated blood transfusions contracted AIDS. In CentralAfrica where AIDS is prevalent the disease is also spread by heterosexualcontact. Heterosexual transmission is also becoming more common in the UnitedStates. Considerable research is being conducted to develop a defense againstthe disease. In 1986, a drug, azidothymidine (AZT), was found to be effectiveagainst a form of pneumonia common among AIDS patients. It appears to interferewith reproduction of the virus inside the body cells. In the near future, it ishoped that research will produce a vaccine against the virus. See also Healthand Medicine.
Air sac See Alveolus (air sac).
Albinism An inherited trait requiring two recessive genes resulting inindividuals without pigment.
Algae Simple photosynthetic organisms. Algae may be composed of a single cell,a filament of cells, or a flat plate of cells. Cell walls of algae are made ofcellulose. Algae have some characteristics of animal cells. With the exceptionof the red algae, most algal cells have centrioles. During cell division, mostalgal cells form a cell furrow like those in animal cells.
Alimentary canal See Digestive system.
Allantois An embryonic membranous sac that stores the nitrogenous wastes ofreptiles and birds up to the time of hatching. In mammals, it forms part of theplacenta.
Alleles Two or more genes that have the same positions on homologouschromosomes. Alleles are separated from each other during meiosis and cometogether again at fertilization when homologous alleles are paired, one fromthe sperm cell and one from the egg cell. Two or more alleles determine atrait.
Allergy The sensitivity of some people to substances that are quite harmlessto most other people. The cells of these sensitive people produce antibodies toward off whatever substance affects them. The antibodies become attached to thetissue cells, rendering the person sensitized. Whenever that particularsubstance enters the body again, it reacts with the attached antibodies anddamages the cells. These damaged cells prompt certain symptoms such as itching,sneezing, tearing eyes, red welts, large hives, fever, and a general feeling ofnot being well.
Allopatric speciation Speciation brought about when an ancestral populationbecomes separated by one or more geographical barriers that prevent dispersal.An example is Death Valley where each isolated spring has its own watertemperature and salinity and each is a habitat for a different species ofpupfish.
Alternation of generations The sea lettuce Ulva is a green algae that lives insaltwater. The life cycle of Ulva is described as alternation of generationsbecause one generation of Ulva is produced sexually by gametes while the nextgeneration is produced asexually by zoospores. The gametophyte generation is ahaploid thallus from which small, flagellated gametes are released into thewater. They pair off and fuse. Each fused pair of gametes forms a zygote that,after a short time, becomes a diploid thallus of a new generation of Ulva. Thediploid thallus is the sporophyte generation. It produces haploid zoosporesthat develop and grow into a haploid thallus, which is now the gametophyte.Many other simple organisms also have an alternation of gametophyte andsporophyte generations.
Alveolus (air sac) A minute air sac in the lungs through which oxygen entersthe bloodstream and through which carbon dioxide and water are excreted fromthe bloodstream.
Alzheimer's disease Progressive mental deterioration more often in olderpeople. Recent research has revealed that nerve cells in parts of the brain aredamaged by an accumulation of amyloid proteins.
Amino acids All proteins are built from small molecular units known as aminoacids. The amino acid molecules link together in a particular way throughpeptide bonds. A dipeptide consists of two amino acids. A polypeptide containsmany amino acid molecules. A protein is composed of one or more polypeptidechains.
Amniocentesis The procedure of amniocentesis in which a small amount ofamniotic fluid is removed from a pregnant woman and is used to study cells ofthe embryo. In this way certain chromosomal defects can be determined beforebirth.
Amnion See Embryonic membranes.
Amniotic fluid The fluid that surrounds the developing embryo in mammals,birds, and reptiles.
Amoeba The members of the phylum Sarcodina are described as being amoeboid.Amoeba proteus is the species most often studied. Species included in theSarcodina move by means of pseudopods, flowing extensions of the flexible andamorphous body. The pseudopods also serve in food-catching. Most of thesarcodines live in fresh water. A contractile vacuole, an organelle designed toexpel excess water from the protist cell body, plays an important role inmaintaining water balance. Food is temporarily stored in a food vacuole whereit is digested by the action of enzymes.
Amphibian An amphibian must spend part of its life cycle in the water whereits eggs are laid and fertilized. The eggs develop into a larval stage, ortadpole, that has fish-like characteristics. In tadpoles breathing is by meansof gills, blood is pumped by a two-chambered heart, and swimming is by means oftail and body movements made possible by muscles in the body wall. Mostamphibians undergo metamorphosis into a lung-breathing adult with a 3-chamberedheart.
Anaerobic Referring to a type of cellular respiration that occurs withoutoxygen such as fermentation. Referring also to certain species of bacteria,such as the tetanus bacilli, that live in an atmosphere devoid of oxygen.
Anaphase A stage in mitosis in which the chromosomes are pulled apart. SeeMitosis.
Angiosperms Flowering plants that form seeds inside ovaries. Examples areapple, rose, and dandelion.
Animal kingdom All animals belong to the kingdom Animalia, a grouping of 29phyla. Twenty-eight of these phyla include animals called invertebrates becausethey do not have a vertebral column, or true backbone, the 29th phylum includesthe vertebrates, animals with a vertebral column.
Annelid The annelids are segmented worms that live in soil, fresh water, orthe sea. Most of the annelids are free-living, although some of the marineforms burrow in tubes and some species (class Myzostoma) are parasites onechinoderms. The body of an annelid is divided into a series of similarsegments and is said to be metamerically segmented. Most annelids have a closedcirculatory system where the blood is contained in vessels. Enlarged muscularblood vessels function as hearts and pump the blood through the system ofvessels. Annelids may be dioecious (have separate sexes) or hermaphroditic.Most annelid species go through a ciliated larval stage known as thetrochophore larva. This is a larva of evolutionary importance because the sametype appears in several phyla.
Annual A plant in which the life cycle is completed in a single year orgrowing season, such as corn, tomato, and beans.
Anther Pollen producing organ in the flower.
Antibiotic An organic compound that is made and secreted by a living organism(commonly a mold) and is able to prevent the growth and reproduction of anotherspecies. Example: Penicillin is synthesized and secreted by the moldPenicillium notatum.
Antibodies The body produces substances known as antibodies to fightdisease-producing agents. Antibody production is a relatively slow process.First of all, the body cells must recognize the invading agent as "foreign" --an antigen -- and then produce an antibody that is exactly right to immobilizethe protein invader. Finally, the blood cells must go into full scaleproduction of this specific antibody. An antibody is specific against "aparticle" type of germ. For example: diphtheria antibodies will not beeffective against scarlet fever antigen.
Anticoagulant A substance that prevents blood from clotting. Heparin is ananticoagulant.
Anticodon A "triplet" of nucleotides in transfer RNA that is able to pair witha complementary triplet (a codon) in messenger RNA, thus arranging transfer RNAto the proper site on the messenger RNA.
Antigen A foreign protein that stimulates the formation of antibodies by theimmune system.
Antitoxin An antibody that works against a specific toxin.
Anus The body opening at the posterior end of the alimentary canal.
Aorta A large artery that carries blood away from the heart. See Heart.
Apical dominance A process in which the terminal bud (top of the stem) inplants inhibits the growth of buds at the sides of the stem.
Appendix A small protruding pouch without function positioned where the smallintestine joins the large intestine. See Digestive system.
Arachnids The class to which spiders belong. The body of the spider is dividedinto the cephalothorax and the abdomen. The cephalothorax has six pairs ofjointed appendages. Spinnerets at the end of the abdomen are projectionsthrough which the spider spins webs. Most spiders breathe by book lungs. Othermembers of the class Arachnida are mites, ticks, scorpions, horseshoe crabs,and harvestmen.
Archegonium The multicellular, water-retaining structure, in which a singleegg is produced and fertilized in the liverworts, mosses, and hornworts.
Archenteron The cavity appearing in the early embryo during the gastrula stagethat ultimately becomes the gut cavity.
Arteriole A small artery.
Artery A blood vessel that transports blood from the heart to the organs andtissues of the body.
Arthropods The arthropod group is by far the largest group of animals withrespect to the number of species it contains. They share the commoncharacteristics of having segmented bodies, jointed appendages, andexoskeletons.
Asexual reproduction Involves only one parent. The parent may divide andbecome two new cells, thus obliterating the parent generation. Or the newindividual may arise from a part of the parent cell; in such a case, the parentremains. Types of asexual reproduction are binary fission, sporulation budding,regeneration, and parthenogenesis.
Assimilation Involves the changing of certain nutrients into the protoplasm ofcells.
Atoms Elements are made of invisible building blocks called atoms. Each atomhas a central nucleus surrounded by a definite number of moving negativelycharged electrons.
ATP (Adenosine triphosphate) The compound that stores energy produced duringcellular respiration and releases this energy when needed for the cell's work.Energy produced during cellular respiration is passed on to adenosinediphosphate (ADP), which then becomes upgraded to ATP.
Atrium The chamber of the heart (also known as the auricle) that receivesblood and passes it to the ventricle for pumping.
Autonomic nervous system See Nervous system, autonomic.
Autosome A chromosome that does not determine sex. Human cells have 22 pairsof autosomes.
Autotrophs Organisms that are able to change inorganic materials into organiccompounds. Among these are the photosynthetic bacteria and the green plantsthat use light energy to produce food. The chemosynthetic bacteria are capableof oxidizing the inorganic compounds of ammonia, nitrites, sulfur, or hydrogengas into high-energy, organic compounds without the need of light energy.
Auxins Plant hormones called auxins are produced by actively growing planttissues like the growing tips of roots and branches, developing leaves, orflowers and fruits. They promote cell enlargement, which is one phase of growth(the other is cell division).
Axon Extension of the neuron (nerve cell) that can carry impulses; is oftenthe longest and least branched process of the cyton (cell body) and usuallycarries impulses away from the cell body of the neuron.
B cell A white blood cell known as a lymphocyte produced in the bone marrowthat works with T cell lymphocytes to destroy germs.
Back cross (test cross) Plant and animal breeders mate an individual with aparticular dominant trait to an individual that is recessive to determine ifthe dominant trait is pure or hybrid. As a result of this mating, if organismsappear that have the recessive trait, then the breeder knows that the parentindividual is hybrid (heterozygous) for the dominant trait.
Bacteria Bacteria are the smallest living organisms. They range in length from0.2-7 micrometers; in diameter from 0.2-2 micrometers. As you recall, the unitused to measure bacteria is the micrometer and is equivalent to 1/1000 of amillimeter. The smallest cells known are the mycoplasmas, which have onlyone-half the DNA of other bacterial cells. The mycoplasmas are bacteria thatlive only as parasites on or in the bodies of plants and animals. Despite theirsmall size bacteria are true cells: they provide their own genetic material(DNA and RNA) and the necessary cytoplasm for their own reproduction; they havemultienzyme systems to control biochemical activities necessary for the life ofthe cell; and they build their own ATP molecules and use the stored energy tosynthesize other organic compounds. Bacterial shapes include spheres (cocci),rods (bacilli), and spirals (spirilla). See also Eubacteria -- true bacteria.
Bacteriophage A virus that infects a bacterial cell.
Biennial A plant for which the life cycle is two years. Vegetative growthoccurs during the first year; flowering, the second. Beets and carrots areexamples.
Bilateral symmetry Two sided, as demonstrated by animals that have an anterior(head end) and a posterior (tail end), a right and left side that are similar.For example: a human being shows bilateral symmetry, as does a horse. If sliceddown the middle, each has two identical halves.
Binary fission A form of asexual reproduction in which a parent organismdivides into two identical daughter cells. The nucleus goes through mitosis.The cytoplasm divides equally.
Binomial nomenclature The scientific naming of species by a double name. Thefirst name in the binomial is the genus name. This system was developed byCarolus Linnaeus (1707-1788) and has become a worldwide standard forclassifying and naming organisms.
The Genus QuercusScientific Name Common NameQuercus alba white oakQuercus coccinea scarlet oakQuercus montana chestnut oakQuercus rubra red oakQuercus suber cork oakQuercus virginiana live oak
Biogenesis The doctrine that living things come only from other living thingsof like kind.
Biogeochemical cycles Certain compounds cycle through the abiotic portion andthe biotic communities of ecosystems. These compounds contain elements that arenecessary to the biochemical processes that are carried out in living cells.Among these elements are carbon, hydrogen, oxygen, and nitrogen. In elementalform, they are useless to cells and must be combined in chemical compounds. Letus trace the pathways of some of the vital compounds from the earth to livingorganisms to the atmosphere and back to earth. This cycle of events is bestdescribed by the term biogeochemical cycles. See figure.
Biology The study of living things; an extensive science including botany,zoology, bacteriology, genetics, physiology, anatomy, and many others.
Biome A climax community in a broad geographical area having one type ofclimate. Examples of biomes are the Taiga, coniferous forests of Canada; theDesert, regions where the annual rainfall is less than 6.5 centimeters; theGrasslands, annual rainfall is low and irregular; the Tropical Rain Forest,characterized by high temperatures and constant rainfall; and the Marine biome,the sea.
Biomes, world A climax community in a broad geographical area having one typeof climate is known as a biome. The earth is divided into several biomes.
The Tundra. Vast stretches of treeless plains surrounding the Arctic Oceanwhere cold is the limiting factor. Plant life consists of lichens, mosses,grasses, and sedges. Animal life includes the musk ox, caribou, polar bears,wolves, foxes, and some marine mammals.
The Taiga. The coniferous forests of Canada and Russia where spruce and firtrees predominate. The kinds of mammals that live in this region are the blackbear, the wolf, lynx, and squirrel.
The Deciduous Forest. These are forests of temperate regions, wherebroad-leaved trees that lose their leaves in the winter predominate. The typesof animals that live in these forests are deer, fox, squirrel, skunk,woodchuck, and raccoon.
The Desert. Deserts form in regions where the annual rainfall is less than 6.5centimeters and where evaporation of water is high. Creosote, sagebrush, andcacti are plants adapted for the desert. The animals include lizards, insects,kangaroo rats, and arachnids.
The Grasslands. These regions have low annual rainfall. Grasslands are locatedin regions that are sheltered from moisture-laden rainfall. The animals thatpredominate in temperate grasslands, called steppes or prairies, are bobcats,badgers, hawks, kit foxes, owls, and coyotes. Typical animals of tropicalgrasslands or savannas are zebras, giraffes, baboons, and gazelles.
The Tropical Rain Forest. The tropical rain forest is characterized by hightemperatures and constant rainfall. This type of biome is found in Central andSouth America, in Southeast Asia, and in West Africa. The trees are tall andthe vegetation is so thick that the forest floor is shaded from light. Theanimals of the rain forest include monkeys, lizards, snakes, and birds.
The Sea. Ocean waters cover almost three fourths of the earth's surface andsupport the greatest abundance and diversity of organisms in the world.Averaging 3.5-4.5 kilometers in depth, a marine biome constitutes the thickestlayer of living things in the biosphere. The dominating physical factorsdetermine the type of living organisms that compose its communities.
Biometrics The science that combines mathematics and statistics needed to dealwith the facts and figures of biology. Biologists handle enormous numbers thatmust be organized and simplified so that they become useful in the analysis ofdata.
Biotic environment The part of the environment that is living and has someeffect on other living organisms.
Bipedalism The ability to walk on two legs instead of four. Bipedalism hasfreed the forearms for doing work.
Birds Birds are terrestrial vertebrates with feathers. Feathers are thedistinctive feature of birds: all birds have them and no other animals are socovered. The forelimb is modified into wings for flight, leaving the hindlimbsfor walking (bipedal locomotion). Birds are built for flight; specialadaptations in body structure effect lightness in weight, efficiency, andstrength. Not only are the feathers light in weight and easily moved and liftedby wind, but they also create warmth next to the body. Body heat warms the airthat is in contact with the bird's body. Warm air becomes lighter and rises.Other adaptations for flight are the compact, but hollow, bones, numerous airsacs occupying all available body spaces, reduced rectum, loss of teeth, andfeathers replacing a bony tail.
