atoms in a MOLECULAR ENTITY(omitting any distinc-
tion from their spatial arrangement).
consumer Any organism that ingests matter and
energy of other organisms.
contact ion pair SeeION PAIR.
contact process A method in which sulfur trioxide
and sulfuric acid are produced from sulfur dioxide.
continuous spectrum A spectrum in which there are
no absorption or emission lines. It contains all wave-
lengths in a specified region of the electromagnetic
spectrum.
contrast agent A PARAMAGNETIC (or FERROMAG-
NETIC) metal complex or particle causing a decrease in
the RELAXATIONtimes (increase in relaxivity) of nuclei
detected in an IMAGE, usually of water. Contrast agents,
sometimes referred to as “dyes,” are used in medical
imaging techniques such as computed tomography
(CT) scans to highlight specific areas such as blood ves-
sels, organs, or tissues to make them more visible.
contributing structure The definition is based on
the valence-bond formulation of the quantum mechani-
cal idea of the wave function of a molecule as com-
posed of a linear combination of wave functions, each
representative of a formula containing bonds that are
only single, double, or triple, with a particular pairing
of electron spins. Each such formula represents a con-
tributing structure, also called “resonance structure” to
the total wave function, and the degree to which each
contributes is indicated by the square of its coefficient
in the linear combination. The contributing structures,
also called “canonical forms,” themselves thus have a
purely formal significance: they are the components
from which wave functions can be built. Structures can
be covalent (or nonpolar) or ionic (or polar). The rep-
resentation is frequently kept qualitative so that we
speak of important or major contributing structures
and minor contributing structures. For example, two
major nonequivalent contributing structures for the
conjugate base of acetone are
CH 2 C(CH 3 )–O–↔H 2 C––C(CH 3 )O
See also DELOCALIZATION; KEKULÉ STRUCTURE;
RESONANCE.control rod A rod containing neutron-absorbing
materials (boron or cadmium). Control rods are used
to move in and out of the core of the reactor to control
the rate of the nuclear reaction.control test A test to increase the conclusiveness of
an experiment.convection Fluid or air circulation driven by temper-
ature gradients; the rising of warm air and the sinking
of cool air. The transfer of heat by circulation or move-
ment of heated liquid or gas.cooperativity The phenomenon that binding of an
effector molecule to a biological system either
enhances or diminishes the binding of a successive
molecule, of the same or different kind, to the same
system. The system may be an ENZYMEor a protein
that specifically binds another molecule such as oxy-
gen or DNA. The effector molecule may be an enzyme
SUBSTRATEor an ALLOSTERIC EFFECTOR. The enzyme
or protein exists in different CONFORMATIONs, with
different catalytic rates or binding affinities, and the
binding of the effector molecule changes the propor-
tion of these conformations. Enhanced binding is
named positive cooperativity; diminished binding is
named negative cooperativity. A well-known example
of positive cooperativity is in HEMOGLOBIN. In BIO-
CATALYSIS, it was originally proposed that only multi-
SUBUNITenzymes could respond in this way. However,
single-subunit enzymes may give such a response (so-
called mnemonic enzymes).coordinate covalence(coordinate link) See COOR-
DINATION.coordinate covalence 59