F1 (first filial generation)	The first filial or hybrid offspring in a genetic cross-fertilization.
	F2 (second filial generation)	Offspring resulting from interbreeding of the hybrid F1 generation.
	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. May exist as a plasmid or integrated into the bacterial chromosome.
	facilitated diffusion	The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients.
	facultative anaerobe (fak-ul-tay-tiv an-uh-robe)	An organism that makes ATP by aerobic respiration if oxygen is present but that switches to fermentation under anaerobic conditions.
	FAD	Abbreviation of flavin adenine dinucleotide, a coenzyme that functions as an electron acceptor in the Krebs cycle.
	Fallopian tube	See Oviduct.
	family	A taxonomic grouping of related, similar genera; the category below order and above genus.
	fat (triacylglycerol) (tri-ah-sil-glis-er-all)	A biological compound consisting of three fatty acids linked to one glycerol molecule.
	fatty acid	A 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.
	feedback inhibition	A method of metabolic control in which the end-product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway.
	feedback systems	Control mechanisms whereby an increase or decrease in the level of a particular factor inhibits or stimulates the production, utilization, or release of that factor; important in the regulation of enzyme and hormone levels, ion concentrations, temperature, and many other factors.
	fermentation	A 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.
	fetus [L., pregnant]	An unborn or unhatched vertebrate that has passed through the earliest developmental stages; a developing human from about the second month of gestation until birth.
	fiber	A lignified cell type that reinforces the xylem of angiosperms and functions in mechanical support; a slender, tapered sclerenchyma cell that usually occurs in bundles.
	fibril [L. fibra, fiber]	Any minute, threadlike structure within a cell.
	fibrin (fy-brin)	The activated form of the blood-clotting protein fibrinogen, which aggregates into threads that form the fabric of the clot.
	fibroblast (fy-broh-blast)	A type of cell in loose connective tissue that secretes the protein ingredients of the extracellular fibers.
	fibrous protein	Insoluble structural protein in which the polypeptide chain is coiled along one dimension. Fibrous proteins constitute the main structural elements of many animal tissues.
	filament [L. filare, to spin]	(1) A chain of cells. (2) In flowers, the stalk of a stamen.
	filtrate	Fluid 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.
	filtration	The first stage of kidney function; blood plasma is forced, under pressure, out of the glomerular capillaries into Bowman's capsule, through which it enters the renal tubule.
	first law of thermodynamics (thur-moh-dy-nam-iks)	The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed.
	fitness	The genetic contribution of an individual to succeeding generations relative to the contributions of other individuals in the population.
	fixed action pattern	A highly stereotypical behavior that is innate and must be carried to completion once initiated.
	flaccid (flas-id)	Limp; walled cells are flaccid in isotonic surroundings, where there is no tendency for water to enter.
	flagellum pl. flagella (fla-jell-um) [L. flagellum, whip]	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.
	flower	The reproductive structure of angiosperms; a complete flower includes sepals, petals, stamens (male structures), and carpels (female structures).
	fluid-feeder	An animal that lives by sucking nutrient-rich fluids from another living organism.
	fluid mosaic model	The 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.
	follicle (fol-eh-kul)	A 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.
	food chain	The pathway along which food is transferred from trophic level to trophic level, beginning with producers.
	food web	The elaborate, interconnected feeding relationships in an ecosystem.
	fossil [L. fossilis, dug up]	The remains of an organism, or direct evidence of its presence (such as tracks). May be an unaltered hard part (tooth or bone), a mold in a rock, petrification (wood or bone), unaltered or partially altered soft parts (a frozen mammoth).
	founder effect	A cause of genetic drift attributable to colonization by a limited number of individuals from a parent population.
	fovea [L., pit]	A small area in the center of the retina in which cones are concentrated; the area of sharpest vision.
	fragile X syndrome	A hereditary mental disorder, partially explained by genomic imprinting and the addition of nucleotides to a triplet repeat near the end of an X chromosome.
	frameshift mutation	A mutation occurring when the number of nucleotides inserted or deleted is not a multiple of 3, thus resulting in improper grouping into codons.
	free energy	A quantity of energy that interrelates entropy (S) and the system's total energy (H); symbolized by G. The change in free energy of a system is calculated by the equation G = H - T 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 [L. fructus, fruit]	A mature ovary of a flower that protects dormant seeds and aids in their dispersal.
	function [L. fungor, to busy oneself]	Characteristic role or action of a structure or process in the normal metabolism or behavior of an organism.
	functional group	A specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions.
		A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
		Glossary from Biology, Fifth Edition By Campbell, Reece, Mitchell © 1998, Benjamin Cummings. Reprinted with Permission. All Rights Reserved