Bivalves Animals, such as clams, that have the body encased within two hingedshells. Lining the inner surfaces of the shells is a membranous mantle. Thecavity inside of the shells is the mantle cavity.
Blastula An early stage in animal embryology; a hollow ball of cellssurrounding a central cavity. See Cleavage.
Blood A liquid tissue consisting of a liquid medium called plasma and threekinds of cells: red blood cells (erythrocytes), white blood cells (leucocytes),and platelets. There are about 25 trillion red blood cells in the human body.For each 600 red blood cells there is one white blood cell. Blood cells numberin the billions. The red blood cells carry oxygen. The white blood cellsfunction in immune reactions.
Blood clotting Platelets are the smallest blood particles. When a capillary iscut, the platelets collect at the site of the injury. There they break intosmaller fragments and initiate the complicated chemical process of bloodclotting in which more than 15 factors, including thromboplastin, calcium(Ca27), and fibrinogen, are involved in the formation of a clot containingblood cells in a fibrin meshwork.
Blood types The main types of blood are A, B, AB, and O. Transfusions of bloodare possible only when the blood types are compatible. If the blood types arenot compatible proteins in the plasma will recognize foreign antigens on redblood cells and respond by causing the cells to agglutinate, or clump, acondition that causes blockage in small blood vessels and often death. Thefollowing table summarizes the blood proteins involved in blood types.
Proteins of Blood TypesBlood Cell PlasmaType Antigen AntibodyA A bB B aAB AB noneO none a and b
Note: Type AB -- universal recipient Type O -- universal donor
Bone The living tissue that comprises the skeleton. Bone tissue is made ofcells that are surrounded by hardened calcium phosphate.
Bowman's capsule See Nephron.
Brain The brain and the spinal cord compose the central nervous system. In thevertebrate body, the organs of the central nervous system are well protected bybeing wrapped in connective tissue and enclosed in bone. The brain, covered bythe membranous meninges, rests in the skull cavity where it is enclosed by thecranium. The spinal cord, also covered by connective tissue, is circled by thevertebral column. The human brain is divided into many parts, each with specialfunctions. Among the most important parts are the cerebrum, cerebellum, andmedulla.
Bronchi (Bronchus) Two tubes made of cartilage rings that extend from thewindpipe (trachea) into the lungs (See Lungs).
Bronchioles Small tubes in the lungs that branch off from the bronchi. Eachbronchiole ends in an air sac called the alveolus. See Respiratory system.
Bryophytes The bryophytes are the first green land plants. They are primitive,small, and inconspicuous. Although multicellular, the tissue differentiation isquite simple. Bryophyte species have no tissues that are specialized forwater-carrying and no cambium specialized for growing new cells. Bryophytespecies do not have true stems, leaves, or roots. Simple rootlike structurescalled rhizoids anchor the plants to the ground and absorb moisture from thesoil.
Budding A form of asexual reproduction in which the parent cell body givesrise to a bud. The bud has the same number and kind of chromosomes as theparent cell, but has much less cytoplasm than the parent cell. The bud developsinto a new individual. Example: yeast cells reproduce by budding.
Calorie The amount of heat required to raise the temperature of one gram ofwater by one degree Celsius (1 degree C). (A kilocalorie is the amount of heatneeded to raise the temperature of one kilogram of water by 1 degree C.)
Calvin cycle The second major stage of photosynthesis involves reductivecarbon dioxide fixation. Because the cyclic reactions that function at thistime do not require light as a source of energy, the term dark reaction hasbeen used to designate this phase of photosynthesis. Calvin and his associatesdetermined the path of carbon in the carbon dioxide by the use of 14C. SeePhotosynthesis.
Cambium See Woody stems.
Capillary The smallest blood vessel.
Carbohydrates Are characteristically composed of carbon, hydrogen, and oxygenin the proportion of CH2O. The hydrogen and oxygen are in the same proportionin carbohydrates as in water.
Carboxyl group See Amino acids.
Carcinogen Any substance or radiation that causes cancer; asbestos, theherbicide dioxin, or radiation from radioactive materials, sunlight, or X raysare examples.
Carnivores Flesh eaters such as snakes, frogs, hawks, and coyotes.
Carrier-facilitated diffusion Transport of a substance across the plasmamembrane by carrier molecules but without energy. This process cannot effectthe net transport of a substance from a region of low concentration to a regionof high concentration. This is a form of passive transport.
Carrying capacity In ecology, the largest number of organisms of a givenspecies that can be maintained indefinitely in a particular part of theenvironment.
Cartilage A specialized type of dense connective tissue not as hard as bone,in which the cells are contained in a rubbery matrix that is smooth, firm, andflexible: occurs in joints, at the end of bones, and in the ears, nose, andwindpipe.
Catalyst A chemical substance that speeds up a reaction without itself beingused up in the overall course of the reaction. Enzymes are biologicalcatalysts.
Cell Each cell is a living unit. Whether living independently as a protist orconfined in a tissue, a cell performs many metabolic functions to sustain life.Each cell is a biochemical factory using food molecules for energy, repair oftissues, growth, and ultimately, reproduction. On the chemical level, the cellcarrries out all of the life functions. Living organisms function the way theydo because their cells have the properties of life.
Cell division When a cell reaches a certain size it divides into two newcells, identical to each other and very similar to the original parent cell.The new cells are known as daughter cells. The events marking cell divisiondiffer in prokaryotes and eukaryotes. See Mitosis.
Cell membrane The outer boundary of the cell, also called the plasma membrane,about 10 nanometers in width. The cell membrane controls the movement ofsubstances into and out of the cell in a process known in general as transport.Highly selective as to the substances that cross its boundary, the cellmembrane is said to be semi-permeable. Using the concept demonstrated by thefluid mosaic model, biologists explain how some molecules are able to penetratethe cell membrane while others cannot. According to this model, the core of themembrane is made up of phospholipids. Large circular proteins are set into themembrane; smaller proteins lie on the surface. The proteins and thephospholipids have special functions and form certain structural pathways thatserve to admit or deny passage to specific molecules.
Cellulose A straight chain polymer of glucose molecules secreted by plants andused as structural supporting material.
Cell wall A relatively rigid structure composed of cellulose that encloses thecells of plants. The cell wall gives these cells their shape and limits theirexpansion in hypotonic media.
Central nervous system See Brain.
Centrioles Paired structures that lie just outside of the nucleus of nearlyall animal cells and some cells of lower plants. They are absent in cells ofhigher plants. Under the light microscope, the centrioles look like twoinsignificant granules, but the electron microscope demonstrates that they havea very intricate structure.
Cerebellum See Brain.
Chemosynthesis The production of high-energy organic compounds from inorganicraw materials without the aid of light energy by some bacteria that live onammonia, nitrogen, and sulfur.
Chitin Tough, nitrogen-containing polysaccharide that is present in theexoskeletons of insects and in the cell walls of most species of fungi.
Chlorophyll See Chloroplasts.
Chloroplasts A group of structures that has the general name plastid. Plastidsare membrane-bound organelles found only in plant cells. Usually plastids arespherical bodies that float freely in the cytoplasm, holding pigment moleculesor starch. Chloroplasts contain the green pigment chlorophyll, a substance thatgives plants the green color. Chlorophyll is a special molecule that has theability to trap light and to convert it to a form of energy that plants can usein carrying out the chemical steps of the food-making process known asphotosynthesis. Each chloroplast is surrounded by a double membrane. Inside thechloroplast are numerous flattened membranous sacs called thylakoids (formerlycalled grana). The thylakoids are the structures that contain the chlorophylland it is within these sacs that photosynthesis takes place. Stroma is the namegiven to the dense ground substance that cushions the thylakoids. Animal cellsdo not have chloroplasts and therefore cannot make their own food. The figureshows the fine structure of a chloroplast.
Chordates Set apart from lower animals by several distinguishingcharacteristics in addition to having a notochord. First, all chordate embryoshave the three primary germ layers from which all specialized tissues andorgans develop. Second, chordates are bilaterally symmetrical animals withanterior-posterior differentiation. Third, the body has a true coelom and adigestive tract that begins with a mouth and ends with an anus. Othercharacteristics that differentiate the chordates from other animals are thepresence of pharyngeal gill slits and the dorsal hollow nerve cord.
Chromatography A technique of separating substances, such as proteins in acomplex liquid, by varying their rates of absorption on media such as filterpaper or in a column of silicia gel.
Chromosomes In the nucleus of the nondividing cell is a tangle of very finethreads that absorb stain quite readily. In the granular stage these threadsare known as chromatin. The chromatin threads come together, shorten andthicken forming chromosomes that can be seen quite prominently in the dividingcell.
Chromosome Numbers of SomeCommon SpeciesOrganism Haploid No. Diploid No.mosquito 3 6fruit fly 4 8gall midge 20* 8*evening primrose 7 14onion 8 16corn 10 20grasshopper (female) 11 22grasshopper (male) 10 21**frog 13 26sunflower 17 34cat 23 38human 23 46plum 24 48dog 39 78sugar cane 40 80goldfish 47 94
* In the fertilized egg of the gall midge, 32 chromosomes become nonfunctionalleaving 8 functional chromosomes.** The male grasshopper has only one sex chromosome.
Cilia See Flagella and cilia.
Circulation Distribution of blood pumped from the heart, through the arteries,arterioles, and capillaries to the body tissues. Blood is carried back to theheart by veins.
Circulatory system The human circulatory system consists of the heart and thesystem of blood vessels that transport blood throughout the body. See Heart.
Cistron The genetic unit of function, considered equivalent to a gene. Eachcistron contains the genetic information for a single polypeptide chain.
Classification The design of the classification system is a simple andpractical one that easily lends itself to the addition of new names oforganisms as they are discovered. Each group of organisms within the scheme isknown as a taxon (plural, taxa). The classification groupings are as follows:kingdom, the largest and most inclusive group, followed by the phylum, class,order, family, genus, and species.
Cleavage The fertilized egg goes through a series of cell divisions in whichthere is no growth in size of the zygote nor separation of the cells.
Climax community A stable ecological community where one or two large treespredominate.
Clone A population of cells (or whole organisms) that has descended from anoriginal parent cell, which was stimulated to reproduce by asexual means.
Codominance A form of inheritance in which neither of the allelic genes thatdetermine a characteristic is dominant over the other. The result of thisinheritance is a blend. When red-flowered primroses are crossed withwhite-flowered primroses, the offspring have pink flowers.
Codon A "triplet" of three nucleotides in messenger RNA that directs the orderof a particular amino acid in a protein molecule.
Coenzyme An organic compound, not a protein, that supports the catalyticactivity of an enzyme. Vitamins are coenzymes.
Coenocyte A cell bounded by a single plasma membrane but containing manynuclei.
Cold-blooded animal Poikilotherm. An animal whose body temperature changeswith the external environment. Examples: fish, amphibians, reptiles.
Commensalism See Nutritional relationships.
Community All of the plant and animal populations living and interacting in agiven environment are known as a community.
Compounds Classified as organic or inorganic. Organic compounds are generallythought of as most compounds of carbon. Inorganic compounds are compounds madeof other elements and a few carbon compounds similar to earthlike substances,like calcium carbonate (CaC03). Organic compounds are called organic because ofthe original belief that they came from living organisms. Today thousands oforganic compounds not found in nature are being synthesized in laboratories.Both organic and inorganic compounds are necessary to life.
Conditioned behavior A type of learned response in which a new responsebecomes associated with an original stimulus.
Conifers (cone bearers) See Gymnosperms.
Conjugation A form of sexual reproduction that is occasionally demonstrated bythe ciliates. Two organisms will join together at the oral groove. Themicronucleus of each will undergo meiosis, producing several cells. All but twoof these in each organism disintegrate. One of these haploid micronucleiremains in each cell, while the other migrates into the other cell, fusing withthe stationary gamete. The new nucleus -- which is now diploid and contains anew genetic combination -- goes through cell division producing a newmacronucleus and a new micronucleus.
Consumers Primary consumers -- herbivores, or plant-eaters. Herbivores come inall sizes: crickets, leaf cutters, deer, and cattle. The carnivores, orflesh-eaters, such as snakes, frogs, hawks, and coyotes are secondary consumersbecause they feed on the herbivores. The tertiary consumers are those that feedon the smaller carnivores and herbivores as well. There are also scavengers inthe ecosystem. Earthworms and ants feed on particles of dead organic matterthat have decayed in the soil. Vultures eat the bodies of dead animals.
Contractile vacuole See Paramecium.
Cotyledons The first leaves of a plant; they are often strikingly differentfrom later leaves. In some plants like the bean, they contain large quantitiesof stored food that get the embryo off to a good start until it can shift foritself. They are the two halves of the bean seed. Cotyledons usually persistfor only a short while after germination.
Crossing over Genes are linked on chromosomes and are inherited in a group ona particular chromosome. However, linkage groups are broken by crossing over, aphenomenon that may occur during meiosis when homologous chromosomes areintertwined during synapsis. It is at this time that chromosomes may exchangehomologous parts and thus assort linkage groups.
Crustacea Derived from the Latin crusta meaning crust, the name Crustaceadescribes the lobsters and their relatives aptly. The body is covered by atough exoskeleton arranged in the form of arched plates that thin out at thejoints to permit maximum movement. The lobster is representative of this class.
Cyclic AMP (cAMP; cyclic adenosine monophosphate) A compound formed from ATPthat regulates the effects of numerous hormones in animals (second messenger).
Cyclosis The circulation of cytoplasm within cells. This is especially true inplant cells where there are large vacuoles. Protoplasm flows around the marginsand in cytoplasmic strands that sometimes extend through the vacuoles.Protoplasm in adjacent cells may flow in opposite directions, or it may reversedirections in any particular cell. Light and temperature especially influencethis action in plant cells, but the exact cause of movement is unknown.Circulation results in a thorough mixing of protoplasm and its contents.Cyclosis of protoplasm in palisade cells of leaves may prevent the overexposureof chloroplasts to light, since they remain in a position of maximum exposureonly momentarily.
Cytochromes Iron-containing red proteins; molecules of the electron-transfermachinery in photosynthesis and respiration.
Cytokinesis The division of cytoplasm occurring during the last state(telophase) of mitosis.
Cyton Nerve cell body. See Nerve cell (neuron).
Cytoplasm The ground substance of the cell that supports all of the cell'sorganelles. See Cell.
Cytosine A nitrogen base that pairs with guanine in DNA and RNA.
Dark reaction The stage of photosynthesis in which carbon dioxide fixationoccurs, resulting in the formation of sugar. See Calvin cycle.
Daughter cells See Cell division.
Deciduous forest The forests of the temperate regions are dominated bybroad-leaved trees that lose their leaves in the winter. Examples of the kindsof trees that compose these hardwood forests are oak, hickory, chestnut, beech,maples, willows, cottonwood, and sycamore. The types of animals that inhabitthese forests are deer, fox, squirrel, skunk, woodchuck, and raccoon.
Decomposers Decomposers form an important part of ecosystems. Bacteria andfungi are organisms that break down dead organic matter and release from itorganic compounds and minerals that are returned to the soil. Many of thematerials returned to the soil are used by the producers in the process offood-making. Without the work of the decomposers the remains of dead plants andanimals would pile up, not only occupying space needed by living organisms, butalso keeping trapped within their dead bodies valuable minerals and compounds.
Dehydration synthesis As the two molecules join, a molecule of water isproduced during the process in addition to the double sugar. A synthesis ofthis type is known as dehydration synthesis. Within living cells,carbohydrates, proteins, and fats are formed by dehydration synthesis.
Dendrite See Nerve cell (neuron).
Denitrifying bacteria Soil bacteria that change nitrates back to atmosphericnitrogen. The cycle then repeats.
Deoxyribonucleic acid (DNA) DNA molecules are the particular type of nucleicacid out of which genes are made. Genes are the bearers of hereditary traitsfrom parent to offspring. See also Replication.
