daily torpor

A daily decrease in metabolic activity and corresponding body temperature during

times of inactivity for some small mammals and birds. The physiological changes during resting periods enable these organisms to survive on energy stores in their tissues.

dalton A measure of mass for atoms and subatomic particles.

Darwinian fitnessThe contribution an individual makes to the gene pool of the next generation, relative to

the contributions of other individuals.

daughter cells Either of the two cells that result when a cell divides.

day-neutral plant A plant whose flowering is not affected by photoperiod.

decapodsA relatively large group of crustaceans that

includes lobsters, crayfish, crabs, and shrimp.

decomposersAny of the saprotrophic fungi and bacteria

that absorb nutrients from nonliving organic material such as corpses, fallen plant

material, and the wastes of living organisms, and convert them into inorganic forms.

decomposition The breakdown of organic materials into inorganic ones.

deep greenAn international initiative focusing on the

deepest phylogenetic branching within the plant kingdom to identify and name the major

plant clades.

deep-sea hydrothermal vents

A dark, hot, oxygen-deficient environment associated with volcanic activity. The food

producers are chemoautotrophic prokaryotes.

dehydration reaction

A chemical reaction in which two molecules covalently bond to each other with the

removal of a water molecule.

deletion(1) A deficiency in a chromosome resulting

from the loss of a fragment through breakage. (2) A mutational loss of one or

more nucleotide pairs from a gene.

demographic transition

A shift from zero population growth in which birth rates and death rates are high to zero population growth characterized instead by

low birth and death rates.

demography The study of statistics relating to births and deaths in populations.

denaturation

For proteins, a process in which a protein unravels and loses its native conformation, thereby becoming biologically inactive. For

DNA, the separation of the two strands of the double helix. Denaturation occurs under

extreme conditions of pH, salt concentration, and temperature.

dendriteOne of usually numerous, short, highly branched processes of a neuron that

conveys nerve impulses toward the cell body.

desnity The number of individuals per unit area or volume.

density dependent Any characteristic that varies according to an increase in population density.

density-dependent factor

Any factor that has a greater impact on a population as the population density

increases.

density-dependent inhibition

The phenomenon observed in normal animal cells that causes them to stop dividing when

they come into contact with one another.

density-independent factor

Any factor that affects a population by the same percentage, regardless of density.

deoxyribonucleic acid (DNA)

A double-stranded, helical nucleic acid molecule capable of replicating and

determining the inherited structure of a cell's proteins.

deoxyriboseThe sugar component of DNA, having one less hydroxyl group than ribose, the sugar

component of RNA.

depolarization

An electrical state in an excitable cell whereby the inside of the cell is made less negative relative to the outside than at the

resting membrane potential. A neuron membrane is depolarized if a stimulus decreases its voltage from the resting

potential of 270 mV in the direction of zero voltage.

depolarizedThe condition of a membrane that is more

negatively charged on one side than on the other.

deposit-feederA heterotroph, such as an earthworm, that eats its way through detritus, salvaging bits

and pieces of decaying organic matter.

dermal tissue system

The protective covering of plants; generally a single layer of tightly packed epidermal cells

covering young plant organs formed by primary growth.

descent with modification

Darwin's initial phrase for the general process of evolution.

desmosome A type of intercellular junction in animal cells that functions as an anchor.

determinate cleavage

A type of embryonic development in protostomes that rigidly casts the

developmental fate of each embryonic cell very early.

determinate growthA type of growth characteristic of animals, in

which the organism stops growing after it reaches a certain size.

determinationThe progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo

develops.

detritivore A consumer that derives its energy from nonliving organic material.

detritus Dead organic matter.

deuterostomes

One of two distinct evolutionary lines of coelomates, consisting of the echinoderms and chordates and characterized by radial,

indeterminate cleavage, enterocoelous formation of the coelom, and development of

the anus from the blastopore.

development The sum of all of the changes that progressively elaborate an organism's body.

diacylglycerol (DAG)A second messenger produced by the

cleavage of a certain kind of phospholipid in the plasma membrane.

diaphragm

(1) A sheet of muscle that forms the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs. (2) A dome-shaped

rubber cup fitted into the upper portion of the vagina before sexual intercourse. It serves

as a physical barrier to block the passage of sperm.

diapsids One of three groups of amniotes based on key differences between their skulls.

diastoleThe stage of the heart cycle in which the

heart muscle is relaxed, allowing the chambers to fill with blood.

diastolic pressure Blood pressure that remains between heart contractions.

diatomThe stage of the heart cycle in which the

heart muscle is relaxed, allowing the chambers to fill with blood.

dicotA subdivision of flowering plants whose members possess two embryonic seed

leaves, or cotyledons.

differentiation See cellular differentiation.

diffusionThe spontaneous tendency of a substance to move down its concentration gradient from a

more concentrated to a less concentrated area.

digestionThe process of breaking down food into molecules small enough for the body to

absorb.

dihybrid

An organism that is heterozygous with respect to two genes of interest. A dihybrid

results from a cross between parents doubly homozygous for different alleles. For

example, parents of genotype AABB and aabb produce a dihybrid of genotype AaBb.

dikaryonA mycelium of certain septate fungi that

possesses two separate haploid nuclei per cell.

dikaryotic A mycelium with two haploid nuclei per cell, one from each parent.

dinoflagellateA unicellular photosynthetic alga with two flagella situated perpendicular grooves in

cellulose plates covering the cell.

dinosaursAn extremely diverse group of ancient

reptiles varying in body shape, size, and habitat.

diociousReferring to a plant species that has

staminate and carpellate flowers on separate plants.

dipeptidase An enzyme found attached to the intestinal lining. It splits small peptides.

diploblastic Having two germ layers.

diploid cell A cell containing two sets of chromosomes (2n), one set inherited from each parent.

