Some lichens are held on their substrate by medullary
fungal hyphae, but many foliose lichens have a lower
cortex and special structures called rhizines to hold
them in place on tree bark or rock.
There are only about 100 algal partners, but there
are as many fungal partners as there are lichen species,
which number about 14,000. The fungus receives sug-
ars and other nutrients from the photobiont and pro-
vides a stable and secure structure in which it lives,
provides water, and shields the photobiont from the
sun’s desiccating rays. Lichens reproduce vegetatively in
three basic ways: fragmentation of the thallus, and by
two kinds of tiny vegetative propagules that contain
both partners—soredia, the more common, and isidia,
which are slightly larger and heavier. Isidia and soredia
are dispersed by wind, water, and animal agents.
Lichens are slow growers and are particularly good
pioneer species in disturbed habitats.
Lichens have been used as food, especially by cari-
bou in winter; as medicine and dyes; as dwarf “trees”
in model railroad setups and architectural plans; and in
holiday wreaths and arrangements. And many harsh
landscapes are much enriched by their subtle beauty.
See alsoALGAE.
life table Atabular and numerical representation of
mortality of births for each age group and includes var-
ious groups of information such as probabilities of
death, survival, and life expectancies.
ligament One component of the musculoskeletal sys-
tem that also consists of tendons, bones, and muscles.
Ligaments are collagen-rich connective structures, a
band of fibrous tissue that attaches and binds bone to
bone and to cartilage and that supports organs in place.
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. The root of the word is sometimes con-
verted into the verb to ligate,meaning to coordinate as
a ligand, and the derived participles ligatingand ligat-
ed.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 radi-
cal 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.
See alsoCOORDINATION.ligand field Ligand field theory is a modified CRYS-
TAL FIELDtheory that assigns certain parameters as
variables rather than taking them as equal to the val-
ues found for free ions, thereby taking into account
the potential covalent character of the metal-LIGAND
bond.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 can act as a channel for the
passage of a specific ion across the membrane. When
activated by a signal molecule, it allows or blocks the
passage of the ion. This results in a change in ion con-
centration 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 alsoEC NOMENCLATURE FOR ENZYMES.ligating SeeLIGAND.ligating 205