cyanobacteria Bacteria, formerly known as blue-
green algae; aquatic and photosynthetic organisms that
live in water and manufacture their own food. Their
fossils go back more than 3.5 billion years, making
them the oldest known species, and they are the con-
tributors to the origin of plants.
cybernetics The science that studies the methods to
control behavior and communication in animals (and
machines).
cybotactic region That part of a solution in the
vicinity of a solute molecule in which the ordering of
the solvent molecules is modified by the presence of the
solute molecule. The term solvent “cosphere” of the
solutehas also been used
See alsoSOLVATION.
cyclic AMP(cAMP or 3′,5′-AMP) Cyclic adenosine
monophosphate. A compound synthesized from ATP
(by the enzyme adenylyl cyclase) in living cells that acts
as an intercellular and extracellular second messenger
mediating peptide and amine hormones.cyclic electron flow Two photosystems are present
in the thylakoid membrane of chloroplasts: photosys-
tem I and photosystem II. The two photosystems work
together during the light reactions of PHOTOSYNTHESIS.
The light-induced flow of electrons beginning with and
returning to photosystem I to produce ATP without
production of NADPH (nicotine adenine dinucleotide
phosphate with hydrogen) is cyclic electron flow. The
generation of ATP by this process is called noncyclic
photophosphorylation.cyclin A protein found in dividing cells that activates
protein kinases (cyclin-dependent protein kinases), an
enzyme that adds or removes a phosphate group from
a target protein and controls the progression of one
phase of the cell cycle to the next. The concentration of
the cyclin increases and decreases during the cell cycle.cyclin-dependent kinase A protein kinase, an
enzyme involved in regulating cell growth and division,
that must be attached to cyclin to occur.cyclization Formation of a ring compound from a
chain by formation of a new bond.
See alsoANNULATION.cycloaddition A reaction in which two or more
unsaturated molecules (or parts of the same molecule)
combine with the formation of a cyclic ADDUCT, in
which there is a net reduction of the bond multiplicity.
The following two systems of notation have been
used for the more detailed specification of cycloaddi-
tions, of which the second, more recent system
(described under (2)) is preferred:
(1) An (i+j+ ...) cycloaddition is a reaction in
which two or more molecules (or parts of the same
molecule) provide units of i, j,... linearly connected
atoms. These units become joined at their respective
termini by new sigma bonds so as to form a cycle con-
taining (i+j+ ...) atoms. In this notation, (a) a Diels-66 cyanobacteria
Curtin-Hammett principle. Transformation of interconverting iso-
mers A′and A′′into products X and Y.