Diplopoda The animal class that includes millipedes.

Dipnoi The class of lungfishes.

directional selection Natural selection that favors individuals at one end of the phenotypic range.

disaccharideA double sugar, consisting of two

monosaccharides joined by dehydration synthesis.

dispersal The distribution of individuals within geographic population boundaries.

dispersion The pattern of spacing among individuals within geographic population boundaries.

dissociation curveA chart showing the relative amounts of oxygen bound to hemoglobin when the

pigment is exposed to solutions varying in their partial pressure of dissolved oxygen.

distal tubuleIn the vertebrate kidney, the portion of a

nephron that helps refine filtrate and empties it into a collecting duct.

disturbanceA force that changes a biological community

and usually removes organisms from it. Disturbances, such as fire and storms, play pivotal roles in structuring many biological

communities.

disulfide bridgeStrong covalent bonds formed when the

sulfur of one cysteine monomer bonds to the sulfur of another cysteine monomer.

diversifying selection

Natural selection that favors extreme over intermediate phenotypes.

diversity Variety in organisms.

DNA fingerprintAn individual's unique collection of DNA

restriction fragments, detected by electrophoresis and nucleic acid probes.

DNA ligaseA linking enzyme essential for DNA

replication; catalyzes the covalent bonding of the 39 end of a new DNA fragment to the 59

end of a growing chain.

DNA methylationThe addition of methyl groups (—CH3) to bases of DNA after DNA synthesis; may

serve as a long-term control of gene expression.

DNA microarray assays

A method to detect and measure the expression of thousands of genes at one time. Tiny amounts of a large number of

single-stranded DNA fragments representing different genes are fixed to a glass slide.

These fragments, ideally representing all the genes of an organism, are tested for

hybridization with various samples of cDNA molecules.

DNA polymeraseAn enzyme that catalyzes the elongation of

new DNA at a replication fork by the addition of nucleotides to the existing chain.

DNA probeA chemically synthesized, radioactively

labeled segment of nucleic acid used to find a gene of interest by hydrogen-bonding to a

complementary sequence.

DNA-binding domain A part of the three-dimensional structure of a transcription factor that binds to DNA.

domainA taxonomic category above the kingdom

level. The three domains are Archaea, Bacteria, and Eukarya.

dominance hierarchy

A linear "pecking order" of animals, where position dictates characteristic social

behaviors.

dominant allele In a heterozygote, the allele that is fully expressed in the phenotype.

dominant phenotype

The phenotype that results from having at least one dominant allele.

dominant speciesThose species in a community that have the

highest abundance or highest biomass. These species exert a powerful control over

the occurrence and distribution of other species.

dopamine A biogenic amine closely related to epinephrine and norepinephrine.

dormancyA condition typified by extremely low

metabolic rate and a suspension of growth and development.

dorsal Pertaining to the back of a bilaterally symmetrical animal.

dorsal lip The dorsal side of the blastopore.

double circulationA circulation scheme with separate

pulmonary and systemic circuits, which ensures vigorous blood flow to all organs.

double covalent bond

A type of covalent bond in which two atoms share two pairs of electrons; symbolized by a

pair of lines between the bonded atoms.

double fertilizationA mechanism of fertilization in angiosperms, in which two sperm cells unite with two cells

in the embryo sac to form the zygote and endosperm.

double helixThe form of native DNA, referring to its two

adjacent polynucleotide strands wound into a spiral shape.

Down syndromeA human genetic disease resulting from

having an extra chromosome 21, characterized by mental retardation and

heart and respiratory defects.

downy mildewsMembers of the group Oomycota, they are

heterotrophic stramenopiles that lack chloroplasts, typically have cell walls made of cellulose, and generally live on land as

parasites of plants.

Drosophila Fruit fly.

Duchenne muscular dystrophy

A human genetic disease caused by a sex-linked recessive allele; characterized by

progressive weakening and a loss of muscle tissue.

duodenumThe first section of the small intestine, where

acid chyme from the stomach mixes with digestive juices from the pancreas, liver,

gallbladder, and gland cells of the intestinal wall.

duplicationAn aberration in chromosome structure

resulting from an error in meiosis or mutagens; duplication of a portion of a

chromosome resulting from fusion with a fragment from a homologous chromosome.

dyneinA large contractile protein forming the side-

arms of microtubule doublets in cilia and flagella.

