filopodium (plural filopodia) (microspike) Thin, spike-like protrusion with an actin filament core, generated on the leading edge of a crawling animal cell. (Figure 16–21) firing rule   Important principle governing synapse reinforcement and elimination during development of the nervous system: when two (or more) neurons synapsing on the same target cell fire at the same time, they reinforce their connections to that cell; when they fire at different times, they compete, so that all but one of them tend to be eliminated.
flagellum (plural flagella) Long, whiplike protrusion whose undulations drive a cell through a fluid medium. Eukaryotic flagella are longer versions of cilia. Bacterial flagella are smaller and completely different in construction and mechanism of action. Compare cilium.
fluorescence microscope   Microscope designed to view material stained with fluorescent dyes or proteins. Similar to a light microscope but the illuminating light is passed through one set of filters before the specimen, to select those wavelengths that excite the dye, and through another set of filters before it reaches the eye, to select only those wavelengths emitted when the dye fluoresces. (Figure 9–12) fluorescence recovery after photobleaching (FRAP) Technique for monitoring the kinetic parameters of a protein by analyzing how fluorescent protein molecules move into an area of the cell bleached by a beam of laser light. (Figure 9–29) fluorescence resonance energy transfer (FRET) Technique for monitoring the closeness of two fluorescently labeled molecules (and thus their interaction) in cells. Also known as Förster resonance energy transfer. (Figure 9–26) focal adhesion kinase (FAK) Cytoplasmic tyrosine kinase present at cell–matrix junctions (focal adhesions) in association with the cytoplasmic tails of integrins.
focal adhesion kinase (FAK) Cytoplasmic tyrosine kinase present at cell–matrix junctions (focal adhesions) in association with the cytoplasmic tails of integrins.
follicular helper T cell (TFH) Type of T cell located in lymphoid follicles that secretes various cytokines to stimulate B cells to undergo antibody class switching and somatic hypermutation.
formin   Dimeric protein that nucleates the growth of straight, unbranched actin filaments that can be cross-linked by other proteins to form parallel bundles.
free energy (G) (Gibbs free energy) The energy that can be extracted from a system to drive reactions. Takes into account changes in both energy and entropy. (Panel 2–7, pp. 102–103) free ribosome   Ribosome that is free in the cytosol, unattached to any membrane.
free-energy change (∆G) see ΔG.
Frizzled   Family of cell-surface receptors that are seven-pass transmembrane proteins that resemble GPCRs in structure but do not generally work through the activation of G proteins. Activated by Wnt binding to recruit the scaffold protein Dishevelled, which helps relay the signal to other signaling molecules.
fungus (plural fungi) Kingdom of eukaryotic organisms that includes the yeasts, molds, and mushrooms. Many plant diseases and a relatively small number of animal diseases are caused by fungi.
fusion protein   Engineered protein that combines two or more normally separate polypeptides. Produced from a recombinant gene.
∆G   Change in the free energy during a reaction: the free energy of the product molecules minus the free energy of the starting molecules. A large negative value of ∆G indicates that the reaction has a strong tendency to occur. (Panel 2–7,  pp. 102–103)
G0   State of withdrawal from the eukaroytic cell-division cycle by entry into a quiescent digression from the G1 phase. A common, sometimes permanent, state for differentiated cells.