The Biology Place Glossary: G


	G protein	A GTP-binding protein that relays signals from a plasma-membrane signal receptor, known as a G-protein linked receptor, to other signal-transduction proteins inside the cell. When such a receptor is activated, it in turn activates the G protein , causing it to bind a molecule of GTP in place of GDP. Hydrolysis of the bound GTP to GDP inactivates the G protein.
	G-protein linked receptor	A signal receptor protein in the plasma membrane that responds to the binding of a signal molecule by activating a G protein.
	G₁ phase	The first growth phase of the cell cycle, consisting of the portion of interphase before DNA synthesis begins.
	G₂ phase	The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs.
	gametangium pl. gametangia (gam-eh-tanj-ee-um) [Gk. gamein, to marry + L. tangere, to touch]	The reproductive organ of bryophytes, consisting of the male antheridium and female archegonium; a multichambered jacket of sterile cells in which gametes are formed.
	gamete (gam-eet) [Gk., wife]	A haploid egg or sperm cell; gametes unite during sexual reproduction to produce a diploid zygote.
	gametophyte (guh-mee-toh-fite)	The multicellular haploid form in organisms undergoing alternation of generations, which mitotically produces haploid gametes that unite and grow into the sporophyte generation.
	ganglion (gang-lee-un) pl. ganglia (gang-lee-on) [Gk. ganglion, a swelling]	A cluster (functional group) of nerve cell bodies in a centralized nervous system.
	gap junction	A type of intercellular junction in animal cells that allows the passage of material or current between cells.
	gap phases	In the cell cycle, the phases that precede (G₁) and follow (G₂) the synthesis (S) phase in which DNA is replicated; in the G₁ phase, the cell doubles in size, and its enzymes, ribosomes, and other cytoplasmic molecules and structures increase in number; in the G₂ phase, the replicated chromosomes begin to condense and the structures required for mitosis or meiosis are assembled.
	gastric [Gk. gaster, stomach]	Pertaining to the stomach.
	gastrin	A digestive hormone, secreted by the stomach, that stimulates the secretion of gastric juice.
	gastrovascular cavity [Gk. gaster, stomach + L. vasculum, a small vessel]	The central digestive compartment, usually with a single opening that functions as both mouth and anus.
	gastrula (gas-troo-la) [Gk. gaster, stomach]	The two-layered, cup-shaped embryonic stage.
	gastrulation (gas-truh-lay-shun)	The formation of a gastrula from a blastula.
	gated ion channel	A specific ion channel that opens and closes to allow the cell to alter its membrane potential.
	Gause's principle	See Competitive exclusion.
	gel electrophoresis (jell eh-lek-troh-for-ee-sis)	The separation of nucleic acids or proteins, on the basis of their size and electrical charge, by measuring their rate of movement through an electrical field in a gel.
	gene [Gk. genos, birth, race; L. genus, birth, race, origin]	A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).
	gene amplification	The selective synthesis of DNA, which results in multiple copies of a single gene, thereby enhancing expression.
	gene cloning	The production of multiple copies of a gene.
	gene flow	The loss or gain of alleles from a population due to the emigration or immigration of fertile individuals, or the transfer of gametes, between populations.
	gene pool	The total aggregate of genes in a population at any one time.
	genetic code	The system of nucleotide triplets in DNA and RNA that carries genetic information; referred to as a code because it determines the amino acid sequence in the enzymes and other protein molecules synthesized by the organism.
	genetic drift	Changes in the gene pool of a small population due to chance.
	genetic isolation	The absence of genetic exchange between populations or species as a result of geographic separation or of premating or postmating mechanisms (behavioral, anatomical, or physiological) that prevent reproduction.
	genetic map	An ordered list of genetic loci (genes or other genetic markers) along a chromosome.
	genetic recombination	The general term for the production of offspring that combine traits of the two parents.
	genome (jee-nome)	The complete complement of an organism's genes; an organism's genetic material.
	genomic imprinting	The parental effect on gene expression. Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
	genomic library	A set of thousands of DNA segments from a genome, each carried by a plasmid, phage, or other cloning vector.
	genotype (jee-noh-type)	The genetic makeup of an organism.
	genus pl. genera (jee-nus) [L. genus, race, origin]	A taxonomic category above the species level, designated by the first word of a species' binomial Latin name.
	geographical range	The geographic area in which a population lives.
	geological time scale	A time scale established by geologists that reflects a consistent sequence of historical periods, grouped into four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
	germ cells [L. germinare, to bud]	Gametes or the cells that give rise to gametes
	germination [L. germinare, to bud]	In plants, the resumption of growth or the development from seed or spore.
