F

F-factor: A fertility factor in bacteria; a DNA segment that confers the ability to form a pili for conjugation and associated function required for the transfer of DNA from donor to recipient. It may exist as a plasmid or be integrated into the bacterial chromosome.
F plasmid: The plasmid form of a F-factor.
F1 generation: The first filial, or hybrid, offspring in a series of genetic crosses.
F2 generation: Offspring resulting from interbreeding of the hybrid F1 generation.
Facilitator: A species that have a positive effect on the survival and reproduction of other species in a community and it contributes to community structure.
Facilitated diffusion: The spontaneous passage of molecules and ions. bound to specific carrier proteins, across a biological membrane down their concentration gradients.
Facultative anaerobe: An 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.
Fast muscle fibers: Muscle cells used for rapid powerful contractions.
Fat (triacylglycerol): A biological compound consisting of three fatty acids linked to one glycerol molecule.
Fate map: Territorial diagram of embryonic development that reveals the future development of individual cells and tissues.
Fatty acid: A long carbon chain carboxylic acid. Fatty acids vary in length and in the number and location of double bonds.
Feedback inhibition: A method of metabolic control in which the end product of a metabolic pathway acts an inhibitor of an enzyme within that pathway.
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.
Fertilization envelop: The swelling of the vitelline layer away from the plasma membrane.
Fetus: A developing human from the ninth week of gestation until birth; has all the major structures of an adult.
Fiber: A lignified cell type that reinforces the xylem of angiosperms and functions in mechanical supports.
Fibrin: The activated form of the blood-clothing protein fibrinogen, which aggregates into threads that forms the fabric of the clot.
Fibrinogen: The inactive form of the plasma protein that is converted to the active form fibrin.
Fibroblast: A type of cell in lose 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 system: A root system common to monocots consisting of a mat of thin roots spreading out below  the soil surface.
Filament: The stalk of a stamen.
Filtrate: Fluid extracted by the excretory system from the body fluid. The excretory system produces urine from the filtrate after extracting valuable solutes from it and concentrating it.
Filtration: The extraction of water and small solutes, including metabolic waste, from the body fluid into the excretory system.
Fimbria: A short, hairlike prokaryotic appendage that functions in adherence to the substrate or to other cells.
First Law of Thermodynamics: The principle of conservation of energy. Energy can be transferred  and transformed but can not be created or destroyed.
Fission: The separation of a parent into two or more individuals of approximately equal size.
Fitness: The number of offspring left by an individual after one generation relative to the contribution of other individuals. The fitness of an allele is the average fitness of individuals that carry that allele.
Fitness components: Traits, such a survival, mating success, and reproduction that combine to determine fitness.
Fitness landscape:  Either a graph of fitness as a function of individual genotype or phenotype or of population mean fitness as a function of allele frequencies or trait means.
Fitness, mean: The average fitness of a population.
Fitness, relative: The fitness divided by the mean fitness or by the fitness of a reference genotype.
Fixed: When all copies of a gene carry the same allele, that allele is said to be fixed.
Flaccid: Limp. A walled cell is flaccid in surroundings where is no tendency for water to enter.
Flagellum: A long cellular appendage specialized for locomotion. The flagella of prokaryotes and eukaryotes differ in both structure and function.
Florigen:  A flowering signal, not yet chemically identified, that may be a hormone or may be a change in relative concentrations of multiple hormones.
Flower: In an angiosperm, a short stem with up to four sets of modified leaves, bearing structures that function in sexual reproduction.
Fluctuation test:  An experimental method for measuring mutation rates and for determining whether mutations arise prior or in response to selection.
Fluid feeder: An animal that lives by sucking nutrient-rich fluids from another living organism.
Fluid mosaic model: The current accepted model of cell membrane structure, which envisions the membrane as a mosaic of individual protein molecules drifting laterally in a fluid bilayer of phospholipids.
Follicle: A microscopic structure in the ovary that contains the developing ovum and secretes estrogen.
Follicle-stimulating hormone (FSH):  A tropic hormone produced and secreted by the anterior pituitary that stimulates the production of eggs by the ovaries and sperm by the testis.
Follicular phase: That part of the ovarian cycle during which follicles are growing and oocytes maturing
Food chain: The 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: a series of organisms related by predator-prey and consumer-resource interactions. The entirety of interrelated food chains in an ecological community.
Foot: (1) The portion of a bryophyte sporophyte that gathers sugars, amino acids, water, and minerals from the parent gametophyte via transfer cells. (2) One of the three main parts of a mollusc; a muscular structure usually used for movement.
Foraging: Behavior necessary to recognize, search for, capture, and consume food.
Foraminera: An aquatic protist that secrets a hardened shell containing calcium carbonate and extends pseudopodia through pores in the shell
Forebrain: One 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 record: The chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata.
Founder effect:  The loss of genetic variation and the consequent change of genotype frequencies that occurs when small number of individuals found a new population.
Fovea: An eye’s center of focus and the place on the retina where photoreceptors are highly concentrated.
Fragmentation: A means of asexual reproduction whereby a single parent breaks into parts that regenerates into whole new individuals.
Frameshift mutation:  An insertion or deletion mutations that leads to a change in the reading frame in a protein coding gene.
Free energy: The portion of a system’s energy that can perform work when temperature and pressure are uniform through the system.
Free energy of activation:  The amount of energy that reactants must absorb before a chemical reaction will start; also called activation energy.
Frequency-dependent selection: Selection that occurs when relative fitness depends on genotype frequencies.
Frequency spectrum: The distribution of allele frequencies.
Fruit: A mature ovary of a flower that protects dormant seeds and aids in their dispersal.
Functional group:  A specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions.
Fungi: The eukaryotic kingdom that includes organisms that absorbs nutrients after decomposing organic material.
Fusiform initials: Cells within the vascular cambrium that produce elongated cells such as tracheids, vessel elements, fibers, and sieve-tube members.
Fusion, chromosomal: A mutation in which two chromosomes are connected at their ends. The product is a metacentric chromosome.