Desert Deserts form in regions where the annual rainfall is less than 6.5centimeters, or where rain occurs unevenly during the year and the rate ofevaporation is high. The temperature changes drastically from hot days to coldnights. Plants that survive in the desert have specific adaptations for lowmoisture and high temperature. Examples of desert plants are creosote,sagebrush, cacti, and cheat grass. Examples of desert animals are lizards,insects, kangaroo rats, and arachnids.
Diaphragm The muscular structure that separates the chest cavity from theabdominal cavity in mammals; in a microscope, the part that regulates lightentering the lens system.
Dicotyledon (dicot) A type of angiosperm plant in which the embryo plant iscontained between two seed leaves or cotyledons.
Differentiation See Embryo.
Diffusion The movement of molecules from an area of greater concentration toan area of lesser concentration. Diffusion is a type of passive transport.
Digestion Begins in the mouth. Teeth grind the food while three pairs ofsalivary glands pour salivary juice (saliva) into the mouth. Saliva containsthe enzyme salivary amylase (ptyalin), which begins the digestion of starch.The moistened, chewed food is swallowed and moves through the throat into thefood tube, or esophagus. The esophagus has no digestive function but moves thefood into the stomach by waves of muscle contractions called peristalsis.Chemical digestion is known as hydrolysis. Extracellular digestion takes placeoutside of cells. Intracellular digestion takes place inside of cells withincell vacuoles.
Digestive system The human digestive system begins with a mouth and ends withan anus, and is often described as a "tube within a tube." Variously called thegut, alimentary canal, or the gastrointestinal tract, the digestive systemextends from the lower part of the head region through the entire torso.Essentially, this system carries out five separate jobs that have to do withthe processing and distribution of nutrients. First, it governs ingestion orfood intake. Second, it transports food to organs for temporary storage. Third,it controls the mechanical breakdown of food and its chemical digestion. Afourth function is the absorption of nutrient molecules. Its final piece ofwork is the temporary storage and then elimination of waste products.
Dihybrid Mixed genes for two traits. See Heredity, Mendelian.
Dinoflagellates Dinoflagellates are small protists and usually unicellular.Most of these organisms have two unequal flagella, one extending longitudinallyfrom the posterior end of the cell, the other encircling the central part ofthe cell. Some dinoflagellates extend trichocysts like the paramecium; othershave nematocysts, stinging cells common in the coelenterates. Some species --Noctiluca, for example -- are bioluminescent, giving off light like a firefly.
Diploid number The full complement of chromosomes in somatic (body) cellsdesignated by the symbol 2N; also known as the species number or chromosomenumber.
Disease A disease is a disorder that prevents the body organs from working asthey should. In general, diseases can be classified as being infectious ornoninfectious. Infectious diseases are caused by organisms that invade the bodyand do harm to the cells, tissues, and organs. As a rule, there is diseasespecificity when a specific disease-producing organism causes a particulardisease. Disease-producing organisms are said to be pathogens and are describedas being pathogenic. Noninfectious diseases are caused by factors other thanpathogenic organisms. Among the factors that are responsible for noninfectiousdiseases are genetic causes, malnutrition, exposure to radiation, emotionaldisturbances, organ failure, poisoning, endocrine malfunctioning, andimmunological disorders. Whatever the cause, a disease works counter to thewell being of the diseased organism.
Dominant trait When organisms with contrasting traits are crossed, the traitthat shows up in the F1 generation is called the dominant trait. The trait thatis hidden is called the recessive trait.
Double helix See Deoxyribonucleic acid (DNA).
Droplet infection A common method of passing germs along. Disease germs arepresent in droplets ot water that escape from the nose and mouth when sneezing,coughing, and talking. If these infected droplets are inhaled or taken in bymouth, the germs then enter the body of another person.
Ear and hearing The human ear is made up of three divisions: the outer ear,middle ear, and inner ear. The outer ear catches sound waves and transportsthem to the eardrum, a membrane that stretches across the outer canalseparating it from the middle ear. Sound waves cause the eardrum to vibrate.The middle ear contains three very small bones called the hammer, anvil, andstirrup. These are the smallest bones in the body. These bones accept thevibrations from the eardrum and transmit them to the oval window, one of twosmall membrane-covered openings between the middle ear and the inner ear. Theinner ear, which is entirely encased in bone, has a fluid-filled structurecalled the cochlea, so named because it resembles a snail in shape. The cochleahas numerous canals that are lined with hair cells. The vibrations from theoval window are transmitted to the hair cells in the cochlea and thence on theauditory nerve, which conducts the vibrations to the brain. In the brain,signals are interpreted into sounds.
Echinoderms Spiny-skinned invertebrates that include the starfish, brittlestars, sand dollars, sea urchins, and sea cucumbers. Although they do not lookvery much like vertebrate animals, the development of the echinoderm embryostrongly resembles that of the chordates in the early stages. The larval stageis free-swimming and shows bilateral symmetry.
Ecological niche An important concept of ecology is that of the niche. Anecological niche is a feeding pattern exhibited by species that compose acommunity. A niche is a feeding way-of-life in relationship to other organisms.
Ecology The science that studies the interrelationships between living speciesand their physical environment. The word "ecology" was coined in 1869 by theGerman zoologist Ernst Haeckel to emphasize the importance of the environmentin which living things function. The environment includes living or bioticfactors and nonliving factors referred to as abiotic factors.
Ecosystem The living community and the nonliving environment work together ina cooperative ecological system known as an ecosystem. An ecosystem has no sizerequirement or set boundaries. A forest, pond, and field are examples ofecosystems. So is an unused city lot, small aquarium, the lawn in front of aresidential dwelling, or a crack in a sidewalk. All of these examples reflectareas where interaction is taking place between living organisms and thenonliving environment.
Ectoderm See Embryo.
Effectors The kind of responses that organisms can make is related to theirown body equipment. The responding part is the effector. Most animal effectorsare either muscles or glands. In considering responses one ordinarily thinksabout rapid, visible muscular responses. Glandular responses are much moresubtle.
Embryo The stage in human or animal development following cleavage. Duringembryonic development the tissues and organs are being built. When the embryotakes human or animal form, it is known as a fetus. The process in whichtissues are formed is known as differentiation.
Differentiation of theThree Primary Germ LayersEctoderm Endoderm Mesodermskin lining of lungs musclesnervous system lining of skeleton digestive systemsense organs pancreas heart liver blood vessels respiratory blood system ovaries, testes kidneys
Embryonic membranes The embryo produces several membranes that do not form anypart of the new baby but which are necessary to the development and well beingof the embryo. One of these membranes is the amnion, a waterfilled sac thatcompletely surrounds and protects the embryo. The water absorbs shocks andprevents friction that might damage the embryo. The implanted embryo isattached to the uterus by means of the umbilical cord, a structure thatcontains blood vessels that function in carrying nutrients and oxygen to theembryo and transporting wastes away from the embryo. The umbilical cordconnects with the placenta, a vascularized organ made up of tissues of themother and embryo. See Allantois.
Endocrine system Made up of the pituitary gland, thyroid gland, parathyroidglands, the adrenal gland, the isles of Langerhans in the pancreas, the thymusgland, the pineal gland, and the gonads -- testes in the male and ovaries inthe female. Certain secretions of the stomach and small intestine are alsohormones and thus part of the endocrine system. Through their secretions theendocrine glands regulate growth, rate of metabolism, response to stress, bloodpressure, muscle contraction, digestion, immune responses, and the developmentand functioning of the reproductive system. Hormones exert their influence bybecoming involved with the genetic machinery of cells and by affecting themetabolic activities of cells working through the cellular respirationpathways.
Endoderm See Embryo.
Endoplasmic reticulum Spreading throughout the cytoplasm, extending from thecell membrane to the membranes of the nucleus is a network of membranes thatform channels, tubes, and flattened sacs; this network is named the endoplasmicreticulum. One function of the endoplasmic reticulum is the movement ofmaterials throughout the cytoplasm and to the plasma membrane. The endoplasmicreticulum has other important functions related to the synthesis of materialsand their packaging and distribution to sites needed.
Enzyme-substrate complex An enzyme affects the rate of reaction of thesubstrate molecule that fits the enzyme's activity site. In order for this tohappen, a close physical association must take place between enzyme andsubstrate. This association is called the enzyme-substrate complex. SeeSubstrate.
Enzymes (organic catalysts) A catalyst is a molecule that controls the rate ofa chemical reaction but is itself not used up in the process. Enzymes areproteins that control the rate of chemical reactions that take place in cells,tissues, and organs. Each chemical reaction that occurs in a living systemrequires the assistance of a specific enzyme (enzyme specificity).
Epinephrine The hormone secreted by the medulla of the adrenal gland; alsocalled adrenaline. It is secreted as a result of stress and produces effects onthe circulatory system and on glucose mobilization.
Epithelial tissue See Tissues.
Estrus The period of maximum sexual receptivity in the female mammal. Estrusis also the time of the release of eggs in the female.
Eubacteria -- true bacteria This group, referred to as the true bacteria,represents a large number of species. All of these bacteria have thick andrigid cell walls. Some of the species are nonmotile (nonmoving), while othersare motile, using flagella or a sling motion to move from place to place.Species belonging to the eubacteria are identified by their shapes. Therod-shaped bacteria are known as bacilli (bacillus, sing.), the round bacteriaas the cocci (coccus, sing.), and the spiral-shaped as spirillae (spirillum,sing.). Some species typically remain attached: diplococci occur in pairs,streptococci in chains, and staphylococci in clusters. See also Bacteria.
Eukaryotes Organisms whose cells contain the genetic material (DNA) enclosedin a nucleus; includes all organisms above the level of bacteria and blue-greenalgae.
Evolution Concerns the orderly changes that have shaped the earth and thathave modified the living species that inhabit the earth. Evolution is a fusionof biological and physical sciences that have provided supporting data thatconfirm the fact that over periods of time major changes have occurred in theinterior of the earth and on its surface, accompanied by modifications inclimate. All of the changes in the earth are classified as nonbiological orinorganic evolution. Changes that have taken place in living organisms areknown as biological or organic evolution.
Evolution, evidences of Evidence that evolution -- gradual change over aperiod of time -- has occurred in living things is provided by many sciencesand includes facts from the geologic record, the study of fossils, and evidencefrom cell studies, biochemistry, comparative anatomy, and comparativeembryology.
Evolution, theories of Since the eighteenth century several theories have beenproposed to explain evolution. Among these are the use and disuse theory ofLamarck and the theory of natural selection formulated by Charles Darwin.Recent advances in genetics cell biology, and functional anatomy andbiochemistry have led to the formulation of a modern theory of evolution basedon Darwin's concept of natural selection.
Excretion Removes waste products of cellular respiration from the body. Thelungs, skin, and kidneys are excretory organs in humans that remove carbondioxide, water, and urea from the blood and other body tissues. Guttation isthe excretion of drops of water from plants during periods of high humidity.
Excretory system In human beings, the lungs, skin, and urinary system work toexpel the wastes produced in metabolic activities. The lungs excrete carbondioxide and water. The skin expels water and salts from the sweat glands and asmall amount of oil from the sebaceous glands. The urinary system handles themajor work of excretion.
Exocrine glands Glands, such as salivary and sweat glands, from which thesecretions are discharged through ducts directly into an organ or onto thesurface of the body.
Exoskeleton A hard covering on the outside of the animal body. The exoskeletonof arthropods has the same functions of support as the bony internal skeletonof vertebrates.
Eye The human eyeball measures about 2.5 centimeters in diameter. Most of theeyeball rests in the bony eyesocket of the skull. Only about one sixth of theeye is exposed. External structures associated with the eye are eyelids,lashes, and eyebrows. A transparent protective membrane, the cornea, covers theeye. Six small muscles attach the eye to the eyesocket. Secretions from tearglands help to keep the eye moist. The lens is a transparent body that focuseslight on the light-sensitive retina where images are formed.
Fats Organic compounds that supply cells with energy. Fats are usuallyinsoluble in water. A fat results from the combination of one glycerol andthree long-chain fatty acids joined through dehydration synthesis. Certain fatsare essential to the structure and function of body cells, to the building ofcell membranes, and to the synthesis of certain hormones. Fats also aid in thetransport of fat-soluble vitamins. Foods rich in fats include butter, bacon,egg yolk, cream, and certain cheeses.
Fermentation See Anaerobic.
Ferns The fern plant used in flower bouquets is the sporophyte generation. Thesporophyte generation produces asexual spores. The mature fern has true roots,leaves, and stem. Ferns growing in temperate climates have an underground stemcalled a rhizome that grows in a horizontal position. The rhizome not onlystores food materials, but also gives rise to new fern plants that grow alongits length. The stems of tropical species grow upright in a vertical positionand serve as trunks of tree ferns.
Fertilization, birds Fertilization is accomplished during mating at which timethe male and female place their cloacas close together. Sperm swim from thecloaca of the male into the female cloaca and up into the oviduct.Fertilization takes place high up in the oviduct before the albumen and theother surrounding membranes are secreted by the oviduct cells. Most birds lay aclutch of less than six eggs. However, ducks may lay as many as 15 eggs at onetime. See Allantois.
Fertilization, fish Many sperm never reach the eggs and many fertilized eggsdie before development. Hence there is an overproduction of gametes to ensurethe survival of the species. In a few fish species fertilization is internaland parental care is given to the fertilized eggs. The stickleback male, forexample, takes care of the fertilized eggs in nests and the male seahorsecarries them around in a brood pouch.
Fertilization, frogs In frogs, fertilization is external. Sperm leave thetestes through tubules called vasa efferentia that communicate with the kidney.The sperm cells then pass into the Wolffian duct that leads to the cloaca, apassageway that opens to the outside of the body. In the female, large eggmasses are released into the body cavity from two ovaries, located at theanterior end of each kidney. Beating cilia sweep the eggs into coiled tubulesknown as oviducts where they are propelled to the cloaca and then out of thebody. As the eggs pass through the oviducts they are coated with a thin layerof jelly-like material. At the time when the female is depositing eggs in theshallow waters of a pond or brook, the male deposits sperm over them. The spermswim to the eggs and as each sperm reaches an egg, it digests its way throughthe jelly and into the egg, effecting fertilization. After fertilization thejelly coating on the eggs swells due to the absorption of large amounts ofwater. The swelling of the black jelly causes the eggs to adhere together andprotects them from predation by fish and other animals. The fertilized egg, orzygote, undergoes cleavage, forming a tadpole.
Flagella and cilia Fine threads of cytoplasm that extend from the surfaces ofsome cells. Both of these structures are involved in the locomotion of someprotist species. Cilia are relatively short extensions but appear in greatnumbers, usually surrounding the body of the protist. Flagella are much longerthan cilia and appear in fewer numbers. In addition to serving the locomotiveneeds of one-celled organisms, flagella and cilia help functions of other typesof cells. Sperm cells of animals and plants are propelled through fluid mediaby the whip-like actions of their flagella. Tissue cells of the human windpipeare lined with cilia that wave back and forth catching dust particles andpushing them away from the lungs. The microstructure of the flagella and ciliaresembles that of the centrioles.
Flatworms The simplest of the flatworms demonstrate bilateral symmetry. Thisphylum represents a step up the evolutionary scale showing a recognizable headend and definite development of excretory, nervous, and reproductive systems.Most of the flatworms are hermaphrodites, as well as parasites. Some flatwormsare serious parasites of humans and other animals. Among the flatworms are theplanaria, flukes, and tapeworms.
Flower, parts of The reproductive structure of the angiosperm is the flowerthat encloses the male and female sex organs. The green leaflike sepals protectthe flower when in the bud stage. Collectively, sepals are known as the calyx.Just inside of the calyx are colored petals. All of the petals in a flower areknown as the corolla. The stamens are the male reproductive structures; pollengrains are produced in the anther. The pistil is in the center of the flower.The top portion of the pistil is the stigma. The style is the long stalk thatleads to the rounded portion of the pistil called the ovary. Inside of theovary are one or more ovules. The pistil and its many parts compose the femaleportion of the flower.
Flower, reproduction in Involves the maturation of the pollen grain duringwhich three nuclei are produced. One is a pollen tube nucleus; the other twoare sperm nuclei. As the sperm nuclei are traveling down the style, each ovuleis going through a maturation process that results in the formation of a viableegg cell and a double nucleus. The sperm nuclei fertilize the egg cell and alsothe double nucleus. The zygote goes through a series of changes that lead toseed formation.
