hydrophobic tails face inwards, away
from the external aqueous environment.
lipoic acid A vitamin of the *vitamin B complex. It is one of the
*coenzymes involved in the decarboxylation of pyruvate by the enzyme pyruvate dehydrogenase. Good
sources of lipoic acid include liver
and yeast.
lipolysis The breakdown of storage
lipids in living organisms. Most longterm energy reserves are in the form
of triglycerides in fats and oils. When
these are needed, e.g. during starvation, lipase enzymes convert the
triglycerides into glycerol and the
component fatty acids. These are
then transported to tissues and oxidized to provide energy.
lipoprotein See lipid.
lipowitz alloy A low-melting
(70–74°C) alloy of bismuth (50%), lead
(27%), tin (13%), and cadmium (10%).
liquation The separation of mixtures of solids by heating to a temperature at which lower-melting
components liquefy.
liquefaction of gases The conversion of a gaseous substance into a liquid. This is usually achieved by one
of four methods or by a combination
of two of them:
(1) by vapour compression, provided
that the substance is below its *critical temperature;
(2) by refrigeration at constant pressure, typically by cooling it with a
colder Ûuid in a countercurrent heat
exchanger;
(3) by making it perform work adiabatically against the atmosphere in a
reversible cycle;
(4) by the *Joule–Thomson effect.
Large quantities of liqueÜed gases
are now used commercially, especially *liqueÜed petroleum gas and
liqueÜed natural gas.
liqueÜed natural gas (LNG) See
liquefied petroleum gas.
liqueÜed petroleum gas (LPG)
Various petroleum gases, principally
propane and butane, stored as a liquid under pressure. It is used as an
engine fuel and has the advantage of
causing very little cylinder-head deposits.
LiqueÜed natural gas (LNG) is a similar product and consists mainly of
methane. However, it cannot be
liqueÜed simply by pressure as it has
a low critical temperature of 190 K
and must therefore be cooled to
below this temperature before it will
liquefy. Once liqueÜed it has to be
stored in well-insulated containers. It
provides a convenient form in which
to ship natural gas in bulk from oil
wells or gas-only wells to users. It is
also used as an engine fuel.
liquid A phase of matter between
that of a crystalline solid and a *gas.
In a liquid, the large-scale threedimensional atomic (or ionic or molecular) regularity of the solid is absent but, on the other hand, so is the
total disorganization of the gas. Although liquids have been studied for
many years there is still no comprehensive theory of the liquid state. It
is clear, however, from diffraction
studies that there is a short-range
structural regularity extending over
several molecular diameters. These
bundles of ordered atoms, molecules,
or ions move about in relation to
each other, enabling liquids to have
almost Üxed volumes, which adopt
the shape of their containers.
liquid crystal A substance that
Ûows like a liquid but has some order
in its arrangement of molecules. Nematic crystals have long molecules all
aligned in the same direction, but
lipoic acid
328
l
www.AzShimi.ir www.AzShimi.com
from the external aqueous environment.
lipoic acid A vitamin of the *vitamin B complex. It is one of the
*coenzymes involved in the decarboxylation of pyruvate by the enzyme pyruvate dehydrogenase. Good
sources of lipoic acid include liver
and yeast.
lipolysis The breakdown of storage
lipids in living organisms. Most longterm energy reserves are in the form
of triglycerides in fats and oils. When
these are needed, e.g. during starvation, lipase enzymes convert the
triglycerides into glycerol and the
component fatty acids. These are
then transported to tissues and oxidized to provide energy.
lipoprotein See lipid.
lipowitz alloy A low-melting
(70–74°C) alloy of bismuth (50%), lead
(27%), tin (13%), and cadmium (10%).
liquation The separation of mixtures of solids by heating to a temperature at which lower-melting
components liquefy.
liquefaction of gases The conversion of a gaseous substance into a liquid. This is usually achieved by one
of four methods or by a combination
of two of them:
(1) by vapour compression, provided
that the substance is below its *critical temperature;
(2) by refrigeration at constant pressure, typically by cooling it with a
colder Ûuid in a countercurrent heat
exchanger;
(3) by making it perform work adiabatically against the atmosphere in a
reversible cycle;
(4) by the *Joule–Thomson effect.
Large quantities of liqueÜed gases
are now used commercially, especially *liqueÜed petroleum gas and
liqueÜed natural gas.
liqueÜed natural gas (LNG) See
liquefied petroleum gas.
liqueÜed petroleum gas (LPG)
Various petroleum gases, principally
propane and butane, stored as a liquid under pressure. It is used as an
engine fuel and has the advantage of
causing very little cylinder-head deposits.
LiqueÜed natural gas (LNG) is a similar product and consists mainly of
methane. However, it cannot be
liqueÜed simply by pressure as it has
a low critical temperature of 190 K
and must therefore be cooled to
below this temperature before it will
liquefy. Once liqueÜed it has to be
stored in well-insulated containers. It
provides a convenient form in which
to ship natural gas in bulk from oil
wells or gas-only wells to users. It is
also used as an engine fuel.
liquid A phase of matter between
that of a crystalline solid and a *gas.
In a liquid, the large-scale threedimensional atomic (or ionic or molecular) regularity of the solid is absent but, on the other hand, so is the
total disorganization of the gas. Although liquids have been studied for
many years there is still no comprehensive theory of the liquid state. It
is clear, however, from diffraction
studies that there is a short-range
structural regularity extending over
several molecular diameters. These
bundles of ordered atoms, molecules,
or ions move about in relation to
each other, enabling liquids to have
almost Üxed volumes, which adopt
the shape of their containers.
liquid crystal A substance that
Ûows like a liquid but has some order
in its arrangement of molecules. Nematic crystals have long molecules all
aligned in the same direction, but
lipoic acid
328
l
www.AzShimi.ir www.AzShimi.com