secondary compound Compounds that are not
directly a function in the process of growth and devel-
opment in a plant but are normal parts of a plant’s
METABOLISM. They often serve to discourage consump-
tion by making it taste bad or having a toxic effect on
the consumer. Examples of secondary compounds are
nicotine and caffeine.
secondary kinetic electrolyte effect See KINETIC
ELECTROLYTE EFFECT.
secondary kinetic isotope effect See ISOTOPE
EFFECT, KINETIC.
secondary productivity The rate of new biomass
production that is nutrient material synthesized by con-
sumers over a specific time frame in an ecosystem.
secondary standard Standard prepared by dilution
of an aliquot of a PRIMARY STANDARDsolution with a
known volume of solvent, or by subsequent serial dilu-
tions; or a standard solution measured by reference to
a primary standard solution.
secondary structure Level of structural organization
in proteins described by the folding of the polypeptide
chain into structural MOTIFs such as ALPHA HELICES
and BETA SHEETs, which involve hydrogen bonding of
backbone atoms. Secondary structure is also formed in
NUCLEIC ACIDs, especially in single-stranded RNAs by
internal BASE PAIRING.
See alsoPRIMARY STRUCTURE.
secondary voltaic cell VOLTAIC CELL that has the
ability to be recharged.
See alsoPRIMARY VOLTAIC CELL.
second law of thermodynamics States that
ENTROPY, a measure of disorder, increases in the uni-
verse and is spontaneous. Elements in a closed system
will tend to seek their most probable distribution, and
entropy always increases; it is a measure of unusable
energy and a gauge of randomness or chaos within a
closed system. Also called the law of increased entropy.
Along with the FIRST LAW OF THERMODYNAMICS, these
two laws serve as the fundamental principles of
physics.second messenger An intracellular METABOLITE or
ion increasing or decreasing as a response to the stimu-
lation of RECEPTORs by AGONISTs, which are consid-
ered the “first messenger.” This generic term usually
does not prejudge the rank order of intracellular bio-
chemical events.second-order reaction A reaction where two reac-
tant molecules come together for the reaction to occur.
The reaction rate is proportional to the square of the
substrate concentration or to the first power of each of
two reactants.
See alsoRATE OF REACTION.selective permeability Refers to the control a cell
membrane has over what can cross, deciding which
specific molecules may enter or leave the cell by using
either PASSIVEor ACTIVE TRANSPORT, or by way of a
vesicle.selectivity The discrimination shown by a reagent in
competitive attack on two or more SUBSTRATEs or on
two or more positions in the same substrate. It is quan-
titatively expressed by ratios of RATE CONSTANTs of the
competing reactions or by the decadic logarithms of
such ratios.
See alsoISOSELECTIVE RELATIONSHIP; PARTIAL RATE
FACTOR; REGIOSELECTIVITY; SELECTIVITY FACTOR;
STEREOSELECTIVITY.selectivity factor A quantitative representation of
SELECTIVITY in AROMATIC SUBSTITUTION REACTIONs
(usually ELECTROPHILIC for monosubstituted benzene
derivatives). If the PARTIAL RATE FACTOR, f,expresses
the reactivity of a specified position in the aromaticselectivity factor 243