E siteOne of three binding sites for tRNA during

translation, it is the place where discharged tRNAs leave the ribosome; E stands for exit

site.

ecdysone A steroid hormone that triggers molting in arthropods.

ecdysozoa One of two distinct clades within the protostomes. It includes the arthropods.

echinodermsSessile or slow-moving animals that include sea stars, sea urchins, brittle stars, crinoids,

and basket stars.

ecological efficiencyThe ratio of net productivity at one trophic level to net productivity at the next lower

level.

ecological footprint

A method to use multiple constraints to estimate the human carrying capacity of

Earth by calculating the aggregate land and water area in various ecosystem categories that is appropriated by a nation to produce

all the resources it consumes and to absorb all the waste it generates.

ecological niche(nich) The sum total of a species' use of the

biotic and abiotic resources of its environment.

ecological species concept

The idea that ecological roles (niches) define species.

ecological succession

Transition in the species composition of a biological community, often following

ecological disturbance of the community; the establishment of a biological community in

an area virtually barren of life.

ecology The study of how organisms interact with their environments.

ecosystemAll the organisms in a given area as well as the abiotic factors with which they interact; a

community and its physical environment.

ecosystem ecologyThe study of energy flow and the cycling of

chemicals among the various biotic and abiotic factors in an ecosystem.

ectodermThe outermost of the three primary germ

layers in animal embryos; gives rise to the outer covering and, in some phyla, the

nervous system, inner ear, and lens of the eye.

ectomycorrhizae

A type of mycorrhizae in which the mycelium forms a dense sheath, or mantle, over the

surface of the root. Hyphae extend from the mantle into the soil, greatly increasing the

surface area for water and mineral absorption.

ectoparasites Parasites that feed on the external surface of a host.

ectothermAn animal, such as a reptile, fish, or

amphibian, that must use environmental energy and behavioral adaptations to

regulate its body temperature.

ectothermicOrganisms that do not produce enough

metabolic heat to have much effect on body temperature.

ediacaran period The last period of the Precambrian era.

effector cellA muscle cell or gland cell that performs the

body's responses to stimuli; responds to signals from the brain or other processing

center of the nervous system.

efferent arteriole The blood vessel draining a nephron.

egg-polarity genesAnother name for maternal effect genes,

these genes control the orientation (polarity) of the egg.

ejaculatory ductThe short section of the ejaculatory route in mammals formed by the convergence of the

vas deferens and a duct from the seminal vesicle. The ejaculatory duct transports

sperm from the vas deferens to the urethra.

elastic fibersLong threads made of the protein elastin.

Elastic fibers provide a rubbery quality to the extracellular matrix that complements the nonelastic strength of collagenous fibers.

electrocardiogram (ECG or EKG)

A record of the electrical impulses that travel through cardiac muscle during the heart

cycle.

electrochemical gradient

The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion

across a membrane and its tendency to move relative to the membrane potential.

electroencephalogram (EEG)

A medical test that measures different patterns in the electrical activity of the brain.

electrogenic pump An ion transport protein generating voltage across the membrane.

electromagnetic receptor

Receptors of electromagnetic energy, such as visible light, electricity, and magnetism.

electromagnetic spectrum

The entire spectrum of radiation ranging in wavelength from less than a nanometer to

more than a kilometer.

electronA subatomic particle with a single negative

charge; one or more electrons move around the nucleus of an atom.

electron microscope

A microscope that focuses an electron beam through a specimen, resulting in resolving

power a thousandfold greater than that of a light microscope. A transmission electron

microscope (TEM) is used to study the internal structure of thin sections of cells. A

scanning electron microscope (SEM) is used to study the fine details of cell surfaces.

electron shell An energy level representing the distance of an electron from the nucleus of an atom.

electron transport chain

A sequence of electron carrier molecules (membrane proteins) that shuttle electrons

during the redox reactions that release energy used to make ATP.

electronegativity The attraction of an atom for the electrons of a covalent bond.

electroporationA technique to introduce recombinant DNA into cells by applying a brief electrical pulse to a solution containing cells. The electricity creates temporary holes in the cells' plasma membranes, through which DNA can enter.

element Any substance that cannot be broken down to any other substance.

elimination The passing of undigested material out of the digestive compartment.

embryo New developing individuals.

embryo sacThe female gametophyte of angiosperms, formed from the growth and division of the

megaspore into a multicellular structure with eight haploid nuclei.

embryonic lethal Mutations with phenotypes leading to death at the embryo or larval stage.

embryophyteAnother name for land plants, recognizing that land plants share the common derived trait of multicellular, dependent embryos.

emulsification The process that keeps tiny fat droplets from coalescing.

enantiomer Molecules that are mirror images of each other.

endangered speciesA species that is in danger of extinction

throughout all or a significant portion of its range.

endemic species Species that are confined to a specific, relatively small geographic area.

endergonic reactionA nonspontaneous chemical reaction in which free energy is absorbed from the

surroundings.

endocrine gland A ductless gland that secretes hormones directly into the bloodstream.

endocrine system

The internal system of chemical communication involving hormones, the

ductless glands that secrete hormones, and the molecular receptors on or in target cells

that respond to hormones; functions in concert with the nervous system to effect

internal regulation and maintain homeostasis.

