F₂ (second filial generation) • The immediate progeny of a mating between members of the F₁ generation.

Facilitated diffusion • Passive movement through a membrane involving a specific carrier protein; does not proceed against a concentration gradient. (Contrast with active transport, diffusion.)

Facultative anaerobes • Prokaryotes that can shift their metabolism between anaerobic and aerobic operations depending on the presence or absence of O₂.

FAD • See Flavin adenine dinucleotide.

Fat • A triglyceride that is solid at room temperature. (Contrast with oil.)

Fate map • A map of the blastula showing which blastomers will contribute to specific tissues and organs in the mature body.

Fatty acid • A molecule with a long hydrocarbon tail and a carboxyl group at the other end. Found in many lipids.

Fauna (faw´ nah) • All of the animals found in a given area. (Contrast with flora.)

Feces [L. faeces: dregs] • Waste excreted from the digestive system.

Feedback control • Control of a particular process induced, directly or indirectly, by the presence or absence of a product of that process.

Fermentation (fur men tay´ shun) [L. fermentum: yeast] • The anaerobic degradation of a substance such as glucose to smaller molecules with the extraction of energy.

Fertilization • Union of gametes. Also known as syngamy.

Fertilization membrane • A membrane surrounding an animal egg which becomes rapidly raised above the egg surface within seconds after fertilization, serving to prevent entry of a second sperm.

Fetus • The latter stages of an embryo that is still contained in an egg or uterus; in humans, the unborn young from the eighth week of pregnancy to the moment of birth.

Fiber • An elongated, tapering cell of flowering plants, usually with a thick cell wall. Serves a support function.

Fibrin • A protein that polymerizes to form long threads that provide structure to a blood clot.

Filter feeder • An organism that feeds upon much smaller organisms, that are suspended in water or air, by means of a straining device.

Filtration • In the excretory physiology of some animals, the process by which the initial urine is formed; water and most solutes are transferred into the excretory tract, while proteins are retained in the blood or hemolymph.

First law of thermodynamics • Energy can be neither created nor destroyed.

Fission • Reproduction of a prokaryote by division of a cell into two comparable progeny cells.

Fitness • The contribution of a genotype or phenotype to the genetic composition of subsequent generations, relative to the contribution of other genotypes or phenotypes. (See inclusive fitness.)

Fixed action pattern • A behavior that is genetically programmed.

Flagellum (fla jell´ um) (plural: flagella) [L. flagellum: whip] • Long, whiplike appendage that propels cells. Prokaryotic flagella differ sharply from those found in eukaryotes.

Flavin adenine dinucleotide (FAD) • A coenzyme involved in redox reactions and containing the vitamin riboflavin (B₂).

Flexor • A muscle that flexes an appendage.

Flora (flore´ ah) • All of the plants found in a given area. (Contrast with fauna.)

Floral meristem • Meristem that forms the sexual parts of flowering plants (sepals, petals, stamens, and carpels).

Florigen • A plant hormone (not yet isolated) involved in the conversion of a vegetative shoot apex to a flower.

Flower • The total reproductive structure of an angiosperm; its basic parts include the calyx, corolla, stamens, and carpels.

Fluid mosaic model • A molecular model for the structure of biological membranes consisting of a fluid phospholipid bilayer in which suspended proteins are free to move in the plane of the bilayer.

Fluorescence • The emission of a photon of visible light by an excited atom or molecule.

Follicle [L. folliculus: little bag] • In female mammals, an immature egg surrounded by nutritive cells.

Follicle-stimulating hormone • A gonadotropic hormone produced by the anterior pituitary.

Food chain • A portion of a food web, most commonly a simple sequence of prey species and the predators that consume them.

Food vacuole • Membrane enclosed structure formed by phagocytosis in which engulfed food particles are digested by the action of lysosomal enzymes.

Food web • The complete set of food links between species in a community; a diagram indicating which ones are the eaters and which are eaten.

Forb • Any broad-leaved herbaceous plant. Especially applied to such plants growing in grasslands.

Fossil • Any recognizable structure originating from an organism, or any impression from such a structure, that has been preserved over geological time.

Fossil fuel • A fuel (particularly petroleum products) composed of the remains of organisms that lived in the remote past.

Founder effect • Random changes in allele frequencies resulting from establishment of a population by a very small number of individuals.

Fovea [L. fovea; a small pit] • The area, in the vertebrate retina, of most distinct vision.

Fragmented habitat • Habitat patches that have become smaller and more isolated due to the destruction of larger habitats by human activities.

Frame-shift mutation • A mutation resulting from the addition or deletion of one or two consecutive base pairs in the DNA sequence of a gene, resulting in misreading mRNA during translation and production of a nonfunctional protein. (Contrast with missense mutation, nonsense mutation, synonymous mutation.)

Free energy • That energy which is available for doing useful work, after allowance has been made for the increase or decrease of disorder.

Frequency-dependent selection • Selection that changes in intensity with the proportion of individuals in a population having the trait.

Fruit • In angiosperms, a ripened and mature ovary (or group of ovaries) containing the seeds. Sometimes applied to reproductive structures of other groups of plants.

Fruiting body • A structure that bears spores.

Functional genomics • The assignment of functional roles to genes first identified by sequencing entire genomes.

Functional group • A characteristic combination of atoms that contribute specific properties when attached to larger molecules.

Functional mRNA • Eukaryotic mRNA that has been modified after transcription by the removal of introns and the addition of a 5´ cap and a 3´ poly(A) tail.

Fungus • A member of the kingdom Fungi, a (usually) multicellular eukaryote with absorptive nutrition. (Yeasts are unicellular fungi.)