Fluid Mosaic Model See Cell membrane.
Food chain The flow of energy through an ecosystem can be studied by way offood chains which show how energy is transferred from one organism to anotherthrough feeding patterns. An example of a food chain on a cultivated fieldmight be as follows: Lettuce --> Rabbit --> Snake --> HawkThe flow of energy in a food chain is in a straight line pattern. Most of theenergy is concentrated in the level of the producer. At each succeeding levelthe energy is decreased. However, the feeding relationships among organisms inan ecosystem are not usually this simple, and, in actuality, are more complex,forming a food web.
Food pyramid Another way of illustrating energy flow in an ecosystem. Theautotrophs at the base of the pyramid support all of the heterotrophs(consumers) that exist at each nutritional level and there is a decrease ofavailable energy at each nutritional level.
Food web Shows that there are several alternative energy pathways in a foodweb. It is the alternative pathways that enable an ecosytem to keep itsstability. One species does not eradicate another in the quest for food.
Fossils The preserved remains of plants and animals, usually found insedimentary rock. The age of fossils is estimated by the use of carbon dating,the ratio of radioactive carbon (carbon 14) to nonradioactive carbon (carbon12). The fossil records contained in the layers of sedimentary rock providereliable evidence of change in plant and animal species. The lower down therock layer, the older the fossil. Top layers contain more recent fossil remainsof more complex species.
Fruit A ripened (mature) ovary bearing one or more seeds. A simple fruitdevelops from a single ovary; examples are tomatoes, plums, and pears. Anaggregate fruit develops from a group of ovaries produced in a single flower;examples are raspberries and blackberries. A multiple fruit develops from theovaries of a cluster of flowers that are carried on the same stalk; examplesare pineapples, figs, and mulberries.
Fungi Eukaryotic, multicellular, and multinucleate organisms. Yeasts areunicellular forms. The cells of fungi are different from those of other speciesbecause the boundaries separating the cells are either entirely missing or onlypartially formed. Thus fungi are primarily coenocytic organisms; this meansthat the cells have more than one nucleus in a single mass of cytoplasm.However, the characteristic that most distinguishes the fungi from otherorganisms is their mode of nutrition. Fungi are saprophytes, absorbing organicnutritive matter from decaying plant and animal bodies.
Gametes Sex cells; male sex cells are called sperm and are produced in gonadscalled testes or spermaries. Female sex cells are the eggs or ova and areproduced in female gonads called ovaries. All gametes have the haploid(monoploid) number of chromosomes.
Gametophyte In alternation of generations, the gametophyte generation produceshaploid gametes. Fusion of the haploid gametes forms a diploid zygote, whichgrows into a sporophyte (plant). The sporophyte produces haploid reproductivespores. Each spore grows into a multicellular haploid plant, the gametophyte.The cycle repeats.
Gamma globulin A protein in the blood plasma from which antibodies are made.Antibodies, produced by the lymphocytes in the immune system, are moleculesthat inactivate or destroy antigens.
Ganglia (ganglion, sing.) Functional groups of nerve cell bodies that lieoutside of the brain and spinal cord, allowing parts of the nervous system tocoordinate activities without involving the whole system.
Gastrula A stage in embryonic development when the three primary tissue layers(ectoderm, mesoderm, and endoderm) develop and the primitive gut (archenteron)forms. See Cleavage.
Gene Around 1911, Thomas Hunt Morgan introduced the tiny fruit fly Drosophilamelanogaster as the new experimental organism for work in the field ofheredity. The experimental work of Morgan resulted in the discovery of thechromosome as the means by which hereditary traits are transmitted from onegeneration to another. Morgan's chromosome theory of inheritance includes theconcept that chromosomes are composed of discrete units called genes. Genes arethe actual carriers of specific traits and move with the chromosomes in mitoticand meiotic cell divisions. Morgan further proposed that genes control thedevelopment of traits in each organism. When genes change, or mutate, thetraits they control change. Morgan's work was based on the chromosome theory ofinheritance, which began to take shape in 1902 from the work of Walter S.Sutton.
Genetic code The DNA molecule carries coded instructions for controlling allfunctions of the cell. At the present time, scientists know most about thefunctions of the genetic code that control protein synthesis. They havedetermined that triplet combinations of bases code for each of 20 amino acids.The coded sequence of amino acids determines the formation of different typesof proteins. The code for proteins is present in messenger RNA molecules thatare complementary to DNA molecules. For example: let us suppose that a portionof a DNA molecule carries a code such as AAC GGC AAA TTT. Its mRNA complementwould be as follows: UUG CCG UUU AAA.
Genetics The science of heredity.
Genome The complete store of an organism's genetic material, which consists ofgenes on chromosomes.
Genotype The genetic makeup of an individual. For a given trait an individualmay have two like genes or two unlike genes. Genotype determines whether thedominant or recessive trait will show and whether or not the individual isdominant or recessive for the trait.
Geotropism Different parts of a plant may respond differently to the samestimulus. Thus the stem and leaves will grow upward, away from gravity; theyshow negative geotropism. On the other hand, the roots will grow downward,toward gravity; they show positive geotropism.
Germs Organisms that invade the body of animals and plants and cause disease.Organisms that cause disease are bacteria, spirochetes, viruses, and parasiticworms.
Gibberellin Plant growth substance isolated from the fungus Gibberellafujikuroi that has the following effects on plants:
1. It causes corn, wheat, and many other plants to grow very rapidly, showingan increase in height that is three to five times the normal in a short periodof time.
2. It makes dwarf plants that by heredity should always be stunted, such asdwarf pea or dwarf corn, grow to the size of normal plants.
3. Seeds that are soaked overnight in it germinate ahead of time.
4. Biennial plants such as foxglove and carrot, that flower in the second yearof their life cycle, burst into bloom in only one year.
5. Tomatoes and cucumbers develop from flowers that are not pollinated if theflower buds are sprayed with it.
6. Garden and house plants, such as geranium and petunias, bloom ahead of time,and have large flowers.
Glycogen A multibranched glucose storage polysaccharide deposited in the liverand muscles of animals; also known as "animal starch."
Glycolysis The anaerobic degradation of glucose into pyruvic acid; the initialstage of cellular respiration. See Anaerobic; Respiration.
Gonads The sex organs of male (testes) and female (ovaries). Sex cells(gametes) are produced in the sex organs. Male sex cells are called sperm;female, ova (ovum, sing.)
Grasshopper Belongs to a large group of organisms classified as arthropods.The name arthropod in literal translation means "jointed foot," a distinctivecharacteristic of this group, expressed traditionally as "jointed appendages."The arthropods are segmented animals protected by an exoskeleton made ofprotein and the flexible but tough carbohydrate chitin. The chitinousexoskeleton is fashioned in articulating plates held together by hingescovering both the body and the appendages, and attached to muscles that makepossible quick and unencumbered movements.
Greenhouse effect The amount of carbon dioxide in the atmosphere has beenfound to be increasing. Much of this is coming from the burning of fossil fuels(coal, oil, and natural gas) by factories, homes, and automobiles. Since treesuse carbon dioxide in photosynthesis, the cutting down of the world's forestsover the years is preventing some of this C02 from being absorbed. The actualeffects of this increase are not fully known. Some scientists claim that carbondioxide, like the glass of a greenhouse, allows visible sunlight to passthrough to the earth. As the earth warms up, it gives off infrared rays. Theseare absorbed by the carbon dioxide in the atmosphere, instead of being givenoff into space. It is believed that this "greenhouse effect," as it is called,will cause the earth's atmosphere to eventually warm up. Some of the possibleeffects of such an increase in atmospheric temperature might be: spread ofdesert areas; reduction of food crop production; warmer climate; melting ofpolar ice, with the raising of sea levels and the consequent flooding ofheavily populated areas along the coast.
Growth Describes the increase of cell size and increase of cell numbers. Thelatter process occurs when cells divide in response to a sequence of eventsknown as mitosis.
Guard cells Cells in the epidermis of the leaf that regulate the opening andclosing of the stomates. See Leaf cross section.
Gymnosperms Gymnosperms are cone-bearers. They are woody plants, chieflyevergreens, with needle-like or scale-like leaves. Cone-bearing plants grow inmany parts of the world including tropical climates. However, most species arefound in the cooler parts of temperate regions. Examples of gymnosperm speciesare pines, spruces, firs, cedars, yews, California redwoods, bald cypresses,and Douglas firs. Most biologists think that the gymnosperms evolved directlyfrom progymnosperms present during the Devonian period.
Haploid number The organism's sex cells, or gametes -- eggs and sperm, containhalf the species number of chromosomes. This number is called the haploidnumber and abbreviated N. Meiosis is the kind of nuclear division that leads tothe formation of sperm and egg cells.
Hardy-Weinberg principle A population includes all members of a species thatlive in a given location. Modern geneticists are concerned about the factors inpopulations that affect gene frequencies. All of the genes that can beinherited (heritable genes) in a population are known collectively as the genepool. The Hardy-Weinberg principle uses an algebraic equation to compute thegene frequencies in human populations. The conditions set by the Hardy-Weinbergprinciple for determining the stability of a gene pool are as follows: largepopulations, random mating, no migration, and no mutation.
Heart The human heart lies in the chest cavity behind the breastbone andslightly to the left. The heart is a bundle of cardiac muscles specialized forrhythmic contractions and relaxations known as heartbeat. The rate of averageheartbeat is 72 times per minute. The inside of the heart is divided into fourchambers. The two chambers at the top are the receiving chambers, or the atria.The lower chambers, the ventricles, are pumping chambers. Each atrium isseparated from the ventricle below by a valve. The atrium and the ventricle onthe right are separated from the left atrium and ventricle by a thick wall ofmuscle called the septum.
Hemoglobin An iron-protein complex in red blood cells that functions as anoxygen carrier.
Heredity, Mendelian Gregor Mendel, an Austrian monk, began the first organizedand mathematical study of how traits are inherited. Using the garden pea as thetest organism, Mendel identified seven different traits that were easilyrecognizable in this self-pollinating plant. He called each of these traitsunit characters. Mendel not only identified characteristics that seemed to beinherited, but for each unit character, he identified an opposite trait. Forexample: if the unit character was height, the opposite traits were short andtall. If the unit character was seed coat color, the opposite traits wereyellow and green. Mendel formulated three major laws or principles ofinheritance.
The Law of Dominance: If two organisms that exhibit contrasting traits arecrossed, the trait that shows up in the first filial generation (F1) is thedominant trait. For example: when a pure-bred tall pea plant is crossed with ashort pea plant, all of the offspring will be tall. The offspring will not bepure tall, however, and are therefore known as hybrids. The factor forshortness is hidden. We say today that the phenotype of the F1 plants is tall.A phenotype refers to the traits that we can see. The genotypes of geneticmakeup of these plants is said to be hybrid or heterozygous, meaning mixed.
The Law of Segregation: When hybrids are crossed, the recessive traitsegregates out at a ratio of three individuals with the dominant trait to oneindividual with the recessive trait. The 3:1 ratio is known as the phenotypicratio, because it refers to the traits that can be seen and not those factorshidden in the germplasm. The hybrid cross is also known as the F1 cross and theoffspring produced by this cross are known as the second filial generation orF2. In terms of modern knowledge, the F2 generation also produces another typeof ratio called the genotypic ratio which refers to gene makeup. The genotypicratio is 1:2:1, translated into 1 homozygous dominant (pure) individual: 2heterozygous (hybrid) individuals: 1 homozygous recessive. Only the homozygousrecessive shows the recessive trait.
The Law of Independent Assortment: Mendel believed that each trait is inheritedindependently of others and remains unaltered throughout all generations. Wenow know that Mendel's "factors" are genes that are linked together onchromosomes and that if genes are on the same chromosome, they are inheritedtogether.
Hermaphrodite An animal that has both male and female reproductive organs andproduces both eggs and sperm. Lifestyles such as burrowing (earthworms), livingattached to objects (barnacles), or living within the body of another organism(tapeworms) make it difficult for such organisms to meet with a member of theopposite sex. Hermaphroditism solves the problem of reproduction. Mosthermaphrodites mate with another member of the same species each donating spermto fertilize the eggs of the other. Sequential hermaphroditism is areproductive pattern exhibited in some reef fish that are able to change theirsex from male to female or vice versa as conditions warrant.
Heterotroph An organism that cannot synthesize its food from inorganicmaterials such as carbon dioxide and water. A heterotroph must obtain itsnutrition by the intake of preformed organic molecules. All animals areheterotrophs.
Heterotroph-autotroph hypothesis The first organisms on earth were probablyheterotrophs utilizing the organic pool for nutrition. As time went on, theearly heterotrophs must have faced a serious crisis because oxygen fromphotodissociation (atmospheric reactions) began to increase in concentration.This changed the atmosphere. Oxygen and hydrogen peroxide (H2O2) destroyed manyheterotrophs. Eventually, photosynthetic organisms -- autotrophs such asblue-green algae -- evolved, increasing the oxygen content of the atmosphereand thereby threatening the continued existence of the heterotrophs. An ozonelayer in the atmosphere developed from the high concentration of oxygen, whichfurther diminished the organic compounds available to the beleagueredheterotrophs. Some heterotrophs developed pathways for utilizing oxygen inenergy production (aerobic respiration). The carbohydrates produced byautotrophs and the oxygen of the atmosphere supplied the new heterotrophs withthe nutritive materials necessary for survival (heterotroph-autotrophhypothesis). Thus the stage was set for the development of life on the scalethat is known today.
Homeostasis The term used to describe the stable internal environment of thecell and the organism as a whole. Homeostasis is a condition necessary forlife. The internal chemical balance of the cell must be maintained at a steadystate to promote innumerable biochemical activities that foster the productionand use of energy. The concept of homeostasis was initially developed by thenineteenth century physiologist Claude Bernard.
Homologous structures A comparative study of the bone structures and bodysystems of animals from the various phyla reveals a great deal of similarity. Acomparative study of the skeletal systems of vertebrates shows that many of thebones are very much alike. Much of our evidence for evolution comes from astudy of homologous structures. Homologous structures are bones that look alikeand have the same evolutionary origin although they may be used for differentpurposes. The flipper of a whale, the arm of a human, and the wing of a birdare homologous structures having the same evolutionary origin and maintainingsimilarity of structure.
Hormones Secretions from the endocrine glands that regulate the activities ofbody organs. Examples: growth hormone regulates growth of the long bones;thyroxin regulates body metabolism; adrenaline controls the release of sugarfrom the liver. Hormones are secreted directly into the bloodstream, which thencarries them to their target organs.
Hybrid An organism having mixed genes for a trait.
Hydra A freshwater coelenterate that is representative of a genus of the samename. Hydra is a polyp and has no medusa form in its life history. In length,each hydra is about 12 millimeters and has about eight tentacles that surroundthe mouth-anus. Hydras move about by somersaulting, end over end. The animal'slocomotion is made possible by cells that have contractile fibers calledmyonemes. These epitheliomuscular cells have locomotor and sensory functions.Reproduction in hydra is sexual and asexual. A single organism produces bothegg and sperm, which are discharged into the water where fertilization takesplace. Asexual reproduction occurs by budding.
Hydrolysis See Digestion.
Hypothalamus The region of the brain that controls body temperature,osmoregulatory activities, maturity, thirst, hunger, and sex drive. Thehypothalamus is also the region where the nervous and hormonal systemsinteract.
Immunity Ability to resist the attack of a particular disease-producingorganism. Immunity to one kind of disease germ does not automatically make aperson immune to other types of disease germs. Active immunity is brought aboutby antibody production by a person's own body cells. Active immunity can bestimulated in either of two ways; by getting the disease and recovering from itor by being immunized against the disease. Immunization that produces activeimmunity involves the injection of weakened disease agents that stimulateantibody production but produce only mild symptoms or none at all. Activeimmunity is longlasting because the body cells continue to produce theantibodies. An injection of gamma globulins can give a person temporaryimmunity against certain specific diseases. This means that a person hasborrowed antibodies in the blood and not those made by his (her) own cells.This kind of immunity is called passive immunity. It lasts only as long as theantibodies last; when they are used up, the immunity ceases.
