epidermis Epithelial layer covering the outer surface of the body. Has different structures in different animal groups. The outer layer of plant tissue is also called the epidermis.
epigenetic inheritance Inheritance of phenotypic changes in a cell or organism that do not result from changes in the nucleotide sequence of DNA. Can be due to positive feedback loops of transcription regulators or to heritable modifications in chromatin such as DNA methylation or histone modifications. (Figure 7–53) epistasis analysis   Analysis to discover the order in which the genes act, by investigating if a mutation in one gene can mask the effect of a mutation in another gene when both mutations are present in the same organism or cell.
epithelial tissues   Tissues, such as the lining of the gut or the epidermal covering of the skin, in which cells are closely bound together into sheets called epithelia.
epithelium (plural epithelia) Sheet of cells covering the outer surface of a structure or lining a cavity.
equilibrium   State in a chemical reaction where there is no net change in free energy to drive the reaction in either direction. The ratio of product to substrate reaches a constant value at chemical equilibrium. (Figure 2–30) equilibrium constant (K) The ratio of forward and reverse rate constants for a reaction. Equal to the association or affinity constant (Ka) for a simple binding reaction (A + B
AB). See also affinity constant, dissociation constant. (See page 62)
ER lumen Space enclosed by the membrane of the endoplasmic reticulum (ER).
ER resident protein   Protein that remains in the endoplasmic reticulum (ER) or its membranes and carries out its function there, as opposed to proteins that are present in the ER only in transit.
ER retention signal   Short amino acid sequence on a protein that prevents it from moving out of the endoplasmic reticulum (ER). Found on proteins that are resident in the ER and function there.
ER signal sequence   N-terminal signal sequence that directs proteins to enter the endoplasmic reticulum (ER). Cleaved off by signal peptidase after entry.
ER tail-anchored proteins   Membrane proteins anchored in the endoplasmic reticulum (ER) membrane by a single transmembrane α helix contained at their C-terminus.
erythrocyte Small hemoglobin-containing blood cell of vertebrates that transports oxygen to, and carbon dioxide from, tissues. Also called a red blood cell.
erythropoietin   A hormone produced by the kidney that stimulates the production of red blood cells in bone marrow.
ESCRT protein complexes   Four protein complexes (ESCRT-0, ESCRT-1, ESCRT-2, and ESCRT-3) that act sequentially to shepherd mono-ubiquitylated membrane proteins on endosomal membranes into intralumenal vesicles.  ESCRT-3 complex catalyzes the pinching-off reaction.
ethylene   Small gas molecule that is a plant growth regulator influencing plant development in various ways including promoting fruit ripening, leaf abscission, and plant senescence and functioning as a stress signal in response to wounding, infection, and flooding.
euchromatin   Region of an interphase chromosome that stains diffusely; “normal” chromatin, as opposed to the more condensed heterochromatin.
eukaryote   Organism composed of one or more cells that have a distinct nucleus. Member of one of the three main divisions of the living world, the other two being bacteria and archaea. (Figure 1–17) eukaryotic initiation factor (eIF) Protein that helps load initiator tRNA on to the ribosome, thus initiating translation.
excitatory neurotransmitter   Neurotransmitter that opens cation channels in the postsynaptic membrane, causing an influx of Na+, and in many cases Ca2+, that depolarizes the postsynaptic membrane toward the threshold potential for firing an action potential.
executioner caspases Apoptotic caspases that catalyze the widespread cleavage events during apoptosis that kill the cell.