endocytosisThe cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the

substance and pinch off to form an intracellular vesicle.

endodermThe innermost of the three primary germ

layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the

digestive tract.

endodermisThe innermost layer of the cortex in plant

roots; a cylinder one cell thick that forms the boundary between the cortex and the stele.

endomembrane system

The collection of membranes inside and around a eukaryotic cell, related either

through direct physical contact or by the transfer of membranous vesicles.

endometrium The inner lining of the uterus, which is richly supplied with blood vessels.

endomycorrhizaeA type of mycorrhizae that, unlike

ectomycorrhizae, do not have a dense mantle ensheathing the root. Instead,

microscopic fungal hyphae extend from the root into the soil.

endoparasites Parasites that live within a host.

endoplasmic reticulum

An extensive membranous network in eukaryotic cells, continuous with the outer

nuclear membrane and composed of ribosome-studded (rough) and ribosome-free

(smooth) regions.

endorphin A hormone produced in the brain and anterior pituitary that inhibits pain perception.

endoskeletonA hard skeleton buried within the soft tissues

of an animal, such as the spicules of sponges, the plates of echinoderms, and the

bony skeletons of vertebrates.

endospermA nutrient-rich tissue formed by the union of

a sperm cell with two polar nuclei during double fertilization, which provides

nourishment to the developing embryo in angiosperm seeds.

endospore A thick-coated, resistant cell produced within a bacterial cell exposed to harsh conditions.

endosymbiotic theory

A hypothesis about the origin of the eukaryotic cell, maintaining that the forerunners of eukaryotic cells were

symbiotic associations of prokaryotic cells living inside larger prokaryotes.

endotheliumThe innermost, simple squamous layer of

cells lining the blood vessels; the only constituent structure of capillaries.

endothermAn animal that uses metabolic energy to

maintain a constant body temperature, such as a bird or mammal.

endothermicOrganisms with bodies that are warmed by heat generated by metabolism. This heat is usually used to maintain a relatively stable body temperature higher than that of the

external environment.

endotoxinA component of the outer membranes of

certain gram-negative bacteria responsible for generalized symptoms of fever and ache.

energetic hypothesis

The concept that the length of a food chain is limited by the inefficiency of energy transfer

along the chain.

energy The capacity to do work (to move matter against an opposing force).

energy couplingIn cellular metabolism, the use of energy

released from an exergonic reaction to drive an endergonic reaction.

energy level The different states of potential energy for electrons in an atom.

enhancerA DNA sequence that recognizes certain

transcription factors that can stimulate transcription of nearby genes.

enterocoelousThe type of development found in

deuterostomes. The coelomic cavities form when mesoderm buds from the wall of the

archenteron and hollows out.

enterogastrones A category of hormones secreted by the wall of the duodenum.

eneteropeptidaseAn intestinal enzyme that directly or indirectly triggers activation of other enzymes within the intestinal lumen.

entomology The study of insects.

entropy A quantitative measure of disorder or randomness, symbolized by S.

environmental gainAn ecological term for the effect of spatial

variation, or patchiness, relative to the size and behavior of an organism.

enzymeA protein serving as a catalyst, a chemical agent that changes the rate of a reaction without being consumed by the reaction.

epicotyl The embryonic axis above the point at which the cotyledons are attached.

epidermis (1) The dermal tissue system in plants. (2) The outer covering of animals.

epididymis A coiled tubule located adjacent to the testes where sperm are stored.

epigenesist The progressive development of form in an embryo.

epiglottisA cartilaginous flap that blocks the top of the

windpipe, the glottis, during swallowing, which prevents the entry of food or fluid into

the respiratory system.

epinephrine A hormone produced as a response to stress; also called adrenaline.

epiphyteA plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.

episomeA genetic element that can exist either as a

plasmid or as part of the bacterial chromosome.

epistasisA genetic element that can exist either as a

plasmid or as part of the bacterial chromosome.

epithalamusA brain region, derived from the

diencephalon, that contains several clusters of capillaries that produce cerebrospinal

fluid.

epithelial tissue Sheets of tightly packed cells that line organs and body cavities.

epitopseA localized region on the surface of an

antigen that is chemically recognized by antibodies; also called antigenic determinant.

erythrocyteA red blood cell; contains hemoglobin, which

functions in transporting oxygen in the circulatory system.

erythropoietinA hormone produced in the kidney when

tissues of the body do not receive enough oxygen. This hormone stimulates the

production of erythrocytes.

esophagus A channel that conducts food, by peristalsis, from the pharynx to the stomach.

essential amino acids

The amino acids that an animal cannot synthesize itself and must obtain from food.

