Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

ectoderm Outermost of the three primary cell layers of the animal embryo; gives rise to epidermal tissues, the nervous system, and external sense organs. See also endoderm and mesoderm.

effector Ultimate component of a signal transduction pathway that elicits a response to the transmitted signal.

endoderm Innermost of the three primary cell layers of the animal embryo; gives rise to the gut and most of the respiratory tract. See ectoderm and mesoderm.

EF hand A type of helix-loop-helix structural motif that occurs in many Ca²⁺-binding proteins such as calmodulin. (Figure 3-10b)

efferent neurons Nerves that transmit signals from the central nervous system to peripheral tissues such as muscles and endocrine cells.

electric potential The energy associated with the separation of positive and negative charges. An electric potential is maintained across the plasma membrane of nearly all cells.

electrochemical gradient The driving force that determines the energetically favorable direction of transport of an ion (or charged molecule) across a membrane. It represents the combined influence of the ion’s concentration gradient across the membrane and the membrane potential.

electron carrier Any molecule or atom that accepts electrons from donor molecules and transfers them to acceptor molecules in coupled oxidation and reduction reactions. (Table 12-4)

electron transport Flow of electrons via a series of electron carriers from reduced electron donors (e.g., NADH) to O₂ in the inner mitochondrial membrane, or from H₂O to NADP⁺ in the thylakoid membrane of plant chloroplasts. (Figures 12-19 and 12-38)

electron-transport chain Set of four large multiprotein complexes in the inner mitochondrial membrane plus diffusible cytochrome c and coenzyme Q through which electrons flow from reduced electron donors (e.g., NADH) to O₂. Each member of the chain contains one or more bound electron carriers. (Figure 12-22)

electrophoresis Any of several techniques for separating macromolecules based on their migration in a gel or other medium subjected to a strong electric field. (Figure 3-38)

G-9

elongation, transcription Addition of nucleotides to a growing polynucleotide chain, as templated by a complementary DNA coding strand. (Figure 5-11)

elongation factor (EF) One of a group of nonribosomal proteins required for continued translation of mRNA (protein synthesis) following initiation. (Figure 5-25)

embryonic stem (ES) cells A line of cultured cells derived from very early embryos that can differentiate into a wide range of cell types either in vitro or after reinsertion into a host embryo. (Figure 21-5)

ENCODE (Encyclopedia of DNA Elements) A comprehensive, publicly available database of human DNA control elements and the transcription factors that bind to them in different cell types, histone post-translational modifications mapped by ChIP-seq and other related methods, DNase I hypersensitive sites, and regulatory lncRNAs and their sites of association in the genome, as well as newly discovered regulatory elements “that control cells and circumstances in which a gene is active.”

endergonic Referring to reactions and processes that have a positive G and thus require an input of free energy in order to proceed; opposite of exergonic.

endocrine Referring to signaling mechanism in which target cells bind and respond to a hormone released into the blood by distant specialized secretory cells usually present in a gland (e.g., pituitary or thyroid gland).

endocytic pathway Cellular pathway involving. receptor-mediated endocytocis that internalizes extracellular materials too large to be imported by membrane transport proteins and to remove receptor proteins from the cell surface as a way to down-regulate their activity. (Figure 14-29)

endocytosis General term for uptake of extracellular material by invagination of the plasma membrane; includes receptor-mediated endocytosis, phagocytosis, and pinocytosis.

endoplasmic reticulum (ER) Network of interconnected membranous structures within the cytoplasm of eukaryotic cells contiguous with the outer nuclear envelope. The rough ER, which is associated with ribosomes, functions in the synthesis and processing of secreted and membrane proteins; the smooth ER, which lacks ribosomes, functions in lipid synthesis. (Figure 1-12)

endosome One of two types of membrane-bounded compartments: early endosomes (or endocytic vesicles), which bud off from the plasma membrane during receptor-mediated endocytosis, and late endosomes, which have an acidic internal pH and function in sorting of proteins to lysosomes. (Figures 14-1 and 14-29)

endosymbiont Bacterium that resides inside a eukaryotic cell in a mutually beneficial partnership. According to the endosymbiont hypothesis, both mitochondria and chloroplasts evolved from endosymbionts. (Figure 12-7)

endothelium The thin layer of cells that lines the interior surface of blood and lymphatic vessels.

endothermic Referring to reactions and processes that have a positive change in enthalpy, △H, and thus must absorb heat in order to proceed; opposite of exothermic.

energy charge A measure of the fraction of total adenosine phosphates that have “high-energy” phosphoanhydride bonds, which is equal to ([ATP] + 0.5 [ADP])/([ATP] + [ADP] + [AMP]).

enhancer A regulatory sequence in eukaryotic DNA that may be located at a great distance from the gene it controls or even within the coding sequence. Binding of specific proteins to an enhancer modulates the rate of transcription of the associated gene. (Figure 9-23)

enhanceosome Large nucleoprotein complex that assembles from transcription factors (activators and repressors) as they bind cooperatively to their binding sites in an enhancer with the assistance of DNA-bending proteins. (Figure 9-34)

enthalpy (H) Heat; in a chemical reaction, the enthalpy of the reactants or products is equal to their total bond energies.

