irregular Refers to flowers that are bilaterally sym-
metrical, i.e., that are divisible into equal halves only in
one plane.
irruption A rapid and temporary increase in popula-
tion density, often followed by a mass emigration; com-
mon in bird species.
ischemia Local deficiency of blood supply and
dioxygen to an organ or tissue owing to constriction of
the blood vessels or to an obstruction.
isobacteriochlorin 2,3,7,8-Tetrahydroporphyrin. A
reduced PORPHYRINwith two pairs of con-fused satu-
rated carbon atoms (C-2, C-3 and C-7, C-8) in two of
the pyrrole rings.
See alsoBACTERIOCHLORIN.
isoenzymes Multiple forms of ENZYMEs arising from
genetically determined differences in PRIMARY STRUC-
TURE. The term does not apply to those derived by
modification of the same primarySEQUENCE.
isogamy Sexual reproduction involving the fusion of
gametes that are similar in size or are morphologically
indistinguishable, e.g., fungi such as zygomycetes.
isolating mechanism Any environmental, behav-
ioral, mechanical, or physiological barriers or charac-
teristics that will prevent two individuals of different
populations from producing viable progeny. Important
for the development of new species.
isomerase An ENZYME of EC class 5, which cat-
alyzes the isomerization of a SUBSTRATE.
See alsoEC NOMENCLATURE FOR ENZYMES.
isomers Compounds that have the same number and
type of atoms (same molecular formula) but differ in
the way they are combined with each other. They can
differ by the bonding sequence, called structural or
constitutional isomerism, or the way their atoms are
arranged spatially, called stereoisomerism. Other types
include conformational, configurational, geometric,
optical, enantiomers, and diastereomers.isomorphic alternation of generations When
gametophyte and sporophyte generations are morpho-
logically alike, but differ in the number of chromo-
somes.Isoptera Termites, a social order of insects that are
soft bodied and comprise about 2,300 species world-
wide. They obtain their nutrition from eating wood
and digesting cellulose with the help of bacteria and
protozoans found in their intestinal area.isosteres Molecules or ions of similar size containing
the same number of atoms and valence electrons, e.g.,
O2â, Fâ, Ne.
See alsoBIOISOSTERE.isotonic solutions Solutions having identical osmot-
ic pressures, i.e., a solution where cells do not swell or
shrink.isotope A different form of a single element that has
the same number of protons, but has a different num-
ber of neutrons in its nucleus. Radioactive isotopes are
unstable and break down until they become stable.
Carbon 14 is a radioactive isotope of carbon that is
used to date fossilized organic matter.isotropy Lack of ANISOTROPY; the property of
molecules and materials of having identical physical
properties in all directions.Ixodes A genus of ticks. Ixodes scopularisis the vec-
tor for Lyme disease because it can carry aspirochete
known as Borrelia burgdorferithat is transmitted upon
the bite of the tick.182 irregular