of a quencher, and should be called “apparent lifetime”
instead.
See alsoCHEMICAL RELAXATION; HALF-LIFE; RATE
OF REACTION.
ligand A molecule, ion, hormone or compound that
binds to a specific receptor site that binds to another
molecule.
The atoms or groups of atoms bound to the CEN-
TRAL ATOM(seeCOORDINATION). The root of the word
is sometimes converted into the verb to ligate,meaning
to coordinate as a ligand, and the derived participles,
ligatingand ligated.This use should not be confused
with its use to describe the action of LIGASEs (a class of
ENZYMEs). The names for anionic ligands, whether
inorganic or organic, end in -o. In general, if the anion
name ends in -ide, or -ate, the final -e is replaced by -o,
giving -ido and -ato, respectively. Neutral and cationic
ligand names are used without modification. Ligands
bonded by a single carbon atom to metals are regarded
as radical substituents, their names being derived from
the parent hydrocarbon, from which one hydrogen
atom has been removed. In general, the final letter -e of
the name is replaced by -yl.
In biochemistry the term ligand has been used
more widely: if it is possible or convenient to regard
part of a polyatomic molecular entity as central, then
the atoms or groups or molecules bound to that part
may be called ligands.
ligand field Ligand field theory is a modified CRYS-
TAL FIELD theory that assigns certain parameters as
variables rather than taking them as equal to the values
found for free ions, thereby taking into account the
potential covalent character of the metal-LIGANDbond.
ligand field splitting Removal of a degeneracy of
atomic or molecular levels in a molecule or ion with a
given symmetry induced by the attachment or removal
of ligands to produce reduced symmetries.
ligand-gated ion-channel receptor Ion channels are
specialized pores in the cell membrane that help control
and transfer electrical impulses (action potentials) in
the cell. They regulate the flow of sodium, potassium,
and calcium ions into and out of the cell. The ligand-
gated ion-channel receptor is a signal receptor protein
in a cell membrane that may act as a channel for the
passage of a specific ion across the membrane. When
activated by a signal molecule, it will allow or block
the passage of the ion. This results in a change in ion
concentration that often affects cell functioning.ligase An ENZYMEof EC class 6, also known as a
synthetase, that catalyzes the formation of a bond
between two SUBSTRATE molecules coupled with the
HYDROLYSISof a diphosphate bond of a NUCLEOSIDE
triphosphate or similar cosubstrate.
See alsoECNOMENCLATURE FOR ENZYMES.ligating SeeLIGAND.light microscope A common laboratory instrument
that uses optics to bend visible light to magnify images
of specimens placed on an attached platform or other
viewing area.light reactions A major component of photosynthe-
sis in which a group of chemical reactions occurs in the
thylakoid membranes of chloroplasts that harvest
energy from the sun to produce energy-packed chemi-
cal bonds of ATPand NADPHand give off oxygen as a
by-product.light water reactor A term used to describe reactors
using ordinary water as coolant, including boiling
water reactors (BWRs) and pressurized water reactors
(PWRs), the most common types used in the United
States.lignin A complex amorphous polymer in the sec-
ondary cell wall (middle lamella) of woody plant cells
that cements or naturally binds cell walls to help make
them rigid. Highly resistant to decomposition by chem-
ical or enzymatic action. It is the major source material
for coal. It also acts as support for cellulose fibers.lignin 165