group theory A branch of mathematics concerned
with the study of groups.
growth factor A complex family of organic chemi-
cals, especially polypeptides, that bind to cell surface
receptors and act to control new cell division, growth,
and maintenance by bone marrow.
Synthetic growth factors are being used to stimu-
late normal white blood cell production following can-
cer treatments and bone marrow transplants.
Examples of growth factors are: insulin, insulinlike
growth factors (IGF), and GF1 and II, which are
polypeptides similar to insulin; somatomedin; polypep-
tides made by the liver and fibroblasts that, when
released into the blood (stimulated by somatotropin),
help cell division and growth by incorporating sulfates
into collagen, RNA, and DNA synthesis; HGH, human
growth hormone, also called somatotropin, a protein-
like hormone from the pituitary gland that stimulates
the liver to produce somatomedins that stimulate
growth of bone and muscle; platelet-derived growth
factor (PDGF), a glycoprotein that stimulates cell pro-
liferation and chemotaxis in cartilage, bone, and other
cell types; fibroblast growth factor, which promotes the
proliferation of cells of mesodermal, neuroectodermal,
epithelial, or endothelial origin; epidermal growth fac-
tor (EGF), important for cell development as it binds to
receptors on cell surfaces to create a growth signal; and
granulocyte colony-stimulating factor (G-CSF), a
growth factor that promotes production of granulo-
cytes, a type of white blood cell.
Grunwald-Winstein equation The LINEAR FREE-
ENERGY RELATION
log(ks/k 0 ) = mYexpressing the dependence of the rate of SOLVOLYSISof
a substrate on ionizing power of the solvent. The rate
constant k 0 applies to the reference solvent (ethanol-
water, 80:20, v/v) and ksto the solvent s, both at 25°C.
The parameter mis characteristic of the substrate and
is assigned the value unity for tert-butyl chloride. The
value Yis intended to be a quantitative measure of the
IONIZING POWERof the solvent s. The equation was
later extended by Winstein, Grunwald, and Jones
(1951) to the form
log(ks/k 0 ) = mY+ lN
where Nis the NUCLEOPHILICITYof the solvent and l its
susceptibility parameter. The equation has also been
applied to reactions other than solvolysis.
See also DIMROTH-REICHARDT ET PARAMETER;
POLARITY; Z-VALUE.guanylate cyclase An ENZYME catalyzing the con-
version of guanosine 5′-triphosphate to cyclicguanosine
3 ′,5′-monophosphate, which is involved in cellular
REGULATIONprocesses. One member of this class is a
HEME-containing enzyme involved in processes regu-
lated by nitrogen monoxide.guest An organic or inorganic ion or molecule that
occupies a cavity, cleft, or pocket within the molecular
structure of A HOST MOLECULAR ENTITYand forms a
COMPLEXwith it or that is trapped in a cavity within
the crystal structure of a host.
See also CROWN; CRYPTAND; INCLUSION COM-
POUND.gypsum A soft, transparent, mineral composed of
hydrated calcium sulfate. Burnt gypsum commonly
used by artists is known as plaster of paris.120 group theory