Gaia A hypothetical superorganism composed of the Earth's four spheres: the biosphere, hydrosphere, lithosphere, and atmosphere.
gametes Haploid reproductive cells (ovum and sperm). PICTURE
gametophyte The haploid stage of a plant exhibiting alternation of generations, generates gametes by the process of mitosis.
ganglia Clusters of neurons that receive and process signals; found in þatworms and earthworms. PICTURE
gap junctions Junctions between the plasma membranes of animal cells that allow communication between the cytoplasm of adjacent cells.
gastric pits The folds and grooves into which the stomach lining is arranged.
gastrin A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric acids.
gastroesophageal sphincter A ring of muscle at the junction of the esophagus and the stomach that remains closed except during swallowing to prevent the stomach contents from entering the esophagus.
gene pool The sum of all the genetic information carried by members of a population. Note: there is no diving in the deep end of the gene pool!
genera Taxonomic subcategories within families (sing.: genus), composed of one or more species.
genes SpeciÞc segments of DNA that control cell structure and function; the functional units of inheritance. Sequence of DNA bases usually code for a polypeptide sequence of amino acids.
gene therapy The insertion of normal or genetically altered genes into cells through the use of recombinant DNA technology; usually done to replace defective genes as part of the treatment of genetic disorders.
genetic code The linear series of nucleotides, read as triplets, that speciÞes the sequence of amino acids in proteins. Each triplet speciÞes an amino acid, and the same codons are used for the same amino acids in almost all life-forms, an indication of the universal nature of the code. PICTURE
genetic divergence The separation of a population's gene pool from the gene pools of other populations due to mutation, genetic drift, and selection. Continued divergence can lead to speciation.
genetic drift Random changes in the frequency of alleles from generation to generation; especially in small populations, can lead to the elimination of a particular allele by chance alone.
genetic maps Diagrams showing the order of and distance between genes; constructed using crossover information.
genetics The study of the structure and function of genes and the transmission of genes from parents to offspring.
genital herpes A sexually transmitted disease caused by the herpes virus; results in sores on the mucus membranes of the mouth or genitals.
genome 1. The set of genes carried by an individual. 2. The set of genes shared by members of a reproductive unit such as a population or species.
genotype The genetic (alleleic) makeup of an organism with regard to an observed trait.
geographic isolation Separation of populations of a species by geographic means (distance, mountains, rivers, oceans, etc.) that lead to reproductive isolation of those populations.
geographic range The total area occupied by a population.
geological time The span of time that has passed since the formation of the Earth and its physical structures; also, a timescale that focuses on events on the order of thousands of years or more. PICTURE
geotropism Plants' response to gravity: roots grow downward, showing positive geotropism, while shoots grow upward in a negative response. PICTURE
germ cells Collective term for cells in the reproductive organs of multicellular organisms that divide by meiosis to produce gametes.
gestation Period of time between fertilization and birth of an animal. Commonly called pregnancy.
gibberellins A group of hormones that stimulate cell division and elongation in plants. Gibberellic acid (GA), the first of this class to be discovered, causes bolting (extreme elongation) of stems. GA is also applied to certain plants to promote larger fruits.
gill slits Opening or clefts between the gill arches in Þsh. Water taken in by the mouth passes through the gill slits and bathes the gills. Also, rudimentary grooves in the neck region of embryos of air-breathing vertebrates such as humans; a characteristic of chordates.
ginkgos Group of seed plants today restricted to a single genus (Ginkgo biloba); ginkgos were more diverse during the Mesozoic Era.
glial cells Nonconducting cells that serve as support cells in the nervous system and help to protect neurons.
glomerulus A tangle of capillaries that makes up part of the nephron; the site of Þltration.
glucagon A hormone released by the pancreas that stimulates the breakdown of glycogen and the release of glucose, thereby increasing blood levels of glucose. Glucagon and insulin work together to maintain blood sugar levels.
glucocorticoids A group of steroid hormones produced by the adrenal cortex that are important in regulating the metabolism of carbohydrates, fats, and proteins.
glucose A six-carbon single sugar; the most common energy source. PICTURE
glycogen Polysaccharide consisting of numerous monosaccharide glucoses linked together. The animal equivalent of starch.
glycolipids Polysaccharides formed of sugars linked to lipids, a part of the cell membrane. PICTURE
glycolysis The universal cellular metabolic process in the cell's cytoplasm where 6-carbon glucose is split into two 3-carbon pyruvate molecules, and some ATP and NADH are produced. Click here to view the On-Line Biology Book chapter on glycolysis.
glycoproteins Polysaccharides formed of sugars linked to proteins. On the outer surface of a membrane, they act as receptors for molecular signals originating outside the cell. PICTURE
gnetales Group of seed plants restricted to three genera today (Gnetum, Ephedra, and Welwitschia); the possible outgroup for flowering plants.
golden brown algae Common name applied to the protistan division Chrysophyta.
Golgi complex Organelles in animal cells composed of a series of þattened sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum. PICTURE
gonadotropin-releasing hormone (GnRH) A hormone produced by the hypothalamus that controls the secretion of luteinizing hormone.
gonadotropins Hormones produced by the anterior pituitary that affect the testis and ovary; include follicle-stimulating hormone and luteinizing hormone.
gonads The male and female sex organs.
Gondwana Name applied to the ancient (Paleozoic-early Mesozoic) southern hemisphere supercontinent that rifted apart to form present-day Antarctica, India, Africa, Australia, and South America. The southern part of Pangaea.
gonorrhea A sexually transmitted disease that is caused by a bacterium that inþames and damages epithelial cells of the reproductive system.
grana A series of stacked thylakoid disks containing chlorophyll; found in the inner membrane of chloroplasts. PICTURE
grasslands biome Occurs in temperate and tropical regions with reduced rainfall or prolonged dry seasons; characterized by deep, rich soil, an absence of trees, and large herds of grazing animals.
green algae Common name for algae placed in the division Chlorophyta. PICTURE
greenhouse effect The heating that occurs when gases such as carbon dioxide trap heat escaping from the Earth and radiate it back to the surface; so-called because the gases are transparent to sunlight but not to heat and thus act like the glass in a greenhouse.
ground system Plant tissue system, composed mainly of parenchyma cells with some collenchyma and sclerenchyma cells, that occupies the space between the epidermis and the vascular system; is involved in photosynthesis, water and food storage, and support; one of the four main tissue systems in plants.
growth hormone (GH) A peptide hormone produced by the anterior pituitary that is essential for growth.
growth rings Features of woody stems produced by plants growing in areas with seasonal (as opposed to year-long) growth. The growth ring marks the position of the vascular cambium at the cessation of the previous year's growth. PICTURE
guard cells Specialized epidermal cells that flank stomates and whose opening and closing regulates gas exchange and water loss. PICTURE 1 | PICTURE 2
guanine One of the nitrogenous bases in nucleic acids, guanine is one of the two purine bases. PICTURE
gymnosperms Flowerless, seed-bearing land plants; the Þrst seed plants; living groups include the pines, ginkgos, and cycads. Naked seeds.
gynoecium Collective term for all of the carpels (or pistils) in a flower. Some flowers have many pistils that are partially or wholly fused. PICTURE
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