	gibberellin (jib-ur-el-in) [Fr. gibberella, genus of fungi]	A class of related plant hormones that stimulate growth in the stem and leaves, trigger the germination of seeds and breaking of bud dormancy, and stimulate fruit development with auxin.
	gill	A localized extension of the body surface of many aquatic animals, specialized for gas exchange.
	gland [L. glans, glandis, acorn]	A structure composed of modified epithelial cells specialized to produce one or more secretions that are discharged to the outside of the gland.
	glial cell (glee-ul) [Gk. glia, glue + L. cella, a chamber]	A nonconducting cell of the nervous system that provides support, insulation, and protection for the neurons.
	globular protein [L. dim. of globus, a ball]	A polypeptide chain folded into a roughly spherical shape.
	glomerulus (gluh-mare-yoo-lus) [L. glomus, ball]	A ball of capillaries surrounded by Bowman's capsule in the nephron and serving as the site of filtration in the vertebrate kidney.
	glucagon [Gk. glykys, sweet + ago, to lead toward]	A peptide hormone secreted by pancreatic endocrine cells that raises blood glucose levels; an antagonistic hormone to insulin.
	glucocorticoid	A corticosteroid hormone secreted by the adrenal cortex that influences glucose metabolism and immune function.
	glucose [Gk. glykys, sweet]	A six-carbon sugar (C₆H₁₂O₆); the most common monosaccharide in animals.
	glycerol	A three-carbon molecule with three hydroxyl (OH) groups attached; a glycerol molecule can combine with three fatty acid molecules to form a fat or an oil.
	glycocalyx (gly-koh-kay-liks)	A fuzzy coat on the outside of animal cells, made of sticky oligosaccharides.
	glycogen (gly-koh-jen)[Gk. glykys, sweet + genos, race or descent]	An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.
	glycolipids [Gk. glykys, sweet + lipos, fat]	Organic molecules similar in structure to fats, but in which a short carbohydrate chain rather than a fatty acid is attached to the third carbon of the glycerol molecule; as a result, the molecule has a hydrophilic "head" and a hydrophobic "tail." Glycolipids are important constituents of the plasma membrane and of organelle membranes.
	glycolysis (gly-kol-eh-sis) [Gk. glykys, sweet + lysis, loosening]	The splitting of glucose into pyruvate. Glycolysis is the one metabolic pathway that occurs in all living cells, serving as the starting point for fermentation or aerobic respiration.
	glycoprotein [Gk. glykys, sweet + proteios, primary]	A protein with covalently attached carbohydrate.
	Golgi apparatus (goal-jee)	An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum.
	gonadotropins (goh-nad-oh-troh-pinz) [Gk. gone, seed + trope, a turning]	Hormones that stimulate the activities of the testes and ovaries; a collective term for follicle-stimulating and luteinizing hormones.
	gonads [Gk. gone, seed]	The male and female sex organs; the gamete-producing organs in most animals.
	graded potential	A local voltage change in a neuron membrane induced by stimulation of a neuron, with strength proportional to the strength of the stimulus and lasting about a millisecond.
	gradualism	A view of Earth's history that attributes profound change to the cumulative product of slow but continuous processes.
	Gram stain	A staining method that distinguishes between two different kinds of bacterial cell walls.
	granum pl. grana (gran-um) [L., grain or seed]	A stacked portion of the thylakoid membrane in the chloroplast. Grana function in the light reactions of photosynthesis.
	gravitropism (grav-eh-troh-piz-um) [L. gravis, heavy + Gk. trope, turning]	A response of a plant or animal in relation to gravity.
	greenhouse effect	The warming of planet Earth due to the atmospheric accumulation of carbon dioxide, which absorbs infrared radiation and slows its escape from the irradiated Earth.
	gross primary productivity (GPP)	The total primary productivity of an ecosystem.
	gross productivity	A measure of the rate at which energy is assimilated by the organisms in a trophic level, a community, or an ecosystem.
ground meristem [Gk. merizein, to divide]	A primary meristem that gives rise to ground tissue in plants.
ground tissue system	A tissue of mostly parenchyma cells that makes up the bulk of a young plant and fills the space between the dermal and vascular tissue systems.
growth factor	A protein that must be present in the extracellular environment (culture medium or animal body) for the growth and normal development of certain types of cells.
guard cell	A specialized epidermal plant cell that forms the boundaries of the stomata.
guttation	The exudation of water droplets caused by root pressure in certain plants.
gymnosperm (jim-noh-spurm) [Gk. gymnos, naked + sperma, seed]	A vascular plant that bears naked seeds not enclosed in any specialized chambers.
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	Glossary from Biology, Fifth Edition By Campbell, Reece, Mitchell © 1998, Benjamin Cummings. Reprinted with Permission. All Rights Reserved