entropy (S) A measure of the degree of disorder or randomness in a system; the higher the entropy, the greater the disorder.

envelope See nuclear envelope or viral envelope.

enzyme A protein that catalyzes a particular chemical reaction involving a specific substrate or small number of related substrates.

epidermal growth factor (EGF) One of a family of secreted signaling proteins (the EGF family) that is used in the development of most tissues in most or all animals. EGF signals are bound by receptor tyrosine kinases. Mutations in EGF signal transduction components are implicated in human cancer, including brain cancer. See HER family.

epigenetic Referring to a process that affects the expression of specific genes and is inherited by daughter cells but does not involve a change in DNA sequence.

epinephrine A catecholamine secreted by the adrenal gland and some neurons in response to stress; also called adrenaline. It functions as both a hormone and neurotransmitter, mediating “fight or flight” responses (e.g., increased blood glucose levels and heart rate).

epithelial-to-mesenchymal transition (EMT) Describes a developmental program during which epithelial cells acquire the characteristics of mesenchymal cells. Cells lose adhesive properties and acquire motility.

epithelium (pl. epithelia) Sheet-like covering, composed of one or more layers of tightly adhering cells, on external and internal body surfaces. (Figure 20-10)

epitope The part of an antigen molecule that binds to an antigen-specific receptor on B or T cells or to antibody. Large protein antigens usually possess multiple epitopes that bind to antibodies of different specificity.

equilibrium constant (K_eq) Ratio of forward and reverse rate constants for a reaction. For a binding reaction, A + B ⇆ AB, the association constant (K_a) equals K, and the dissociation constant (K_d) equals 1/K.

erythropoietin (Epo) A cytokine that triggers production of red blood cells by inducing the proliferation and differentiation of erythroid progenitor cells in the bone marrow. (Figures 16-8 and 21-23)

E2F transcription factor complex Transcription factor that promotes the transcription of G₁/S phase cyclins and many other genes whose function is required for S phase.

euchromatin Less condensed portions of chromatin present in interphase chromosomes; includes most transcriptionally active regions. See also heterochromatin. (Figure 8-28a)

G-10

eukaryotes Class of organisms, composed of one or more cells containing a membrane-enclosed nucleus and organelles, that constitutes one of the three distinct evolutionary lineages of modern-day organisms; also called eukarya. Includes all organisms except viruses and prokaryotes. (Figure 1-1)

eukaryotic translation initiation factors (eIFs) Proteins required to initiate protein synthesis in eukaryotic cells. (Figure 5-24)

excision-repair system, DNA One of several mechanisms for repairing DNA damage due to spontaneous depurination or deamination or exposure to carcinogens. These repair systems normally operate with a high degree of fidelity and their loss is associated with increased risk for certain cancers.

excitatory synapse A synapse in which the neurotransmitter induces a depolarization of the postsynaptic cell, favoring generation of an action potential.

exergonic Referring to reactions and processes that have a negative △G and thus release free energy as they proceed; opposite of endergonic.

exocytosis Release of intracellular molecules (e.g., hormones, matrix proteins) contained within a membrane-bounded vesicle by fusion of the vesicle with the plasma membrane of a cell.

exon Segment of a eukaryotic gene (or of its primary transcript) that reaches the cytoplasm as part of a mature mRNA, rRNA, or tRNA molecule. See also intron.

exon-junction complex A protein complex (EJC) that assembles at exon-exon junctions following pre-mRNA splicing. EJCs stimulate nuclear export of fully processed nuclear mRNPs and participate in the process of nonsense-mediated decay of improperly processed mRNAs.

exon shuffling Evolutionary process for creating new genes (i.e., new combinations of exons) from preexisting ones by recombination between introns of two separate genes or by transposition of mobile DNA elements. (Figures 8-18 and 8-19)

exoplasmic face The face of a cell membrane directed away from the cytosol. (Figure 7-5)

exosome Large exonuclease-containing complex that degrades spliced-out introns and improperly processed pre-mRNAs in the nucleus or mRNAs with shortened poly(A) tails in the cytoplasm. (Figure 10-1)

exothermic Referring to reactions and processes that have a negative change in enthalpy, △H, and thus release heat as they proceed; opposite of endothermic.

expression vector A modified plasmid or virus that carries a gene or cDNA into a suitable host cell and there directs synthesis of the encoded protein; used to screen a DNA library for a gene of interest or to produce large amounts of a protein from its cloned gene (Figures 6-28 and 6-29)

extracellular matrix (ECM) A complex interdigitating meshwork of proteins and polysaccharides secreted by cells into the spaces between them. It provides structural support in tissues and can affect the development and biochemical functions of cells. (Table 20-1)