linear combination of atomic
orbitalsSee lcao.
linear molecule A molecule in
which the atoms are in a straight
line, as in carbon dioxide, O=C=O.
Linear molecules have only two rota-
tional degrees of freedom.
linear rotor See moment of iner-
tia.
line notation A notation system
for writing the structure of a chemi-
cal compound as a string of letters,
numbers, and symbols. Examples of
line notation are Wiswesser line no-
tation (WLN), SMILES, SYBYL line
notation (SLN), ROSDAL, and
*InChI.
line spectrum See spectrum.
linoleic acidA liquid polyunsatu-
rated fatty acid with two double
bonds, CH 3 (CH 2 ) 4 CH:CHCH 2 -
CH:CH(CH 2 ) 7 COOH. Linoleic acid is
abundant in plant fats and oils, e.g.
linseed oil, groundnut oil, and soya-
bean oil. It is an essential fatty acid.
linolenic acid A liquid polyunsatu-
rated fatty acid with three double
bonds in its structure:
CH 3 CH 2 CH:CHCH 2 CH:CHCH 2 CH:CH-
(CH 2 ) 7 COOH. It occurs in certain
plant oils, e.g. linseed and soya-bean
oil, and in algae. It is one of the es-
sential fatty acids.
linseed oilA pale yellow oil
pressed fromÛax seed. It contains a
mixture of glycerides of fatty acids,
including linoleic acid and linolenic
acid. It is a *drying oil, used in oil
paints, varnishes, linoleum, etc.
Linz–Donawitz process See basic-
oxygen process.
lipaseAn enzyme secreted by the
pancreas and the glands of the small
intestine of vertebrates that catalyses
the breakdown of fats into fatty acids
and glycerol.
lipidAny of a diverse group of or-
ganic compounds, occurring in living
organisms, that are insoluble in
water but soluble in organic solvents,
such as chloroform, benzene, etc.
Lipids are broadly classiÜed into two
categories: complex lipids, which are
esters of long-chain fatty acids and
include the *glycerides (which con-
stitute the *fats and *oils of animals
and plants), *glycolipids, *phospho-
lipids, and *waxes; and simple lipids,
which do not contain fatty acids and
include the *steroids and *terpenes.
Lipids have a variety of functions in
living organisms. Fats and oils are a
convenient and concentrated means
of storing food energy in plants and
animals. Phospholipids and *sterols,
such as cholesterol, are major com-
ponents of cell membranes (see lipid
bilayer). Waxes provide vital water-
prooÜng for body surfaces. Terpenes
include vitamins A, E, and K, and
phytol (a component of chlorophyll)
and occur in essential oils, such as
menthol and camphor. Steroids in-
clude the adrenal hormones, sex hor-
mones, and bile acids.
Lipids can combine with proteins
to form lipoproteins, e.g. in cell
membranes. In bacterial cell walls,
lipids may associate with polysaccha-
rides to form lipopolysaccharides.
A- Information about IUPAC nomenclature
of lipids
lipid bilayerThe arrangement of
lipid molecules in biological mem-
branes, which takes the form of a
double sheet. Each lipid molecule
comprises a hydrophilic ‘head’ (hav-
ing a high afÜnity for water) and a
hydrophobic ‘tail’ (having a low
afÜnity for water). In the lipid bilayer
the molecules are aligned so that
their hydrophilic heads face out-
wards, forming the outer and inner
surfaces of the membrane, while the327 lipid bilayer
l