Infectious diseases Caused by pathogens (germs). These pathogenicmicroorganisms include certain bacteria, protozoans, spirochetes, richettsias,mycoplasmas, and fungi. Parasitic worms and viruses also often produce diseasein humans. Most infectious diseases are contagious -- capable of being passedfrom one person to another by means of body contact or by droplet infection.
Ingestion The taking in or procuring of food. Digestion refers to the chemicalchanges that take place in the body by which nutrient molecules are convertedto forms usable by cells.
Inheritance, intermediate Geneticists have discovered that in many cases atrait is not controlled by a single gene, but rather by the cooperative actionof two or more genes. There are many instances in which Mendel's "law ofdominance" does not hold true. A case in point is what was once called blendinginheritance, or incomplete dominance. It is now known as codominance. Whenred-flowered evening primroses are crossed with white-flowered primroses, thehybrids are pink. Neither red nor white color is dominant and therefore theresult is a blend. In sweet peas, the expression of red or white flowers isdependent upon two genes: a (C) gene for color and an (R) gene for enzyme. If Cand R are inherited together, the flower color is red. If the dominant C ismissing, the flower is white and if the dominant R is missing, the color iswhite. Therefore white flowers are the result of several different genotypes:ccrr, ccRR, CCrr, Ccrr.
Invertebrates Animals without backbones. About 90 percent of all animalspecies are invertebrates. Just like all members of the kingdom Animalia,invertebrates are multicellular. They are composed of cells that lack walls.Most invertebrates are capable of locomotion and have specialized cells withcontractile proteins that facilitate movement. However, the adult forms of somelower invertebrate species are sessile, belonging to a group of filter feeders.These animals use cilia, flagella, tentacles, or gills to sweep smallerorganisms from the currents of water that flow over or through their bodiesinto the digestive cavities. Some of the lower invertebrates reproducevegetatively by budding. Other invertebrate species reproduce sexually,utilizing sperm and egg. Still other species reproduce asexually byparthenogenesis in which an unfertilized egg develops into a completeindividual. Some invertebrates have marvelous powers of regeneration, thegrowing back of lost parts or the production of a new individual from anaggregate of cells or from a piece broken off from the parent organism.Examples of invertebrates are sponges, jelly fish, worms, clams, starfish,insects, crabs.
Involuntary muscle See Muscle, smooth.
Karyotype A technique of producing a photograph of matched chromosome pairsdeveloped in 1956 by Joe Hin Tjio and Albert Levan.
Kidneys Paired organs of the human urinary system located dorsally in theabdomen. The kidneys reabsorb water, sodium, glucose, and some proteins. Excesswater and the protein waste, urea, pass into the collecting duct and are storedtemporarily in the urinary bladder until released from the body. See Urinarysystem.
Kingdom system of classification, five The largest and most inclusiveclassification category is the kingdom. For many decades, living things wereconsidered to be either plants or animals and thus were grouped into one of thetwo established kingdoms. In 1969, the ecologist Robert Whittaker proposed anupdated system of classification in which living things are grouped into one offive kingdoms, based on the extent of their complexity and the methods by whichtheir nutritional needs are met.
Krebs cycle See Respiration.
Kwashiorkor A serious protein deficiency disease is kwashiorkor. This disease,which threatens the lives of many children in Africa, causes misshapen heads,barrel chests, bloated stomachs, spindly legs and arms, decreased mentalabilities, and poor vision.
Leaf The most important function of green leaves is to carry outphotosynthesis, the food-making process in which inorganic raw materials arechanged into organic nutrients. A leaf consists of two parts: a stalk orpetiole and the blade. The petiole attaches the blade to the stem. The blade isthe place where photosynthesis takes place. Leaves vary greatly in shape.
Leaf cross section Study of a leaf cross section under the microscope revealsthree types of tissue: upper and lower epidermis, mesophyll, and the vascularbundles. The mesophyll consists of palisade cells and spongy cells. Theepidermis is a single layer of cells at the upper and lower surfaces of theleaf. The cells have thick walls made of cutin and lack chloroplasts. Theirmain function is to protect the underlying or overlying tissues from drying,bacterial invasion, and mechanical injury. On the underside of the leaf, thelower epidermis has pores known as stomates, the size openings of which areregulated by a pair of guard cells. The stomates serve as passageways foroxygen and carbon dioxide.
Legionnaires' disease An acute, pneumonia-like respiratory infection caused byan air-borne bacterium and associated with water in air conditioning towers.Symptoms are a feeling of malaise, chest pain, muscle aches, shortness ofbreath, and a dry cough. Other symptoms include a high fever, chills, abdominalpain, and sometimes abnormalities of the kidney and liver. If treated promptlywith the antibiotic erythromycin, the infected person recovers. Without prompttreatment, the disease is fatal in 15 percent of cases.
Lichens Lichens are pioneer organisms that can inhabit bare rock and otheruninviting substrates. They live on the barks of trees and even on stone walls.A lichen is a combination of two organisms -- an alga and a fungus -- that livetogether in a mutualistic relationship. The alga carries on photosynthesis,while the fungus absorbs water and mineral matter for its partner. The fungusalso anchors the lichen to the substrate.
Life functions Living things are highly organized systems. They areself-regulating, self-reproducing, and capable of adapting to changes in theenvironment. To satisfy all of the conditions necessary for life, all livingsystems must be able to perform certain biochemical and biophysical activitieswhich collectively are known as life functions. Nutrition is the sum total ofthose activities through which a living organism obtains nutrients (foodmolecules) from the environment. Life functions include nutrition as well asthe processes of ingestion, digestion, and assimilation; respiration whichencompasses breathing and cellular respiration; reproduction of cells;synthesis involving the biochemical processes of cells; regulation encompassingprocesses of control by hormones, enzymes, coenzymes, and utilized by thenervous system and the endocrine system; reproduction, both sexual and asexual.
Light reactions See Photosynthesis.
Linked genes Genes occurring on the same chromosome are inherited together.They are said to form linkage groups. Crossing over breaks linkage groups.
Lipids The lipids are a group of organic compounds that include the fats andfat-like substances. A lipid molecule contains the elements carbon, hydrogen,and oxygen similar to a carbohydrate. Unlike the carbohydrates, however, inlipid molecules the ratio of hydrogen to oxygen is much greater than 2:1. Mostlipids are made up of two basic units: alcohol (usually glycerol) and a classof compounds called fatty acids.
Lock and key A simple analogy is used to explain the specificity of enzymes.Specificity refers to the characteristic of enzymes that permits a particularenzyme to form a complex with a specific substrate molecule only. The "lock andkey" analogy explains enzyme specificity: the substrate is viewed as a padlockand the enzyme as the key able to unlock it. When unlocked (in the analogy) oracted upon by the key, the padlock comes completely apart. The key remainsunchanged and ready to work again on another padlock of the same type.
Lungs The human body has two lungs. Each of these is enclosed in a doublemembranous sac known as the pleural sac. Not only is this sac airtight, but italso contains a lubricating fluid. The pleural sac and the lubricating fluidprevent friction that might be caused by rubbing of the lungs against the chestwall.
Lyme disease A flu-type disease transmitted from ticks to humans. The cause ofthe disease is a species of spiral bacteria injected in the human bloodstreamthrough a bite by the infected deer tick, Ixodes dammini. Early treatment withantibiotics will prevent the development of arthritic symptoms. See also Healthand Medicine.
Lymph The body cells are bathed with tissue fluid called lymph. Lymph comesfrom the blood plasma, diffusing out of the capillaries into the tissue spacesin the body. Lymph differs from plasma in that it has 50 percent fewer proteinsand does not contain red blood cells. Lymph has the important function ofbringing nutrients and oxygen to cells and removing from them the wasteproducts of respiration.
Lymphocytes White blood cells that are produced in the lymph nodes andfunction in immune reactions of the blood.
Malpighian tubules These are long slender tubules, attached at one end to thedigestive tract controlling excretion in insects and certain other arthropods.Nitrogen-containing wastes in the body fluid are changed into uric acid, whichis then moved through the Malpighian tubule to the end of the digestive tractwhere it is ultimately excreted as dry crystals.
Mammals The characteristics that set mammals apart from other animals and madethem adaptable to a wide range of habitats are as follows:
1. Mammals have mammary glands (from which the name mammal) that supply theyoung with milk directly after birth.
2. At some time during the life cycle, all mammals have hair.
3. Mammals are warm-blooded. Constant body temperature is due, in part, to thefour-chambered heart, a device that prevents the mixing of oxygenated anddeoxygenated blood.
4. Most species of mammals have sweat glands that provide a secondary means ofexcreting water and salts.
5. Mammalian teeth have evolved into three different types: incisors fortearing, canines for biting, molars and premolars for grinding.
6. All but a few species have seven vertebrae in the neck. These neck bones areknown as cervical vertabrae.
7. A muscular diaphragm separates the thoracic cavity (containing the lungs andthe heart) from the abdominal cavity (housing part of the digestive system, thereproductive organs, and the excretory system).
Marsupials The marsupials are primitive mammals that do not have a placenta.The young are about 5 centimeters long at birth and are in an extremelyimmature condition. At birth they crawl into the mother's pouch or marsupium.The rounded mouth is attached to a nipple and the mother expresses milk downthe throat of the helpless fetus. As development occurs, the young marsupial isthen able to obtain milk by sucking. There are 29 living genera of marsuspials,28 of which live in Australia. The opossum Didelphys is indigenous to North,South, and Central America and Caenolestes inhabits regions of Central Americaonly. Besides the opossum, other marsupials are the kangaroo, koala, Tasmanianwolf, wombat, wallaby, and native cat.
Mastigophora The Mastigophora are protozoa that have one or more flagella.Some species are free-living and inhabit fresh or salt water. OtherMastigophora species live in a symbiotic relationship with organisms of otherspecies. For example, several species live in the intestines of termites,cockroaches, and woodroaches, where they digest cellulose for these insects.The genus Trypanosoma includes parasites that cause debilitating diseases inhuman beings. Trypanosoma gambiense is the zooflagellate that causes Africansleeping sickness. Humans are infected with the trypanosome by the bite of aninfected tsetse fly.
Meiosis Or reduction division, is cell division that occurs in the primary sexcells leading to the formation of viable egg and sperm cells. Meiosis reducesthe number of chromosomes to one half in each gamete so that upon fertilization(the fusing of sperm and egg nuclei) the species chromosome number is keptconstant.
Menstruation The process in which a nonfertilized egg is discharged from thebody. The vascularized lining of the uterus, known as the endometrium,disintegrates in response to decreased levels of estrogen and progesterone inthe blood. Menstrual bleeding lasts four to seven days in humans.
Messenger RNA The mRNA, carrying a code for a specific protein, moves from thenucleus to the cytoplasm. The mRNA attaches itself to several ribosomes, eachhaving its own ribosomal RNA. Specific transfer RNA (tRNA) molecules bring tothe ribosomes their own kind of activated amino acids. Transfer RNA moleculesthat fit the active sites of mRNA's on the ribosomes temporarily attach tothem. As a result, amino acids are lined up in the proper sequence. The RNAcode is a triplet code with one triplet, or codon, made up of three base codingfor a specific amino acid. See also Protein synthesis.
Metabolism An inclusive term concerning all of the biochemical activitiescarried on by cells, tissues, organs, and systems necessary for the sustainingof life. Metabolic activities in which large molecules are built from smallerones or in which nutrients are changed into protoplasm are called anabolicactivities, or anabolism. Destructive metabolism in which large molecules aredegraded for energy or changed into their smaller building blocks is calledcatabolic activity, or catabolism.
Metamorphosis The change from tadpole to adult frog is known as metamorphosis,a process controlled by the thyroid gland. The adult amphibian loses the gills,lateral line senses, tail, unpaired fins, and muscles controlling them -- thefish characteristics -- and develops structures adapted for life on land. Anadult amphibian breathes by means of lungs and has a three-chambered heart thatis more efficient at pumping blood between the lungs, heart, and rest of thebody. In most species the adult has limbs for movement, but no tail.
Microscope, electron Magnification more than 200,000 times. Using electronsinstead of light and magnets in place of lenses, the electron microscope hasrevolutionized the study of the cell. The scanning electron microscope hasimproved upon the resolution of fine detail made possible by electronmicroscopy. A fine probe directs and focuses electron beams over the materialbeing studied, affording quick scanning and giving finer detail than ispossible with the standard electron microscope.
Microscope, light The best light microscope is capable of magnifying objects2,000 times. The phase contrast microscope makes transparent specimens visible,while the darkfield or the ultramicroscope gives vivid clarity to fragile andtransparent organisms such as the spirochetes that cause syphilis. Theultraviolet microscope is used for photographing living bacteria and naturallyfluorescent substances.
Mitochondrion A membranous cellular organelle. It consists of a smooth outermembrane and a folded inner membrane. The folds are called cristae. Like thecell membrane, the mitochondrion membrane is composed of proteins andphospholipids. The mitochondria are necessary for aerobic respiration to takeplace in cells.
Mitosis Mitosis (also known as karyokinesis) concerns the cell nucleus and itschomosomes. Before the onset of mitosis, the cell is in a stage known asinterphase. During interphase, the chromosomes are exceptionally long and verythin, appearing as fine granules through the light microscope. It is duringthis stage that DNA molecules in the nucleus replicate. The result ofreplication is that each chromosome now has an exact copy of itself. Wheninterphase comes to an end, the cell has enough nuclear material for two cells.The orderly process that divides the chromosomes equally between the twodaughter cells is known as mitosis. There are four stages of mitosis: prophase,metaphase, anaphase, and telophase. The significant events that mark each ofthese stages and interphase are shown below:
Mollusks Soft-bodied, nonsegmented, and usualIy enclosed within a calciumcarbonate shell. They are most abundant in marine waters, although some speciesinhabit fresh water and others live on land. All mollusks have a mantle, aflattened piece of tissue that covers the body and that may secrete thecalcareous shell. The body of the mollusk is described as being a head-foot, amuscular mass having different shapes and functions in the various classes. Themollusks include the chitons, snails, clams, scallops, squids, and octopuses.This is one of the largest animal phyla and includes about 1,000 species.
Monera The monerans are prokaryotic cells. They lack a nuclear membrane,mitochrondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. Theprokaryotes do not have any membrane-bound organelles in the cytoplasm. Themonera are unicellular organisms usually invisible to the naked eye. Mostmonerans live as independent cells, although some may occur in filaments(chains of cells) or colonies of cells held together by a gelatinous coat. Thiskingdom includes bacteria, blue-green bacteria, also known as blue-green algae.
Monotremes The monotremes are primitive egglaying mammals. The eggs, large andfull of yolk, house the developing monotreme embryos. Examples of themonotremes are the "duckbill" or platypus (Ornithorhynchus) indigenous toAustralia and Tasmania; the spiny anteater (Echidna), also an inhabitant ofAustralia; and a long snouted anteater (Proechidna) indigenous to New Guinea.Modified sweat glands of the anteater secrete a milk substitute that the younglick up from tufts of hair on the mother's belly.
Motor neuron See Nerve cell (neuron).
Muscle contraction Muscles contract due to a sliding filament mechanism. Whenthe thick and thin filaments slide past each other, the Z lines of thesarcomeres are pulled closer together: in effect, contracting. Z lines form theboundary between sarcomeres. When the sarcomeres contract, the myofibrilscontract, which causes the contraction of muscle fibers. Myofibrils are thebasic units of muscle fibers. The figure illustrates the sliding filamentmechanism of muscle contraction.
Muscular system Muscles represent 40 percent of the total weight of the humanbody. Muscle tissue is characterized by contractility and electricalexcitability, two distinctive properties that enable it to effect movement ofthe body and its parts. There are three types of muscle tissue: smooth,striated, and cardiac.
