ectoderm Embryonic tissue that is the precursor of the epidermis and nervous
system. effector cell A cell that carries out the final response or function of a particular
process. The main effector cells of the immune system, for example, are
activated lymphocytes and phagocytes the cells involved in destroying
pathogens and removing them from the body. egg The mature female gamete in sexually reproducing organisms. It is usually
a large and immobile cell. elastin Hydrophobic protein that forms extracellular extensible fibres (elastic
fibres) that give tissues their stretchability and resilience. electrochemical gradient The combined influence of a difference in the
concentration of an ion on the two sides of the membrane and the electrical
charge difference across the membrane (membrane potential). It produces a
driving force that causes the ion to move across the membrane. electrochemical proton gradient The result of a combined pH gradient
(proton gradient) and the membrane potential. electron Negatively charged subatomic particle that generally occupies
orbitals surrounding the nucleus in an atom. electron acceptor Atom or molecule that takes up electrons readily, thereby
gaining an electron and becoming reduced. electron carrier Molecule such as cytochrome c, which transfers an electron
from a donor molecule to an acceptor molecule. electron donor Molecule that easily gives up an electron, becoming oxidized
in the process. electron microscope Type of microscope that uses a beam of electrons to
create the image. electron-transport chain Series of electron carrier molecules along which
electrons move from a higher to a lower energy level to a final acceptor
molecule. The energy released during electron movement can be used to power
various processes. Electron-transport chains present in the inner mitochondrial
membrane and in the thylakoid membrane of chloroplasts generate a proton
gradient across the membrane that is used to drive ATP synthesis. elongation factor Protein required for the addition of amino acids to growing
polypeptide chains on ribosomes. embryogenesis Development of an embryo from a fertilized egg, or zygote. embryonic stem cell (ES cell) Cell derived from the inner cell mass of the
early mammalian embryo that can give rise to all the cells in the body. It can
be grown in culture, genetically modified and inserted into a blastocyst to
develop a transgenic animal. endocrine cell Specialized animal cell that secretes a hormone into the blood.
Usually part of a gland, such as the thyroid or pituitary gland. endocytic-exocytic cycle The processes of endocytosis and exocytosis that,
respectively, add and remove plasma membrane from the cell, resulting in no
overall change in the cell's surface area and volume. endocytosis Uptake of material into a cell by an invagination of the plasma
membrane and its internalization in a membrane-bounded vesicle. (See also pinocytosis and phagocytosis.) endoderm Embryonic tissue that is the precursor of the gut and associated
organs. endoplasmic reticulum (ER) Labyrinthine membrane-bounded compartment
in the cytoplasm of eucaryotic cells, where lipids are synthesized and
membrane-bound proteins and secretory proteins are made. endosome Membrane-bounded organelle in animal cells that carries materials
newly ingested by endocytosis and passes many of them on to lysosomes for
degradation. endothelial cell Flattened cell type that forms a sheet (the endothelium) lining
all blood vessels. enhancer Regulatory DNA sequence to which gene regulatory proteins bind,
influencing the rate of transcription of a structural gene that can be many
thousands of base pairs away. entropy Thermodynamic quantity that measures the degree of disorder in a
system; the higher the entropy, the greater the disorder. enveloped virus Virus with a capsid surrounded by a lipid membrane (the
envelope), which is derived from the host cell plasma membrane when the
virus buds from the cell. enzyme Protein that catalyzes a specific chemical reaction. enzyme-linked receptor Major type of cell-surface receptor in which the
cytoplasmic domain either has enzymatic activity itself or is associated with an
intracellular enzyme. In both cases enzymatic activity is stimulated by ligand
binding to the receptor. epidermis Epithelial layer covering the outer surface of the body. It has
different structures in different animal groups. The outer layer of plant tissue is
also called the epidermis. epimerization Reaction that alters the steric arrangement around one atom, as
in a sugar molecule. epinephrine see adrenaline epithelial tissue see epithelium epithelium (epithelia) Coherent cell sheet formed from one or more layers of
cells covering an external surface or lining a cavity. epitope see antigenic determinant equilibrium constant (K) Ratio of forward and reverse rate constants for a
reaction and equal to the association constant. (3 44.) equilibrium State where there is no net change in a system. For example,
equilibrium is reached in a chemical reaction when the forward and reverse
rates are equal. ER lumen The 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
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). It is cleaved off by signal peptidase after
entry. ER see endoplasmic reticulum erythrocyte (red blood cell) Small, hemoglobin-containing blood cell of
vertebrates that transports oxygen and carbon dioxide to and from tissues. erythropoietin Growth factor that stimulates the production of red blood cells.
It is produced by the kidney and acts on precursor cells in bone marrow. ES cell see embryonic stem cell Escherichia coli (E. coli) Rodlike bacterium normally found in the colon of
humans and other mammals and widely used in biomedical research. ester Molecule formed by the condensation reaction of an alcohol group with
an acidic group. Phosphate groups usually form esters when linked to a second
molecule. (See Panel 2 1, 110 111.) ethyl ( CH2CH3) Hydrophobic chemical group derived from ethane (CH3CH3). eucaryote (eukaryote) Organism composed of one or more cells with a
distinct nucleus and cytoplasm. Includes all forms of life except viruses and
procaryotes (bacteria and archea). euchromatin Region of an interphase chromosome that stains diffusely;
"normal" chromatin, as opposed to the more condensed heterochromatin. exocytosis Process by which most molecules are secreted from a eucaryotic
cell. These molecules are packaged in membrane-bounded vesicles that fuse
with the plasma membrane, releasing their contents to the outside. exon Segment of a eucaryotic gene that consists of a sequence of nucleotides
that will be represented in messenger RNA or the final transfer RNA or
ribosomal RNA. In protein-coding genes, exons encode amino acids in the
protein. An exon is usually adjacent to a noncoding DNA segment called an
intron. expression vector A virus or plasmid that carries a DNA sequence into a
suitable host cell and there directs the synthesis of the protein encoded by the
sequence. expression Production of an observable phenotype by a gene usually by
directing the synthesis of a protein. extracellular matrix Complex network of polysaccharides (such as
glycosaminoglycans or cellulose) and proteins (such as collagen) secreted by
cells. Serves as a structural element in tissues and also influences their
development and physiology.