Eight amino acids are essential in the human adult.

essential fatty acids Certain unsaturated fatty acids that animals cannot make.

essential nutrientA chemical element that is required for a

plant to grow from a seed and complete the life cycle, producing another generation of

seeds.

estivationA physiological state characterized by slow

metabolism and inactivity, which permits survival during long periods of elevated

temperature and diminished water supplies.

estrogens

The primary female steroid sex hormones, which are produced in the ovary by the

developing follicle during the first half of the cycle and in smaller quantities by the corpus

luteum during the second half. Estrogens stimulate the development and maintenance

of the female reproductive system and secondary sex characteristics.

estrous cycleA type of reproductive cycle in all female

mammals except higher primates, in which the nonpregnant endometrium is reabsorbed

rather than shed, and sexual response occurs only during midcycle at estrus.

estrus A period of sexual activity associated with ovulation.

estuary The area where a freshwater stream or river merges with the ocean.

ethology The study of animal behavior in natural conditions.

ethyleneThe only gaseous plant hormone,

responsible for fruit ripening, growth inhibition, leaf abscission, and aging.

euchromatin The more open, unraveled form of eukaryotic chromatin that is available for transcription.

eudicotsA large subgroup of traditionally dicot angiosperms including roses, peas,

buttercups, sunflowers, oaks, and maples.

euglenoidA group of protistans, including Euglena and

its relatives, characterized by an anterior pocket or chamber from which one or two

flagella emerge.

eukaryotic cellA type of cell with a membrane-enclosed

nucleus and membrane-enclosed organelles, present in protists, plants, fungi, and

animals; also called eukaryote.

eumetazoa Members of the subkingdom that includes all animals except sponges.

euryhaline Organisms that can tolerate substantial changes in external osmolarity.

eurypteridsMainly marine and freshwater, extinct,

chelicerates. These predators, also called water scorpions, ranged up to 3 meters long.

Eustachian tube The tube that connects the middle ear to the pharynx.

eutherian mammalsPlacental mammals; those whose young complete their embryonic development

within the uterus, joined to the mother by the placenta.

eutrophicPertaining to a highly productive lake, having

a high rate of biological productivity supported by a high rate of nutrient cycling.

evaporationThe removal of heat energy from the surface

of a liquid that is losing some of its molecules.

evaporative coolingThe property of a liquid whereby the surface becomes cooler during evaporation, owing to

a loss of highly kinetic molecules to the gaseous state.

evolutionAll the changes that have transformed life on

Earth from its earliest beginnings to the diversity that characterizes it today.

evolutionary species concept

The idea that evolutionary lineages and ecological roles can form the basis of

species identification.

exaptationA structure that evolves and functions in one environmental context but that can perform additional functions when placed in some

new environment.

excitable cells Cells that have the ability to generate changes in their membrane potentials.

excitatory postsynaptic

potential (EPSP)

An electrical change (depolarization) in the membrane of a postsynaptic neuron caused

by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic neuron to generate an

action potential.

excretion The disposal of nitrogen-containing waste products of metabolism.

exergonic reaction A spontaneous chemical reaction in which there is a net release of free energy.

exocytosisThe cellular secretion of macromolecules by

the fusion of vesicles with the plasma membrane.

exoenzymes Powerful hydrolytic enzymes secreted by a fungus outside its body to digest food.

exonA coding region of a eukaryotic gene. Exons,

which are expressed, are separated from each other by introns.

exoskeletonA hard encasement on the surface of an

animal, such as the shells of mollusks or the cuticles of arthropods, that provides

protection and points of attachment for muscles.

exotoxinA toxic protein secreted by a bacterial cell

that produces specific symptoms even in the absence of the bacterium.

exponential population growth

The geometric increase of a population as it grows in an ideal, unlimited environment.

expression vectorA cloning vector that contains the requisite

prokaryotic promoter just upstream of a restriction site where a eukaryotic gene can

be inserted.

external fertilization The fusion of gametes that parents have discharged into the environment.

exteroreceptorSensory receptors that detect stimuli outside the body, such as heat, light, pressure, and

chemicals.

extracellular digestion

The breakdown of food outside cells.

extracellular matrix (ECM)

The substance in which animal tissue cells are embedded consisting of protein and

polysaccharides.

extraembryonic membranes

Four membranes (yolk sac, amnion, chorion, allantois) that support the developing embryo

in reptiles, birds, and mammals.

extreme halophilesMicroorganisms that live in unusually highly saline environments such as the Great Salt

Lake or the Dead Sea.

extremeophile Microorganisms that thrive in hot environments (often 60–80 degrees C).

5’ capMicroorganisms that live in extreme

environments. They are further classified as either methanogens, extreme halophiles, or

extreme thermophiles.

F factor

A fertility factor in bacteria, a DNA segment that confers the ability to form pili for conjugation and associated functions

required for the transfer of DNA from donor to recipient. It may exist as a plasmid or

integrated into the bacterial chromosome.

F1 generation The first filial, or hybrid, offspring in a genetic cross-fertilization.

F1 plasmidThe F factor and its plasmid that consists of about 25 genes, most of which are required

for the production of sex pili.