Muscle, cardiac Cardiac muscle is present only in the heart, where the cellsform long rows of fibers. Unlike other muscle tissue, cardiac muscle contractsindependently of nerve supply since reflex activity and electrical stimuli arecontained within the cardiac muscle cells themselves.
Muscle, smooth Smooth muscle is present in the walls of the internal organs,including the digestive tract, reproductive organs, bladder, arteries, andveins. Because smooth muscle is contained in organs that do not respond to thewill of a person, these muscles are called involuntary muscles.
Muscle, striated Striated muscle is variously referred to as striped muscle,voluntary muscle, or skeletal muscle -- terms describing its structure andfunction. Located in the legs, arms, back, and torso, striated muscles attachto and move the skeleton; since they are moved by the will of the person, theyare often termed voluntary muscles.
Mutations Genes can change and changes in genes are known as mutations.Mutations are usually recessive and they are usually harmful. Mutations usuallyoccur at random and spontaneously. However, mutations may be induced byradiation or by chemical contamination. There are several types of mutations. Aloss of a piece of a chromosome is known as a deletion. The genes on the brokenoff piece of chromosome are lost. Sometimes a broken piece of chromosome stickson to another chromosome, thus adding too many genes; this type of mutation isknown as duplication. Sometimes a piece of chromosome becomes rearranged in thechromosome where it belongs, thus changing the sequence of the genes on thatchromosome; this is known as an inversion, and it prevents gene for genematching when chromosomes line up during meiosis. Point mutations are changesin individual genes.
Natural selection Charles Darwin (1809-1882) was an English naturalist whotogether with his cousin, Alfred Russel Wallace (1823-1915), developed a theoryof evolution that laid the groundwork for modern biological thinking. Darwin'stheory of natural selection can be summed up thusly: large numbers of newplants and animals are produced by nature. Many of these do not survive becausenature "weeds out" weak and feeble organisms by killing off those that cannotadapt to changing environmental conditions. Only the strongest and mostefficient survive and produce progeny. Specific tenets of the Darwin-Wallacetheory of evolution follow:
Overproduction: Every organism produces more gametes. If every gamete producedby a given species united in fertilization and developed into offspring, theworld would become so overcrowded that there would be no room for successivegenerations. This does not happen because a balance is maintained in thereproduction of all species. Therefore natural populations remain fairlystable.
Competition: There is competition for life among organisms: competition forfood, room, and space. Therefore there is a struggle for existence in whichsome organisms die and the more hardy survive.
Survival of the Fittest: Some organisms are better able to compete for survivalthan others. The differences that exist between organisms of the same speciesmaking one more fit to survive than another can be explained in terms ofvariations. Variations exist in every species and in every trait in members ofthe species. Therefore some organisms can compete more successfully for theavailable food or space in which to grow or can elude their enemies better.These variations are said to add survival value to an organism.
Nematodes See Worm-Parasites.
Nephridia Coiled tubules that serve as excretory organs in the segmentedworms. The nephridia filter out waste materials.
Nephron The unit of structure and function of the kidney is the nephron. Thereare about one million of these microscopic units in each kidney. They activelyremove waste products from the blood and return water, glucose, sodium ions,and chloride ions to the blood. The nephron is made up of several structures.The first of these is a knot of capillaries called the glomerulus. Theglomerulus fits into a second portion -- the Bowman's capsule, a cup-shapedcellular structure that leads into the third part, the kidney tubule. There arefour main parts of each kidney tubule: the proximal convoluted tubule, the loopof Henle, the distal convoluted tubule, and the collecting duct.
Nerve cell (neuron) The basic unit of function of the nervous system is theneuron, or nerve cell. The parts of the nerve cell are the cyton, or cell body,the dendrites, and the axon. The dendrites receive signals from sense organs orfrom other nerve cells and transmit them to the cyton. The cell body passessignals to the axon, which then conducts the signals away from the dendritesand cell body. The axon terminating in end brushes (known also as terminalbranches) is popularly called a nerve fiber. The nervous system has three typesof neurons. Sensory or afferent neurons receive impulses from the sense organsand transmit them to the brain or spinal cord. Associative or interneurons arelocated within the brain or spinal cord. These transmit signals from sensoryneurons and pass them along to motor neurons. Motor or efferent neurons conductsignals away from the brain or spinal cord to muscles or glands, so-calledeffector organs.
Nerve impulse Communication in the nervous system is made possible by signalsor impulses carried in a one-way direction along nerve cells. These impulsesare electrical and chemical in nature. When a neuron is not carrying animpulse, it is at resting potential. When a nerve cell is stimulated to carryan impulse, its electrical charge changes and it has an action potential.Action potentials (nerve impulses) from any one nerve cell are always the same.All impulses are of the same size, there being no graded responses. This isknown as the "all or none response," meaning that a nerve cell will transmit animpulse totally or not at all.
Nerve net The simple nervous system in Hydra consisting of pointed sensorycells scattered throughout the endoderm and ectoderm, specialized to receiveimpulses.
Nervous system, autonomic A network of nerves known as the autonomic nervoussystem controls the body's involuntary activities and the smooth muscles of theinternal organs, glands, and heart muscle. It is composed of motor (efferent)neurons leaving the brain and spinal cord and also of peripheral efferentneurons. The autonomic nervous system is divided into the sympathetic systemand the parasympathetic system. These subsystems are antagonists. When one setof nerves activates the smooth muscles of the body, the other set inhibits theaction. For example: the parasympathetic nerves dilate the blood vessels andslow the heartbeat; the sympathetic nerves constrict the blood vessels andquicken heartbeat.
Nervous system, human The human being has a complex nervous system that iscomposed of the principal functioning units of the central nervous system andthe autonomic nervous system. The central nervous system is composed of thebrain, spinal cord, and the nerves that radiate from the spinal cord. The brainis divided into three main parts: the forebrain, midbrain, and hindbrain. Theforebrain controls all intelligent activities, voluntary muscle movements,speech, and interpretation of sensory data. The midbrain relays sensoryinformation. The hindbrain controls all vital involuntary activities such asbreathing, heartbeat, digestion, and the like. The central nervous system isthe control center for all regulating activities in the body. See Brain.
Nitrogen cycle The cycle of events through which nitrogen in the air becomesuseful to plants and subsequently to heterotrophic organisms.
Nodules Bumps on the roots of leguminous plants (beans, clover, alfalfa) thathouse nitrifying bacteria. The nitrifying bacteria convert ammonia, releasedinto the soil by breakdown of proteins to nitrites and then to nitrates.
Nucleic acids DNA is an important part of the chromosome structure of allcells. DNA is a nucleic acid as is RNA. The unit of structure and function inthe nucleic acid is called a nucleotide. A nucleotide is composed of aphosphate group, a five-carbon sugar, and a protein base. If the five-carbonsugar is ribose, the nucleic acid is ribonucleic acid (RNA). If the five-carbonsugar is deoxyribose, then the nucleic acid is deoxyribose nucleic acid (DNA).The protein bases in nucleic acids are ring compounds. Those bases with singlerings are pyrimidines. Bases with double rings are purines. The pyrimidines innucleic acid are thymine, cytosine, and uracil. The purines are adenine andguanine. The four bases that make up the DNA molecule are adenine (A), guanine(G), thymine (T), and cytosine (C). The four bases that make up the RNAmolecule are adenine (A), guanine (G), cytosine (C), and uracil (U).
Nutrition Is the totality of methods by which an organism satisfies theenergy, fuel, and regulatory needs of its body cells. Those substances thatcontribute to the nutritional needs of cells are the nutrients. Animals takethese nutrients into the body by the ingestion of food. Food, therefore, refersto edible materials that supply the body nutrients. Nutrients needed in largeamounts are classified as macronutrients: carbohydrates, proteins, and fats.Micronutrients -- vitamins and minerals -- are needed in smaller amounts.Vitamins are organic compounds; minerals are inorganic. Malnutrition resultsfrom the improper intake of nutrients.
Nutritional relationships When two different species of organisms livetogether, the relationship is called symbiosis. If the relationship is ofmutual benefit to both species, it is called mutualism. When one speciesbenefits and the other does not but is not harmed by the association, thecondition is known as commensalism. When one species lives at the expense ofanother, doing harm to its host, the relationship is parasitism.Disease-producing organisms are parasites.
Obligate aerobes Most species of the eubacteria are aerobic, using molecularoxygen in the process of breaking down carbohydrates to carbon dioxide andwater. Obligate aerobes are those organisms that can live only in anenvironment that provides free or atmospheric oxygen. An example of an obligateaerobe is Bacillus subtilis.
Obligate anaerobes Some bacteria are obligate anaerobes and derive theirenergy by fermentation. These organisms cannot live in an environment of freeoxygen. Many obligate anaerobes are disease producers; included in this groupare Clostridium tetani, the causative organism of tetanus, and Clostridiumbotulinum, the bacterium that induces food poisoning. Still other bacteria arefacultative anaerobes. These are basically aerobic bacteria, but they can liveand grow in an environment that lacks free oxygen.
Oogenesis The maturation of the egg cell in which the primary oocyte (eggcell) goes through reduction division (meiosis). The outcome of these divisionsis the production of one viable egg cell and two or more polar bodies.
Open circulatory system Characteristic of mollusks and the arthropods such asthe grasshopper in which the blood is not confined in blood vessels and hascontact with body tissues.
Organelles Organized structures that are parts of cells, such as ribosomes,nuclei, mitochondria, chloroplasts, cilia, contractile vacuoles, andendoplasmic reticulum.
Organs Are groups of tissues that work together to carry out a specialfunction. Examples of organs are the heart, lungs, liver, and stomach.
Osmosis Is the diffusion of water through a membrane. Osmosis is a form ofpassive transport.
Ovary Is the female gonad where egg cells (ova) are produced. At times theovary produces hormones and thus functions as an endocrine gland.
Oviparous Egg-laying, as in birds.
Ovoviviparous A condition in which eggs develop but are retained and hatchedinside the mother's reproductive tract, as in cartilaginous fish (sharks) and afew snakes.
Ovulation The release of an egg cell (ovum) from a follicle in the ovary.Within the follicle a mass known as the corpus luteum forms in mammals andsecretes the hormone progesterone. If the egg is not fertilized, the corpusluteum disintegrates and an egg is released from another follicle during thenext menstrual cycle.
Ovules Immature seeds contained within the ovary of the flower.
Paramecium Representative of the ciliata. The cytoplasm in ciliates isdifferentiated into rigid outer ectoplasm and a more fluid inner endoplasm. Apellicle lies just inside of the cell membrane. Some species respond to adverseenvironmental stimuli by discharging elongated threads called trichocysts thatserve as defense mechanisms or a means of anchoring the protist to floatingpond material while feeding. Characteristic of the ciliata is the presence oftwo kinds of nuclei. The macronucleus controls metabolic activities, while thesmaller micronucleus directs cell division. The figure shows the structure ofthe paramecium, a typical representative of the ciliata.
Parasites An organism that lives on or inside of the body of a plant or animalof another species and does harm to the host. Parasites offer physicaldiscomfort to the host and tend to kill slowly, meanwhile having had time toreproduce themselves for several generations. Ectoparasites live on the host'sbody; body lice, dog fleas, ticks. Endoparasites live within the host's bodyand exhibit several adaptations for life in an intestine or in muscle or blood.
Parthenogenesis Although an egg will not develop unless it has been fertilizedby sperm, some lower animals reproduce by parthenogenesis -- in which eggsdevelop without sperm. This is true of aphids, or plant lice, water fleas, andothers. In bees the drone, or male, develops from an unfertilized egg.Artificial parthenogenesis has been accomplished by scientists experimentingwith the eggs of sea urchins and frogs. Jacques Loeb stimulated frog eggs to gothrough cleavage and eventually to form frogs without fertilization by sperms;he used various stimuli, such as pricking the membrane with a needle, andtreatment with salt solutions or acids. Dr. Gregory Pincus was successful inproducing "fatherless rabbits" by removing the ova from female rabbits,treating the ova with salt solutions and implanting them in other femalerabbits. The baby rabbits that developed were females, and were subsequentlymated to produce normal offspring.
Parturition The birth process is known as parturition. In humans the period ofgestation (period of embryo development) is about 9 months or 40 weeks. At theend of that time, the uterus begins to contract in a process called labor toexpel the baby. The onset of uterine contractions is probably caused by therelease of oxytocin into the bloodstream by the posterior pituitary. The humannewborn passes through the neck of the uterus (cervix) head first and thenthrough the vagina to the outside.
Perennials Plants that live for many years, such as trees, shrubs, andgrasses. Fire, disease, and mismanagement by humans causes the death ofperennials.
pH The number of hydrogen ions in a solution is the basis of pH. The pH scaleranges from 0-14. A pH of 7 is neutral. Below the pH of 7, the number of H+ions increases and the solution becomes more acidic. The lower the pH number,the stronger the acid. A pH above 7 indicates that there are more OH- ions thanH+ ions and the solution becomes more alkaline. Blood has a pH of 7.3. Specialindicators are used to show the acidity or alkalinity of a solution.
Phagocytosis A process by which a cell first surrounds and engulfs a particleand then ingests it. Solid particles are ingested by cells through a processknown as phagocytosis. White blood cells ingesting bacteria serve as an exampleof phagocytosis.
Phenotype The traits of an organism that can be seen.
Phloem Water-carrying (vascular) tissue in the plant leaf and stem specializedfor conducting water with dissolved nutrient molecules downward to the root.
Photoperiodism The physiological response made by plants to changes in daylength is known as photoperiodism. Some plants (short-day) flower only if theyare exposed to light for less than a certain amount of time each day, otherplants (long-day) must have a certain minimum length of photoperiod.Researchers explain the ability of plants to measure time by the action ofphytochrome, a light absorbing pigment that is associated with the cellmembrane and with some of the cell's internal membranes.
Photosynthesis Photosynthesis takes place inside of chloroplasts, structureswithin the cells of the leaf. Chloroplasts have fine structures within --flattened membranous sacs named thylakoids. On the membranes of the thylakoids,chlorophyll and the accessory pigments are organized into functional groupsknown as photosystems. Each of these photosystems contains about 300 pigmentmolecules that are involved directly or indirectly in the process ofphotosynthesis. Photosynthesis involves four sets of biochemical events:photochemical reactions, electron transport, chemiosmosis, and carbon fixation.The photochemical reactions and electron transport activities take place on themembranes of the thylakoids. The oval membranes of a thylakoid surround avacuole or reservoir in which hydrogen ions are stored until needed in theCalvin cycle, or carbon fixation. Each thylakoid rests in the stroma or groundsubstance of the chloroplast. The stroma is the place where carbon fixationoccurs.
Pinocytosis Cell drinking. The engulfing of molcules in solution through thecell membrane requiring the use of the cell's energy.
Plants, green Green plants are grouped in the kingdom Plantae. Members of thekingdom Plantae contain the green pigment chlorophyll. Not only doeschlorophyll color plant leaves and some stems green, it, more importantly,traps light energy which is used in the process of photosynthesis. As anoutcome of photosynthesis nutrient molecules are made, serving as food for bothplants and animals. Most species of the kingdom Plantae are nonmotile, anchoredto one place, and unable to move about, but a few of the lower plants aremotile for at least part of the life cycle. However, the evolutionary trendexhibited in green plants is toward stationary organisms that carry out theirlife functions on land in locations where they remain for life. Lower plantspecies equipped to swim live in salt and fresh water. Higher plants areterrestrial (land-dwelling)
Pollination The transfer of pollen from the anther to the stigma. Most flowersare insect- or wind-pollinated. Insect-pollinated flowers have petals andnectar that attract insects. The pollen tends to be sticky and clings moreeasily to insect bodies. Wind-pollinated flowers lack petals; or if present,they are relatively inconspicuous. They seldom produce nectar. Pollen is lightand dry and sometimes has membranous appendages, all these characteristicsmaking it easier to become wind-borne. Windblown pollen is produced in largequantities enhancing the chances for pollination.
