exocytosis   Excretion of material from the cell by vesicle fusion with the plasma membrane; can occur constitutively or be regulated.
exon Segment of a eukaryotic gene that consists of a sequence of nucleotides that will be represented in mRNA or in a final transfer, ribosomal, or other mature RNA molecule. In protein-coding genes, exons encode the amino acids in the protein. An exon is usually adjacent to a noncoding DNA segment called an intron. (Figure 4–15) exosome   Large protein complex with an interior rich in  3′-to-5′ RNA exonucleases; degrades RNA molecules to produce ribonucleotides.
extracellular pathogens Pathogens that disturb host cells and can cause serious disease without replicating in host cells.
extracellular signal molecule   Any secreted or cell-surface chemical signal that binds to receptors and regulates the activity of the cell expressing the receptor.
extrinsic pathway   Pathway of apoptosis triggered by extracellular signal proteins binding to cell-surface death receptors.
facultative pathogens   Bacteria that replicate in an environmental reservoir such as water or soil and only cause disease if they happen to encounter a susceptible host.
FAD/FADH2 (flavin adenine dinucleotide/reduced flavin adenine dinucleotide) Electron carrier system that functions in the citric acid cycle and fatty acid oxidation. One molecule of FAD gains two electrons plus two protons in becoming the activated carrier FADH2. (Figure 2–39)
Fas (Fas protein, Fas death receptor) Transmembrane death receptor that initiates apoptosis when it binds its extracellular ligand (Fas ligand). (Figure 18–5)
Fas ligand Ligand that activates the cell-surface death receptor, Fas, triggering the extrinsic pathway of apoptosis.
fat Energy-storage lipid in cells. Composed of triglycerides— fatty acids esterified with glycerol.
fate map   Representation showing which cell types will later derive from which regions of a tissue; e.g. from the blastula. (Figure 21–28)
Fc receptor   One of a family of cell-surface receptors that bind the tail region (Fc region) of an antibody molecule. Different Fc receptors are specific for different classes of antibodies, such as IgG, IgA, or IgE.
feedback inhibition   The process in which a product of a reaction feeds back to inhibit a previous reaction in the same pathway. (Figures 3–55 and 3–56) fermentation   Anaerobic energy-yielding metabolic pathway involving the oxidation of organic molecules. Anaerobic glycolysis refers to the process whereby pyruvate is converted into lactate or ethanol, with the conversion of NADH to NAD+. (Figure 2–47) fibril-associated collagen Mediates the interactions of collagen fibrils with one another and with other matrix macromolecules to help determine the organization of the fibrils in the matrix. This collagen (including types IX and XII) has a flexible triple-stranded helical structure and binds to the surface of the fibrils rather than forming aggregates.
fibrillar collagen Class of fibril-forming collagens (including type I collagen, the most common type and the principal collagen of skin and bone) that have long ropelike structures with few or no interruptions and which assemble into collagen fibrils.
fibroblast Common cell type found in connective tissue. Secretes an extracellular matrix rich in collagen and other extracellular matrix macromolecules. Migrates and proliferates readily in wounded tissue and in tissue culture.
fibronectin   Extracellular matrix protein involved in adhesion of cells to the matrix and guidance of migrating cells during embryogenesis. Integrins on the cell surface are receptors for fibronectin.