F2 generation Offspring resulting from interbreeding of the hybrid F1 generation.

facilitateThe positive effect of early species on the appearance of later species in ecological

succession.

facilitated diffusionThe spontaneous passage of molecules and

ions, bound to specific carrier proteins, across a biological membrane down their

concentration gradients.

facultative anaerobeAn organism that makes ATP by aerobic respiration if oxygen is present but that

switches to fermentation under anaerobic conditions.

family In classification, the taxonomic category above genus.

fast block to polyspermy

The depolarization of the egg membrane within 1–3 seconds after sperm binding to the vitelline layer. The reaction prevents

additional sperm from fusing with the egg's plasma membrane.

fast muscle fibers Muscle cells used for rapid, powerful contractions.

fat (tracylglycerol) A biological compound consisting of three fatty acids linked to one glycerol molecule.

fate mapsTerritorial diagrams of embryonic

development that reveal the future development of individual cells and tissues.

fatty acidA long carbon chain carboxylic acid. Fatty acids vary in length and in the number and location of double bonds; three fatty acids

linked to a glycerol molecule form fat.

feathers Light epidermal outgrowths that form the external covering of the body of birds.

feces The wastes of the digestive tract.

feedback inhibitionA method of metabolic control in which the end product of a metabolic pathway acts as

an inhibitor of an enzyme within that pathway.

fermentationA catabolic process that makes a limited amount of ATP from glucose without an

electron transport chain and that produces a characteristic end product, such as ethyl

alcohol or lactic acid.

fertilization The union of haploid gametes to produce a diploid zygote.

fertilization envelope

The swelling of the vitelline layer away from the plasma membrane.

fetusA developing human from the ninth week of

gestation until birth; has all the major structures of an atult.

fiberA lignified cell type that reinforces the xylem of angiosperms and functions in mechanical support; a slender, tapered sclerenchyma

cell that usually occurs in bundles.

fibrinThe activated form of the blood-clotting

protein fibrinogen, which aggregates into threads that form the fabric of the clot.

fibrinogenThe inactive form of the plasma protein that is converted to the active form fibrin, which

aggregates into threads that form the framework of a blood clot.

fibroblastA type of cell in loose connective tissue that

secretes the protein ingredients of the extracellular fibers.

fibronectin A glycoprotein that helps cells attach to the extracellular matrix.

fibrous connective tissue

A dense tissue with large numbers of collagenous fibers organized into parallel

bundles. This is the dominant tissue in tendons and ligaments.

fibrous root systemsRoot systems common to monocots

consisting of a mat of thin roots that spread out below the soil surface.

filament The stalk of a stamen.

filtrateFluid extracted by the excretory system from

the blood or body cavity. The excretory system produces urine from the filtrate after

extracting valuable solutes from it and concentrating it.

filtrationIn the vertebrate kidney, the extraction of

water and small solutes, including metabolic wastes, from the blood by the nephrons.

first law of thermodynamics

The principle of conservation of energy. Energy can be transferred and transformed,

but it cannot be created or destroyed.

fixed action pattern (FAP)

A sequence of behavioral acts that is essentially unchangeable and usually carried

to completion once initiated.

flaccidLimp. Walled cells are flaccid in isotonic

surroundings, where there is no tendency for water to enter.

flagellum(plural, flagella) A long cellular appendage specialized for locomotion, formed from a

core of nine outer doublet microtubules and two inner single microtubules, ensheathed in

an extension of plasma membrane.

flowerIn an angiosperm, a short stem with four sets

of modified leaves, bearing structures that function in sexual reproduction.

fluid mosaic modelThe currently accepted model of cell

membrane structure, which envisions the membrane as a mosaic of individually

inserted protein molecules drifting laterally in a fluid bilayer of phospholipids.

fluid-feeder An animal that lives by sucking nutrient-rich fluids from another living organism.

follicleA microscopic structure in the ovary that

contains the developing ovum and secretes estrogens.

follicle-stimulating hormone (FSH)

A protein hormone secreted by the anterior pituitary that stimulates the production of

eggs by the ovaries and sperm by the testes.

follicular phaseThat portion of the ovarian cycle during

which several follicles in the ovary begin to grow.

food chainThe pathway along which food is transferred from trophic level to trophic level, beginning

with producers.

food vacuole A membranous sac formed by phagocytosis.

food web The elaborate, interconnected feeding relationships in an ecosystem.

foot The portion of a moss sporophyte that

gathers sugars, amino acids, water, and minerals from the parent gametophyte via

transfer cells.

foraging Behavior necessary to recognize, search for, capture, and consume food.

foramA marine protozoan that secretes a shell and

extends pseudopodia through pores in its shell.

forebrainOne of three ancestral and embryonic

regions of the vertebrate brain; develops into the thalamus, hypothalamus, and cerebrum.

fossil A preserved remnant or impression of an organism that lived in the past.

fossil fuelsEnergy deposits formed from the remains of extinct organisms; fossil fuels include coal,

oil, and natural gas.

fossil recordThe chronicle of evolution over millions of

years of geologic time engraved in the order in which fossils appear in rock strata.

founder effectGenetic drift attributable to colonization by a limited number of individuals from a parent

population.

foveaAn eye's center of focus and the place on the

retina where photoreceptors are highly concentrated.

fragile X syndromeA hereditary mental disorder, partially

explained by genomic imprinting and the addition of nucleotides to a triplet repeat

near the end of an X chromosome.