Polymorphism The honeybee exhibits a specialized social structure calledpolymorphism, a condition in which individuals of the same species arespecialized for different functions. In a honeybee colony, three classes ofindividuals arise: fertile males called drones; fertile females, or queens; andsterile females, or workers. The workers have a special concave surface on thesecond pair of walking legs called a pollen basket used to carry pollen. Thequeen bee receives sperm from the drone once during her lifetime. The sperm arestored in a special organ called the spermatotheca in which they may live foryears. Fertilized eggs give rise to females, most of which remain workers. Aspecial female may be selected by the colony and fed a diet of "royal jelly"that causes her to grow larger than the others and become fertile. This fertilefemale will become a queen, and either take over the existing colony or start acolony of her own. Drones develop from unfertilized eggs by the process ofparthenogenesis.
Population genetics See Hardy-Weinberg principle.
Predation This is the process of one species feeding upon another. Thepredator is the consumer that seeks out the prey to be consumed. Predators havespecial adaptations that enable them to hunt and catch their food supply: speedand agility, stingers, fangs, poisons, claws, and camouflage. Plant specieshave certain adaptations that protect them against predation such as thorns,microscopic spines, and the production of poisons such as nicotine, morphine,strychnine, mescaline, and aromatic compounds such as cinnamon and cloves.Animal defenses against predation include protective coloration, spines of theporcupine, the odor of the skunk, and the ability to hide through camouflage.
Proteins Complex molecules built from amino acids. About 20 amino acids areessential to living systems. From these a large number of different kinds ofproteins are formed. The great variety of protein molecules is possible becauseof the many ways in which amino acid molecules can be arranged. Changing thesequence of just one amino acid in a chain will change the protein molecule.Much of the work of the cell is concerned with the synthesizing of proteinmolecules. Some of these proteins such as hormones, enzymes, and hemoglobin areused in complex biochemical activities. Other proteins contribute to thestructure of cells such as those that make up the plasma membrane and othercellular membranes.
Protein synthesis The code for proteins is present in messenger RNA molecules,which are complementary to DNA molecules. For example: let us suppose that aportion of a DNA molecule carries a code such as AAC GGC AAA TTT. Its mRNAcomplement would be as follows: UUG CCG UUU AAA. The mRNA, carrying this code,now moves from the nucleus to the cytoplasm. The mRNA attaches itself toseveral ribosomes, each having its own ribosomal RNA. Specific transfer RNA(tRNA) molecules bring to the ribosomes their own kind of activated aminoacids. Transfer RNA (tRNA) molecules, each with a particular amino acidattached, attach temporarily to the codons on the mRNA and their amino acidsare linked together in the correct sequence, one amino acid at a time. Notethat the RNA code is a triplet code with one triplet, or codon, made up ofthree bases coding for a specific amino acid. See also Messenger RNA.
Protists All of the species assigned to the kingdom Protista are eukaryotic,for example, Amoeba and Paramecia. Most protists carry out their lives within asingle cell as free-living organisms. However, some protist species areorganized into colonies where each cell carries out its own life functions andwhere, also, there may be some simple division of labor among the cells in thegrouping. An impressive variety of species are classified as protists, and theyprobably descended from diverse evolutionary lines. The protists themselvesrepresent evolutionary modification and are probably the ancestors of themodern fungi, plants, and animals.
Protozoa Protozoa, meaning "first animals," are one-celled heterotrophs.Species of protozoa number in the thousands. They live in fresh water, saltwater, dry sand, and moist soil. Some species live as parasites on or inside ofthe bodies of other organisms. Reproduction in the protozoans is usuallydescribed as being asexual by means of mitosis, but recent research hasrevealed that many protozoa augment asexual reproduction with a sexual cycle.Usually, the sexual cycle occurs during periods of adverse environmentalconditions, and the cell arising from the fusion of gametes (zygote) can resistunfavorable conditions. The thick wall and the decreased metabolic rate of thecyst permits survival during periods of cold, drought, or famine. The protozoaare divided into four phyla, based primarily on the methods of locomotion.
Punctuated equilibrium There is a time frame for evolution. The concept ofgradualism supports the idea that evolutionary change is slow, gradual, andcontinuous. The concept of punctuated equilibrium sets forth the idea thatspecies have long periods of stability, lasting for four or five million years,and then change as the result of some geological or other environmental change.
Punnett square A Punnett square is a diagrammatic device used to predict thegenotypic and phenotypic ratios that will result when certain gametes fuse.Remember that as a result of meiosis each gamete has only one half the numberof chromosomes that are in the somatic cells.
Problem: In fruit flies, long wing (L) is dominant over vestigial wing (l).What is the result of a cross between two flies that are heterozygous (Ll) forwing length?
Solution:Parents: Male x Female Ll LlGametes: L l L lPunnett square: L l L LL Ll l Ll llF2: LL = 1 homozygous dominant long winged fly Ll = 2 heterozygous dominant long winged flies ll = 1 homozygous recessive short winged fly
Race All humans belong to the species Homo sapiens. This means that thegenetic material of all people is so similar that all humans can interbreed andproduce fertile offspring. The human species is really a group of interbreedingpopulations. Populations that have adapted to certain environments becomegenetically different based on the frequency with which certain genes appear.Skin color, hair texture, body build, and facial bone structure are a few ofthe characteristics that identify human population groups known as races.Although we can make broad generalizations about the identifyingcharacteristics of racial groups, not every member of each group fits thesespecifications. A set of physical characteristics can be drawn up that will fitindividuals of several different races. Therefore, it is difficult forbiologists and anthropologists to agree on the number of human races.
Radioactive dating Scientists have determined that certain elementsdisintegrate by giving off radiations spontaneously and at a regular rate. Suchelements are said to be radioactive. In the process of emitting radiations, theradioactive substance changes to something else. For example: uranium-238changes to lead. The half-life of U-238, the rate at which one half of theuranium in a rock sample will change to lead, is 4.5 billion years. Uranium'srate of decay is not affected by any chemical or physical conditions. Thereforemeasuring the uranium-lead ratio in a sample of rock is a very reliable methodfor estimating the age of the rock. Dating of the oldest rocks found on earthindicates that they are about 3 billion years old. To allow time for theoriginal formation of the rocks, geologists add another 2 billion years to thisfigure, thus arriving at the 4.5-5 billion-year estimate of the age of theearth.
Recessive trait A characteristic that will appear if there are two like(recessive) genes for the trait. A trait that is masked by a dominant gene. Forexample: in sheep, white wool is dominant over black wool. A sheep with blackwool must have two recessive genes for the trait.
Recombinant DNA Recombinant DNA is DNA combined from two different organismsto produce characteristics not found in nature. The technique for doing thisinvolves the use of a class of special enzymes called "restriction enzymes."These enzymes have the ability of splitting a DNA strand. The fragments of DNAhave sticky ends, and when they touch a strip of DNA from another organism,they stick to it. In this way new genes can be introduced into an organism.Once done, the cell can synthesize the protein coded for by the newly acquiredgenes. Some proteins that have been produced in this way are interferon,insulin, and human growth hormone.
Red blood cells (Erythrocytes) The human body contains about 25 trillionerythrocytes, each one lasting about 120 days. New red cells are produced bythe bone marrow at the rate of one million per second. Erythrocytes containhundreds of molecules of the iron-protein compound hemoglobin. In the lungs,oxygen binds loosely to hemoglobin forming the compound oxyhemoglobin. Aserythrocytes pass body cells with low oxygen content, oxygen is released fromhemoglobin and diffuses into tissue cells. Carbon dioxide combines with anotherportion of the hemoglobin molecule and is transported to the lungs where it isexhaled.
Reflex If you accidentally touch a flame, your finger is automatically pulledback. A sensory neuron carries the impulse from the receptor, at the end ofyour finger, to the spinal cord, here an associative neuron receives it andsends it over a motor neuron to a muscle in your arm, which contracts, andpulls your finger away. There is no thought involved in the reaction. It iscentered in the spinal cord. A moment later, you are aware of it, becauseimpulses are sent up the spinal cord to the brain. Fortunately you do not haveto think about such stimuli. The response is inborn and is a factor in yoursurvival. Such a reflex protects you.
Regulation Encompasses all processes that control and coordinate the manyactivities of a living thing. Chemical activities inside of cells arecontrolled by enzymes, coenzymes, vitamins, minerals, and hormones. The nervousand endocrine systems of higher animals integrate and coordinate bodyactivities. Growth and development of plants is regulated by auxins and othergrowth-control substances. Gene activity is regulated by special molecules thatturn the gene on and off.
Replication The process through which a DNA molecule makes an exact duplicateof itself.
Reproduction The process by which new individuals are produced by parentorganisms. Basic to the understanding of reproduction is the concept thatorganisms produce the same kind of individuals as themselves. There are twomajor kinds of reproduction: asexual and sexual.
Reproductive system, female The female reproductive system serves threeimportant functions: the production of egg cells, the disintegration ofnonfertilized egg cells, and the protection of the developing embryo. Thereproductive system has specialized organs to carry out these functions. Twooval-shaped ovaries lie one on each side of the midline of the body in thelower region of the abdomen. On a monthly alternating basis each ovary producesa mature egg. Eggs are located in spaces in the ovary called follicles. As anegg matures, it bursts out of the ovarian follicle and is released into theappropriate branch of the fallopian tube, a tube that leads from the region ofthe ovary to the uterus. If the egg is fertilized, it becomes implanted in theuterus where it goes through a series of cell divisions known as cleavage.
Reproductive system, male In the male reproductive system some organs arelocated outside of the body and others are positioned internally. The scrotumcontains the testes, glands that produce sperm, and the male hormonetestosterone. Also positioned outside of the body is the penis, the organ thatdelivers the sperm into the body of the female. Each testis contains thousandsof seminiferous tubules. Within these tubules, the sperm cells aremanufactured. Each epididymis tubule functions as a storage place for sperm andalso serves as a pathway that carries the sperm to a duct called a vasdeferens. In its travels to the vas deferens, the sperm pass the seminalvesicles where they obtain nutrients. From the vas deferens, the sperm areconducted to the urethra, a single tube that extends from the bladder throughthe penis. Sperm cells leave the body through the penis. See Sperm.
Reptiles Reptiles have a dry leathery skin covered with epidermal scales. Asomewhat flattened skull contains a brain having a cerebrum much larger thanthat of the fish or amphibians. The eyes have secreting glands that keep thesurface moist. Reptiles are air-breathers and have rather well-developed lungs.The heart is composed of two atria and a ventricle; in some species theventricle is almost divided into two compartments, an evolutionary signpostpointing to the four-chambered heart. The body temperature of reptiles is notconstant, changing with the external environment. In popular speech, suchanimals are called cold-blooded; in technical language, poikilotherms orectothermic.
Respiration Consists of breathing and cellular respiration. Breathing refersto the pumping of air into and out of the lungs of air-breathing animals or themovement of water over the gills of fish. During breathing, oxygen diffusesinto the air sacs in the lungs and carbon dioxide moves out of the lungsthrough the nose and mouth. Cellular respiration is a combination ofbiochemical processes that release energy from glucose and store it in ATP(adenosine triphosphate) molecules.
Respiratory system The respiratory system includes the structures throughwhich oxygen comes into the body to reach the bloodstream, and through whichcarbon dioxide and water vapor leave. Summary of route of air throughrespiratory system: (1) nostrils, (2) nasal cavity and sinuses, (3) pharynx,(4) larynx, (5) trachea, (6) bronchi, (7) bronchial tubes in the lungs, (8) airsacs (alveoli).
Retrovirus A virus in which the genetic information is coded in ribonucleicacid (RNA) instead of in deoxyribonucleic acid (DNA). HIV, the virus of AIDS,is a retrovirus that produces reverse transcriptase, an enzyme that enables theretrovirus to replicate, and to make more of itself.
Ribonucleic acid (RNA) A nucleic acid consisting of a large number ofnucleotides, each of which contains ribose sugar and four bases, includinguracil, and a phosphate radical. See also Protein synthesis.
Ribose A sugar of chemical formula C5H10O5: one of the building blocks ofnucleic acid.
Ribosome A small organelle that is the site of protein synthesis.
Rickettsiae The rickettsiae are very small Gram negative intracellularparasites. They were first described in 1909 by Harold Taylor Ricketts, whofound them in the blood of patients suffering from Rocky Mountain spottedfever. The rickettsiae are nonmotile, nonspore forming, nonencapsulatedorganisms. In length, these organisms range from 0.3-1 micrometers. They livein the cells of ticks and mites and are transmitted to humans through insectbites. The rickettsiae are responsible for several febrile (fever-producing)diseases in humans, such as typhus fever, trench fever, and Q fever.
Roots Roots anchor plants to the soil and absorb water and dissolved mineralsfrom the ground. The absorbed materials enter the root by way of root hairswhich are one-cell extensions of the epidermis. From the root hairs, dissolvedmaterials pass through the cortex, endodermis, and pericycle into xylem cells.The xylem cells conduct the dissolved materials upward. The root cortex servesin the storage of food and water.
Saprophytes (saprobes) These organisms obtain their food by absorbingnutrients from dead organic matter. Examples are some species of eubacteria andspecies of fungi. The fungus has specialized structures that secrete digestiveenzymes into the food substrate on which they live, such as dead logs. Thedigestive enzymes liquefy the small portions of the log, releasing organicmolecules that are absorbed by the fungus. Saphrophytic bacteria secretedigestive enzymes onto the food substrate for digestion. The released organicmolecules then are absorbed by the bacteria.
Scientific method Scientific problem solving depends upon accuracy ofobservation and precision of method. Inherent in scientific thinking isorderliness of approach, which invites the forming of conclusions fromhypotheses, theories, principles, generalizations, concepts, and laws.Scientific problem solving follows a pattern of behaviors that are collectivelyknown as the scientific method.
Semi-permeable membrane The cell membrane controls the passage of materialsinto and out of the cell. It is often referred to as a living gatekeeper. Thecell membrane is semi-permeable and highly selective: not every ion or moleculecan cross its boundary. The movement of materials across the cell boundary andinto or out of the cell is given the general term of transport. It iscontrolled by the globular proteins, phospholipids, and pores of the membraneand by the electrochemical nature of protoplasm, the living substance of thecell.
Sense organs The human body has five major senses -- sight, hearing, taste,smell, and touch -- that provide information about the external environment andtransmit the stimuli to sensory nerves and ultimately to the brain forprocessing.
Sex determination In human beings, there are 22 pairs of autosomes,chromosomes that affect all characteristics not involved in sex determination.One pair of chromosomes determines the sex of an individual. In normal females,the sex chromosomes are designated as XX. In normal males, the sex chromosomesare XY. Certain disorders are sex-linked, usually passed from mother to son bya defective gene on the X chromosome; red-green color blindness is one suchsex-linked trait that is found more frequently in males and hardly at all infemales. Hemophilia is another sex-linked trait that affects males with greaterfrequency than females.
Sharks Cartilaginous fish in which the skeleton is made of cartilage insteadof bone. The body of the shark is covered with placoid scales which arise fromthe ectoderm, also forming teeth in the jaws and on the roof of the mouth.Sharks have no swim bladder and the gill slits are uncovered. The fertilizationis internal; the embryos are nourished from food from the egg.
Sinoatrial node Each heartbeat is started by self activating electricalactivity of the heart's pacemaker known as the sinoatrial node (S-A node),positioned in the wall of the right atrium. From the S-A node, the impulsespreads throughout the atrium to the antrioventricular node (A-V node), aspecialized bundle of cardiac muscle located on the atrium near the ventricles.The impulse spreads from the A-V node to all parts of the ventricles causingsimultaneous contractions in the ventricles.
Skeletal system The human skeleton, like that of all vertebrates, is a livingendoskeleton that grows with the body. At birth, the human baby has a body thatis made up of 270 bones. Due to the fusion of separate bones, the matureskeleton is composed of 206 bones. The human skeleton is a magnificent feat ofengineering. The primary purpose of the skeleton is to carry the weight of thebody and to support and protect the internal organs. The skeleton must bestrong and able to absorb reasonable amounts of shock without fracturing. Atthe same time, the body framework must be flexible and light enough in weightto permit movement. Skeletal bones move in response to muscles that work likelevers, allowing a variety of movements such as walking, running, hopping,sitting, bending, lifting, and stooping.
