located in the cytoplasm of the cell, for various parts of
the cell. Named after the Nobel Prize recipient, Italian
neurologist and histologist Camillo Golgi (1843–1926).
They are the formation site of the carbohydrate side
chains of GLYCOPROTEINSand mucopolysaccharides. The
vacuoles release these by migrating through the cell mem-
brane. Some of the vesicles send molecules to the cellular
membrane, where they are excreted, and some are used
for selective excretion.
gonadotropins Agroup of hormones that are pro-
duced in the pituitary gland and regulate the develop-
ment and function of the testis and ovary. The group
includes the follicle-stimulating hormone (FSH), which
promotes male and female gamete formation, and
luteinizing hormone (LH), which stimulates the secre-
tion of the male and female testosterone and estrogen.
Gonadotropin itself is controlled by the gonadotropin
releasing hormone (GnRH), a hormone that controls
the production and release of gonadotropins and is
secreted by the hypothalamus every 90 minutes or so,
which enables the pituitary to secrete LH and FSH.
gonads The male and female sex organs. In the male,
they are glands located inside the scrotum, behind and
below the penis, which produce sperm and are the pri-
mary source of testosterone. Also called the testes. In
the female, they are ovaries, two almond-sized glands
located on either side of the uterus. They produce and
store the oocytes and the female sex hormones estrogen
and progesterone.
Gondwanaland (Gondwana) The large southern
protocontinent, derived from the supercontinent Pan-
gaea, that, when fragmented, formed Africa, South
America, Antarctica, Australia, and India during the
Paleozoic era more than 200 million years ago as an
event of plate tectonics (continental drift).
G protein A class of heterotrimeric proteins important
in signaling pathways in the plasma membrane in mam-
malian cells. Regulated by the guanine nucleotides GDP
(guanosine diphosphate) and GTP (guanosine triphos-
phate), they participate in cell signal pathways by usually
binding a hormone or signal ligand to a seven-pass trans-
membrane receptor protein; activates intracellular mes-
senger systems when the signaling molecule (typically a
hormone) binds to the transmembrane receptor. The pro-
teins are located on the inner surface of the plasma mem-
brane and transmit signals from outside the membrane,
via transmembrane receptors, to adenylate cyclase, which
then catalyzes the formation of the second messenger,
cyclic adenosine monophosphate (AMP), inside the cell.graded potential A nerve impulse that is initially
proportional to the intensity of the stimulus that pro-
duces it, then declines in intensity; membrane potentials
that vary in magnitude.gradualism An Earth model of evolution that
assumes a slow, steady rate of change, with slow steps
instead of quick leaps, and with new variation arising
by mutation and recombination. A view held by
Charles Darwin.grafting The process of attaching two different plant
parts, usually shoots, to each other to create a single
new plant. Used in propagating trees and shrubs.Gram staining An important laboratory technique
to distinguish between two major bacterial groups,
based on stain retention by their cell walls. Bacteria
smears are fixed by flaming, then stained with
crystal violet followed by iodine solution, and then
rinsed with alcohol or acetone, decolorized, and coun-
terstained with safranin. Gram-positive bacteria are
stained bright purple or purple-black, while gram-nega-
tive bacteria are pink. This staining technique is useful
in bacterial taxonomy and identification and in indicat-
ing fundamental differences in cell-wall structure.
Gram-negative bacteria lack peptidoglycan in the cell
wall, while gram-positive bacteria have about 90 per-
cent of their cell wall composed of peptidoglycan.
See alsoBACTERIA.granum A series of disk- or saclike structures called
thaylakoid disks—specialized membrane structuresgranum 151