fragmentationA means of asexual reproduction whereby a

single parent breaks into parts that regenerate into whole new individuals.

frameshift mutationA mutation occurring when the number of

nucleotides inserted or deleted is not a multiple of three, resulting in the improper grouping of the following nucleotides into

codons.

free energy

A quantity of energy that interrelates entropy symbolized by G (S) and the system's total energy (H). The change in free energy of a

system is calculated by the equation: G equals delta H minus T times delta S, where

T is absolute temperature.

free energy of activation

The initial investment of energy necessary to start a chemical reaction; also called

activation energy.

frequency-dependent selection

A decline in the reproductive success of a morph resulting from the morph's phenotype

becoming too common in a population; a cause of balanced polymorphism in

populations.

fruit A mature ovary of a flower that protects dormant seeds and aids in their dispersal.

functional groupA specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical

reactions.

Fungi The kingdom that contains the fungi.

fungus

A heterotrophic eukaryote that digests its food externally and absorbs the resulting

small nutrient molecules. Most fungi consist of a netlike mass of filaments called hyphae.

Molds, mushrooms, and yeasts are examples of fungi.

fusiform initialsThe cambium cells within the vascular

bundles. The name refers to the tapered ends of these elongated cells.

G protein

A GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G-protein linked receptor, to

other signal-transduction proteins inside the cell. When such a receptor is activated, it in

turn activates the G protein, causing it to bind a molecule of GTP in place of GDP.

Hydrolysis of the bound GTP to GDP inactivates the G protein.

G0 phase A nondividing state in which a cell has left the cell cycle.

G1 phaseThe first growth phase of the cell cycle,

consisting of the portion of interphase before DNA synthesis begins.

G2 phaseThe second growth phase of the cell cycle, consisting of the portion of interphase after

DNA synthesis occurs.

gallbladder An organ that stores bile and releases it as needed into the small intestine.

gametangium(plural, gametangia) The reproductive organ

of bryophytes, consisting of the male antheridium and female archegonium; a multichambered jacket of sterile cells in

which gametes are formed.

gameteA haploid cell such as an egg or sperm.

Gametes unite during sexual reproduction to produce a diploid zygote.

gametophores The mature gamete-producing structure of a gametophyte body of a moss.

gametophyteThe multicellular haploid form in organisms undergoing alternation of generations that mitotically produces haploid gametes that

unite and grow into the sporophyte generation.

gamma aminobutyric acid

(GABA)

An amino acid that functions as a CNS neurotransmitter.

ganglion(plural, ganglia) A cluster (functional group) of nerve cell bodies in a centralized nervous

system.

gap genesMutations in these genes cause "gaps" in

Drosophila segmentation. The normal gene products map out the basic subdivisions along the anterior-posterior axis of the

embryo.

gap junctionA type of intercellular junction in animal cells that allows the passage of material or current

between cells; also known as a communicating junction.

gas exchangeThe uptake of molecular oxygen from the environment and the discharge of carbon

dioxide to the environment.

gastric juice The collection of fluids secreted by the epithelium lining the stomach.

gastrinA digestive hormone, secreted by the

stomach, that stimulates the secretion of gastric juice.

gastrovascular cavity

An extensive pouch that serves as the site of extracellular digestion and a passageway to

disperse materials throughout most of an animal's body.

gastrula The two-layered, cup-shaped embryonic stage.

gastrulation The formation of a gastrula from a blastula.

gated channelA protein channel in a cell membrane that opens or closes in response to a particular

stimulus.

gated ion channel A gated channel for a specific ion. By

opening and closing such channels, a cell alters its membrane potential.

gel electrophoresisThe separation of nucleic acids or proteins,

on the basis of their size and electrical charge, by measuring their rate of movement

through an electrical field in a gel.

geneA discrete unit of hereditary information

consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).

gene amplificationThe selective synthesis of DNA, which

results in multiple copies of a single gene, thereby enhancing expression.

gene cloning The production of multiple copies of a gene.

gene flowThe loss or gain of alleles in a population

due to the migration of fertile individuals or gametes between populations.

gene pool The total aggregate of genes in a population at any one time

gene therapy The alternation of the genes of a person afflicted with a genetic disease.

generalized transduction

The random transfer of bacterial genes from one bacterium to another.

genetic drift Changes in the gene pool of a small population due to chance.

genetic engineering The direct manipulation of genes for practical purposes.

genetic map An ordered list of genetic loci (genes or other genetic markers) along a chromosome.

genetic recombination

The general term for the production of offspring with new combinations of traits

inherited from the two parents.

genetically modified (GM) organism

An organism that has acquired one or more genes by artificial means; also known as a

transgenic organism.

genetics The scientific study of heredity and hereditary variation.

genome The complete complement of an organism's genes; an organism's genetic material.

genomic imprintingThe parental effect on gene expression whereby identical alleles have different

effects on offspring, depending on whether they arrive in the zygote via the ovum or via

the sperm.

genomic libraryA set of thousands of DNA segments from a genome, each carried by a plasmid, phage,

or other cloning vector.

genomics The study of whole sets of genes and their interactions.

genotype The genetic makeup of an organism.