Slime molds Slime molds live on the forest floor where they grow in damp soil,on or around rotting logs and on decaying vegetation. They appear to beshapeless globs of slime of varying colors: white, yellow, or red. The lifecycle includes either an amoeboid slime mass called a plasmodium or aplant-like sporangia stage that is supported by stalks called fruiting bodies.A multicellular mass is called a plasmodium. A multinucleated mass is called apseudoplasmodium. The diagram above shows stages in the life cycle of trueslime mold.
Sodium-potassium pump At times molecules are forced out of cells byexocytosis, a means by which they are carried to the cell surface by vacuolesor vesicles. The sodium-potassium pump is a means by which excess sodium ionsare forcibly extruded from nerve cells while potassium ions are pulled into thecell.
Species The basic unit of classification is the species. A species is a groupof similar organisms that can mate and produce fertile offspring. The red wolf,African elephant, red oak, house fly, hair cap moss -- each belong to aseparate and distinct species. For example, the red wolf belongs to the redwolf species in which the male mates with the female and produces fertile redwolf offspring. Upon maturity these red wolf offspring will reproduce just astheir parents did. Species is a reproductive unit, not one defined bygeography. Once brought together, a Mexican male Chihuahua can mate with afemale Chihuahua born in France because they are species compatible. In rareinstances, members of closely related species -- horses and donkeys, forexample -- can mate and produce offspring. But the products of interspeciesmatings are not fertile and therefore cannot reproduce. When a male donkeymates with a female horse (mare), a mule is produced. The mule is an infertilehybrid. The mating of a male horse (stallion) and a female donkey results in ahinny, also an infertile hybrid.
Sperm The male gamete is structured for motility. It leaves the malereproductive system and is able to swim through the female reproductive systemwhere it may fertilize (penetrate) an egg cell. Sperm cells are haploid, havinghalf the number of chromosomes of a fertilized egg.
Spirochetes Most bacteria belonging to this group are anaerobic; many aredisease producers. These bacteria are long, thin, and curved, moving with awriggling, corkscrew-like motion, made possible by an axial filament. In someways, the spirochetes resemble protozoa, but they are nonnucleated. They do notform spores or branches. They reproduce by transverse fission. The spirocheteTreponema pallidum causes syphilis.
Spongy cells See Leaf cross section.
Sporozoa The Sporozoa are parasitic spore-formers. The adult forms areincapable of locomotion, although immature organisms may move by means ofpseudopodia. Some species of sporozoa go through a complicated life cyclerequiring different hosts during different life stages. For example, thespecies Plasmodium vivax -- the agent that causes malaria -- requires twohosts: the Anopheles mosquito and a human.
Starfish The starfish is an excellent representative of the echinoderms. Ithas all of the distinguishing characteristics: pentaradial symmetry, spinyskin, tube feet controlled by a water vascular system, no head, excretory orrespiratory system. Protruding from the wall of the coelom and extending outbetween the calcareous plates into the sea water, are the papulae, sac-likestructures that function as respiratory and excretory organs. The mouth islocated in the center of the disc on the underside of the body. The nervoussystem is composed of a nerve ring located in the disc from which a ventral andradial nerve branch into each arm. Starfish sexes are separate. Starfish haveremarkable powers of regeneration. If an arms breaks off, the arm grows back.Should a piece of the central disc be attached to the amputated arm, a newindividual will grow from the dismembered part. Starfish prey on oysters.
Stem See Woody stems.
Stoma or stomate Small pore in the plant epidermis that permits gas exchange.Each stoma is bounded by a pair of guard cells whose osmotic status regulatesthe size of the opening.
Substrate The molecules on which an enzyme exerts catalytic action. Thesurface that supports any organism. See Enzyme-substrate complex.
Synthesis Involves those biochemical processes in cells by which smallmolecules are built into larger ones. As a result of synthesis, amino acids,the building blocks of proteins, are changed into enzymes, hormones, andprotoplasm.
Tadpole See Fertilization, frogs.
Tapeworm The life cycle of the tapeworm includes two hosts. A matureproglottid contains a sac filled with hundreds of fertilized eggs. When theproglottid walls rupture, the ground becomes infected with fertilized eggs. Ifthese eggs are ingested by a pig, the protective walls surrounding each egg aredigested, releasing developing embryos of the tapeworm into the digestivesystem of the pig. These embryos bore into the pig's capillaries and arecarried by the blood to the muscles where the scolex forms a cyst. The worm nowremains encysted in the muscles (meat) of the pig. If the butchered pig (nowcalled pork) is improperly cooked and eaten by a human, the encysted wormbecomes activated. Its head begins to bud proglottids and the cycle ofinfection repeats.
Taste buds The taste buds, which receive stimuli for the four taste sensationsof sweet, sour, bitter, and salty, are located in different parts of thetongue. The senses of taste are related to that of smell. When we have a badhead cold, or hold our nose, taste is affected.
Testes Male gonads that produce sperm, the male sex cells. Each gonad consistsof tightly coiled seminiferous tubules where sperm production begins.
Thallophyta Most algae are not differentiated into roots, stems, and leaves.The simple body of the plant -- either a single cell or a flat sheet of simplecells -- is known as a thallus. Thallophytes do not have specialized tissues tocarry water, anchor the plants, or to grow new cells. As a rule, the sex cellsof the thallophytes are produced in rather simple sex organs that are notprotected by a surrounding wall of cells. The zygotes of the thallophytes donot develop into embryos that are contained in a female reproductive organ.Most species of thallophytes live in fresh water, although there are a fewsaltwater forms. Some species live in damp soil or on the bark of trees.
Tissues Cells in the body of the multicellular organism are arranged instructural and functional groups called tissues. A tissue is a group of similarcells that work together to perform a particular function. Tissues that aregrouped together and work for a common cause form organs. Groups of organs thatcontribute to a particular set of functions are called systems. The ability ofcells to carry out special functions in addition to the usual work of cellsexemplifies specialization. When different jobs are accomplished by the varioustissues in an organ, we call this division of labor.
Transpiration Loss of water vapor through plant leaves is termedtranspiration, a process that is responsible for the rise of sap in trees.Guttation is the loss of liquid water through the leaves of plants with shortstems. Guttation is caused by the effect of root pressure on water flow and hashardly any physiological advantage to the plant.
Transport Involves the absorption of materials by living things, including themovement and distribution of materials within the body of the organism. Thereare several transport methods, including diffusion, active transport, andcirculation. Diffusion is the flow of molecules from an area where thesemolecules are in great concentration to an area where there are fewer of them.Active transport is the movement of molecules powered by energy. Circulation isthe movement of fluid and its dissolved materials throughout the body of anorganism or within the cytoplasm of a single cell.
Turtles and tortoises The chelonians are the turtles and tortoises. Theskeleton is modified to form a box-like covering, the upper curved portion ofwhich is called the carapace, the lower part, the plastron. The head, tail, andlegs are the only movable parts of the animal. The jaws are horny andtoothless. Chelonians live on land, in freshwater, and in the sea. More turtleslive on the American continent than anywhere else.
Twins There are two types of twins: identical and fraternal. Identical twinsresult from the fertilization of one egg and have the same genetic makeup. Theyare of the same sex and are almost identical in appearance. They develop in acommon chorionic sac and share a common placenta. However, the umbilical cordsare separate. Fraternal twins develop from two separate fertilized eggs. Theydo not share a common genetic makeup and are no more alike than siblings bornat separate times. The sexes may be different. Each fraternal twin has its ownchorionic membrane and its own placenta.
Umbilical cord The implanted embryo is attached to the uterus by means of theumbilical cord, a structure that contains blood vessels that function incarrying nutrients and oxygen to the embryo and transporting waste away fromit. The umbilical cord connects with the placenta, a vascularized organ made upof tissues of the mother and the embryo. The blood of the embryo thatcirculates in the capillaries of the placenta is separated from the blood ofthe mother by layers of cells thin enough to allow diffusion between the twocirculatory systems. There is no mixing of the blood of the mother with theblood of the embryo. (The term fetus is used in mammalian embryos after theorgans have formed.)
Urinary system The human urinary system is located dorsally in the abdomen.This system consists of two kidneys, tubes known as ureters extending from eachkidney to a urinary bladder and a single urethra, a tube that leads out of thebladder.
Uterus A muscular sac inside the body of the female mammal specialized forholding the developing young.
Vacuole A space in the cytoplasm enclosed by a membrane and filled withliquid. Some vacuoles are digestive vesicles; some are storage areas.Contractile vacuole is an organelle that pumps excess water out of freshwaterprotozoans such as the Paramecium.
Vascular plants The vascular plants are truly land-dwelling plants. They havedeveloped adaptations that permit them to live on land independent of bodies ofwater. The word "vascular" means that these plants have a water-carryingsystem. Water is conducted upward from the roots by xylem tubules. Fluidcompounds are conducted downward from the leaves to lower plant organs by thephloem tubules.
Vegetative propagation In some cases, flowering plants can reproduce more oftheir kind without the use of flowers, but from stems, leaves, or roots. Sincethese are the vegetative parts of the plant normally used for nutrition, thismethod of reproduction is known as vegetative propagation. It is a form ofasexual reproduction, since it involves only one parent.
Values of vegetative propagation:
1. The plants are of the same type as the parents. They do not vary, as mightusually be the case in sexual reproduction, where the characteristics of twoparents are inherited.
2. Plants are reproduced much more quickly and in larger numbers, than if theywere grown from seeds.
3. Seedless fruits such as oranges and grapes can be maintained and propagated.
Venereal diseases (VD) Largely spread through sexual intercourse betweenpeople who are infected with the germs that cause the diseases. The infectionof two such diseases, syphilis and gonorrhea, generally takes place through themucous membranes of the reproductive organs or the mouth. A pregnant woman whohas one of these diseases may give birth to a deformed or dead baby. Blindnessmay also affect the newborn. As a precaution, it is common practice inhospitals to treat the eyes of newborn babies with silver nitrate.
Vertebrates Animals that have a true backbone composed of segmented partscalled vertebrae belong to the chordate subphylum Vertebrata. The vertebrae maybe made of cartilage or bone: if made of the latter, cartilage cushions preventthe bones from rubbing together. The backbone is built around the notochord andusually obliterates it. Vertebrates vary in size from large to small, but allhave a living endoskeleton usually made of bone. All vertebrate species havemarked development of the head where a brain is enclosed in a cranium. Blood ispumped through a closed circulatory system by means of a ventral heart, havingat least two chambers: an atrium and a ventricle. The hepatic portal systemcarries blood laden with food from the intestines to the liver before itreaches the body cells. Vertebrate red blood cells contain the iron-bearingpigment hemoglobin that is specialized to carry oxygen. Such a system of closedblood vessels prevents blood from entering the body cavity. Most vertebrates(except humans) have a post-anal tail that is a continuation of the vertebralcolumn. Although there are never more than two sets of paired appendages, someadult vertebrates show only one such set or none at all, the appendages havingbeen lost over evolutionary time. Evidence of lost appendages may be seen inembryonic forms or may be demonstrated by vestigial structures. The coccyx bonein humans is a remnant (vestigial structure) of a post-anal tail. Othercharacteristics of vertebrates include a mouth that is closed by a movablelower jaw and a thyroid gland derived from the ventral wall of the pharynx. Inthe invertebrate chordates, the endostyle is an evolutionary signpost pointingto the development of the thyroid gland.
Viruses Viruses are not living. They are not cells and they do not exhibit thecharacteristics of life as do cells. Viruses can reproduce only within livingcells. The virus particle is known as a virion. It merely consists of a proteincoat and a nucleic acid core. The protein coat, called a capsid, may be shapedlike a rod, be polyhedral, or have a tail with extending fibers. In someviruses, including those that cause influenza, a cytoplasmic membrane surroundsthe protein coat. This surrounding envelope may come from the plasma membraneof the host cell or may be synthesized by the host's cytoplasm. However,virologists have found that the envelope contains proteins that arevirus-specific. Viral nucleic acid may be a single molecule consisting of asfew as five genes or may have as many as several hundred. Viral nucleic acidmay be single or it may be double-stranded; it may be circular or linear. Someviral nucleic acid is made of DNA (deoxyribonucleic acid); others have only anRNA (ribonucleic acid) core. Viruses never contain both DNA and RNA. SeeBacteriophage.
Vitamins Are organic compounds. They are classified as water soluble or fatsoluble. In general, the water soluble vitamins are coenzymes necessary to theproper sequence of biochemical events that occur during cellular respiration.It is interesting to note that the primates (Homo sapiens included) and guineapigs are the only vertebrate animals that cannot synthesize their own vitamin Cfrom carbohydrates. Therefore, the daily requirements of ascorbic acid must bemet through food intake. The functions of the fat soluble vitamins are notclearly understood. For detailed information on vitamins, See Health andMedicine.
Water, conservation of Water is rendered useless for drinking, bathing,irrigation, and as a habitat for fish when polluted by the chemical wastes fromindustry and by human sewage. Sewage treatment plants clean up sewage before itis dumped into waterways. Special treatment must be given to chemical wastes todetoxify them before disposal. Water is a renewable resource. However, peopleon the earth are using more water than ever before in industry, refrigeration,agriculture, and the like. Humans are dependent on rainfall to maintain anadequate water table (level of groundwater) and to replenish water stores inreservoirs. The wasting of water through careless use can have seriousconsequences for human life.
Water cycle There are three ways in which water vapor enters the atmosphere.Water evaporates from land surfaces and from the surfaces of all bodies ofwater. Water vapor enters the air as a waste product of respiration of animalsand plants. For example: every time you exhale, water vapor is released intothe air. Great amounts of water are lost from plants through the openings inthe leaves; this water loss due to evaporation is called transpiration. Watervapor in the air is carried to high altitudes where it is cooled and formsclouds by condensation. Eventually, clouds fall to the earth in the form ofprecipitation: rain, snow, or sleet. Most of the precipitation returns to theoceans, lakes, or stream and less than 1 percent of it falls on land. Of thewater that does fall on land, about 25 percent of it will evaporate from thevarious land surfaces before it can be absorbed by plants or used by animals.Water that does not evaporate enters the soil and becomes available to plantroots and soil organisms. Soil water that is not absorbed by plants seeps downinto the ground until it reaches an impervious layer of rock. The water movesalong this rock as groundwater until it reaches an outlet into a larger body ofwater such as a lake or an ocean. The water cycle repeats.
White blood cells (Leucocytes) There are five types of white blood cells,functioning to protect the body against invading foreign proteins. Theamoeboid-like neutrophils and monocytes behave as phagocytes, engulfingbacteria and other foreign proteins. Eosinophils detoxify histamine-likesecretions. Lymphocytes participate in immune responses and basophils produceanticoagulants.
Woody stems Stems have three major functions. First, they conduct water upwardfrom the roots to the leaves and conduct dissolved food materials downward fromthe leaves to the roots. Second, stems produce and support leaves and flowers.Third, they provide the mechanisms for the storage of food. Woody stems arecomposed of primary and secondary tissues. Primary tissues are those thatdevelop from the meristems (embryonic tissue) of the buds on twigs during thefirst year of growth. After the first year, growth in the woody stem takesplace in the secondary tissues. These are tissues that arise from the cambium.
Worm-ParasitesDiseases Caused by Diseases Caused byFlatworms RoundwormsTapeworm Infection HookwormSheep Liver Fluke Trichinosis Infection Ascaris InfectionChinese Liver Fluke Pinworm Infection Infection Filariasis
Xylem Water-carrying (vascular) tissue in plants that conducts water from theroots upward.
Zoospores Free-swimming cells with flagella that are produced by asexual celldivision of the green algae Chlamydomonas. The zoospores mature, develop alarge chloroplast, and become vegetative Chlamydomonas cells.
Zygote Fertilized egg formed by the union of egg and sperm. The zygote has the2N chromosome number. Following zygote formation, the cell goes into a seriesof divisions known as cleavage.

posted by VICKY @ 7:41 PM,

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