subunit Anindividual polypeptide chain in a pro-
tein containing more than one polypeptide chain. Dif-
ferent types of subunits are frequently designated by
α, ß, γ, etc.
succession The process whereby species are replaced
over time in an ecological community. Primary succes-
sion is when new communities develop on newly
exposed habitat that has no previous life forms, such as
on bare rock. Secondary succession is when communi-
ties take over an area that previously supported life,
such as a recently burned forest or abandoned agricul-
tural land. Succession usually occurs in one direction
and often can be predicted. However, predictable suc-
cession can be altered, such as in fire disclimax commu-
nities like pine barrens, where fire periodically burns
out species that would, if left unchecked, succeed to a
new community type.
succulent A xeric-adapted plant that has fleshy
stems, roots, or leaves that store water, thus preventing
desiccation in harsh, dry environments, e.g., cactus,
aloe.
sugar An organic compound that has the general
chemical formula (CH 2 O)n. All carbohydrates are
sugars.
sulfite reductase ENZYMEs that catalyze the reduc-
tion of sulfite to sulfide. All known enzymes of this
type contain SIROHEMEand IRON-SULFUR CLUSTERs.
superhyperfine SeeELECTRON PARAMAGNETIC RESO-
NANCE SPECTROSCOPY.
superior ovary (plants) SeeHYPOGYNOUS.
superoxide dismutases(SOD) ENZYMEs that cat-
alyze the dismutation reaction of superoxide anion to
dihydrogen peroxide and dioxygen. The enzymes have
ACTIVE SITEs containing either copper and zinc (Cu/Zn-
superoxide dismutase), or iron (Fe-superoxide dismu-
tase), or manganese (Mn-superoxide dismutase).
See alsoDISPROPORTIONATION.suppressor T cell(T8 cell; CD8 cell) The existence
of these cells is a relatively recent discovery, and their
function is still somewhat debated. The basic concept
of suppressor T cells is a cell type that specifically sup-
presses the action of other cells in the immune system,
notably B cells and T cells, thereby preventing the
establishment of an immune response. How this is
done is not known with certainty, but it seems that cer-
tain specific antigens can stimulate the activation of the
suppressor T cells. This suppressor effect is thought to
be mediated by some inhibitory factor secreted by sup-
pressor T cells. The factor is not any of the known lym-
phokines. Another fact that renders the study of this
cell type difficult is the lack of a specific surface mark-
er. Most suppressor T cells are CD8-positive, as are
cytotoxic T cells.supramolecular chemistry This is defined as the
chemistry of molecular assemblies and of the inter-
molecular bond—“chemistry beyond the molecule”—
bearing on the organized entities of higher complexity
that result from the association of two or more chemi-
cal species held together by intermolecular forces.
Thus, supramolecular chemistry can be considered to
represent a generalized COORDINATION chemistry
extending beyond the coordination of TRANSITION ELE-
MENTs by organic and inorganic LIGANDs to the bond-
ing of all kinds of SUBSTRATEs: cationic, anionic, and
neutral species of either inorganic, organic, or biologi-
cal nature.surface tension The cohesive forces between liquid
molecules. Surface tension is typically measured in
dyne/cm, the force in dynes required to break a film of
length 1 cm. Water at 20°C has a surface tension of
72.8 dyne/cm. Surfactants act to reduce the surface ten-
sion of a liquid.survivorship curve Summarizes the pattern of sur-
vival in a population. There are basically three types ofsurvivorship curve 317