genus(plural, genera) A taxonomic category above

the species level, designated by the first word of a species' binomial Latin name.

geographic range The geographic area in which a population lives.

geographic variation The geographic area in which a population lives.

geologic time scaleA time scale established by geologists that reflects a consistent sequence of historical

periods, grouped into four eras: Precambrian, Paleozoic, Mesozoic, and

Cenozoic.

geometric isomersCompounds that have the same molecular

formula but differ in the spatial arrangements of their atoms.

geometric population growth

A rapid J-shaped growth curve that typically occurs when members have access to

abundant food and are free to reproduce at their physiological capacity.

germ layers Three main layers that form the various tissues and organs of an animal body.

gestation Pregnancy; the state of carrying developing young within the female reproductive tract.

gibberellinsA class of related plant hormones that

stimulate growth in the stem and leaves, trigger the germination of seeds and

breaking of bud dormancy, and stimulate fruit development with auxin.

gillA localized extension of the body surface of many aquatic animals, specialized for gas

exchange.

gill circulation The flow of blood through gills.

GinkgophytaA phylum of gymnosperms represented by a

single extant species, Ginkgo biloba, characterized by fanlike leaves that turn gold

and are deciduous in autumn.

glandular epithelia Epithelia that secrete chemical solutions.

glans penis The head end of the penis.

gliaSupporting cells that are essential for the

structural integrity of the nervous system and for the normal functioning of neurons.

glial cellA nonconducting cell of the nervous system

that provides support, insulation, and protection for the neurons.

glomerulusA ball of capillaries surrounded by Bowman's

capsule in the nephron and serving as the site of filtration in the vertebrate kidney.

glucagonA peptide hormone secreted by pancreatic endocrine cells that raises blood glucose

levels; an antagonistic hormone to insulin.

glucocorticoidA corticosteroid hormone secreted by the

adrenal cortex that influences glucose metabolism and immune function.

glutamate An amino acid that functions as a CNS neurotransmitter.

glyceraldehydes-3-phosphate (G3P)

The carbohydrate produced directly from the Calvin cycle.

glycerol A three-carbon alcohol with a hydroxyl group on each carbon.

glycine An amino acid that functions as a CNS neurotransmitter.

glycocalyx A fuzzy coat on the outside of animal cells, made of sticky oligosaccharides.

glycogenAn extensively branched glucose storage

polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.

glycolysisThe splitting of glucose into pyruvate.

Glycolysis is the one metabolic pathway that occurs in all living cells, serving as the

starting point for fermentation or aerobic respiration.

glycoprotein A protein covalently attached to a carbohydrate.

glycosidic linkage A covalent bond formed between two monosaccharides by a dehydration reaction.

gnathostomes The vertebrate subgroup that possess jaws.

GnetophytaA phylum of gymnosperms consisting of just three extant genera that are very different in

appearance.

golden algaeTypically unicellular, biflagellated, algae with yellow and brown carotene and xanthophyll

accessory pigments.

Golgi apparatusAn organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic

reticulum.

gonadotropinsHormones that stimulate the activities of the

testes and ovaries; a collective term for follicle-stimulating and luteinizing hormones.

gonads The male and female sex organs; the gamete-producing organs in most animals.

G-protein linked receptor

graded potential

gradualism

graft versus host reaction

Gram stain

gram-negative

gram-positive

granum

gravitropism

gray crescent

gray matter

green algae

greenhouse effect

gross primary production (GPP)

ground meristem

ground tissue

growth factor

growth hormone

guard cell

gustatory receptors

guttation

gymnosperm

habitats

habituation

hair cell

half-life

Hamilton’s Rule

haploid cell

Hardy-Weinberg equilibrium

Hardy-Weinberg formula

Hardy-Weinberg theorem

haustorium

Haversian system

heart

heart rate

heat

heat of vaporization

heat-shock protein

heavy chains

helicase

heliozoan

helper T cell (TH)

hemocyanin

hemoglobin

hemolymph

hemophilia

hepatic portal vessel

Hepatophyta

herbivore

herbivory

heredity

hermaphrodite

hermaphroditism

heterochromatin

heterochrony

heterocyst

heterogeneity

heterokaryon

heteromorphic

heterosporous

heterotrophy

heterozygote advantage

heterozygous

hibernation

high-density lipoprotein (HDL)

hindbrain

histamine

histone

histone acetylation

HIV (human immunodeficiency

virus)

holdfast

holoblastic cleavage

homeobox

homeosis

homeostasis

homeotic gene

hominid

hominoid

homologous chromosomes

homologous structures

homology

homosporous

homozygous

horizontal cell

hormone

hornworts

host

host range

human chorionic gonadotropin (HCG)

Human Genome Project

humoral immunity

humus

Huntington’s disease

hybridization

hydration shell

hydrocarbon

hydrogen bond

hydrogen ion

hydrolysis

hydrophilic

hydrophobic

hydrophobic interaction

hydrostatic skeleton

hydroxide ion

hydroxyl group

hymen

hyperpolarization

hypertonic

hypha

hypocotyls

hypothalamus

hypotonic solution