acids between cold-water and warm-water species
were even higher in the gangliosides [6].
As position sn-2 of the glycerophospholipids is
in any case mostly occupied by an unsaturated
fatty acid, this must be exchanged for an even
more unsaturated fatty acid; in position sn-1,
there is an increase in the proportion of monoenic
acids. Desaturation often coincides with an
increase in chain length; this occurs, for example,
in Tetrahymena and the rainbow trout [92, 124].
Two mechanisms are proposed for the adaptive
desaturation of fatty acids in membrane lipids.
The direct stimulation of desaturases by a colddependent decrease in fluidity has been postulated for bacteria and Tetrahymena but has not yet
been demonstrated. In contrast, the induction of
desaturases as a result of a decrease in temperature has been clearly shown in bacteria, Tetrahymena and the carp [66, 122,287]. The cells ofthe
homeothermic mammals are generally not
exposed to large fluctuations in temperature;
they become dysfunctional at temperatures below
10-15 °C. During hibernation, the body temperature may fall to a few degrees above zero, and
adaptations of cell membranes are to be expected
in these conditions. In fact, only minor changes
that are indicative of homeoviscous adaptation of
the membranes of hibernating animals are discernible; the issue undoubtedly requires further
study [2].
Because cooling lowers the pH of the body
fluids (~ pH per °C ranges from -0.015 to -0.013)
there are interacting effects of pH and temperature on membrane fluidity [93]. Compared with
the multitude of investigations on the effects of
temperature, the reactions of cell membranes to
other environmental changes have received little
attention. In deep-sea fish, the desaturation of
fatty acids in phosphatidycholine and phosphatidylethanolamine of the liver increases with depth
under water, presumably to compensate for the
increasing pressure [51]. It has been shown in
marine crustaceans that dilution of the aqueous
environment brings about an increase both in the
contents of cardiolipin, phosphatidylethanolamine and phosphatidylserine in the posterior
osmoregulation-active gills, and in the degree of
desaturation of phospholipids. However, in the
absence of investigations on isolated membranes
or cell organelles, these results are difficult to
interpret [37].
15.4.1 Phospholipids
599
15.4.1 Phospholipids
Phospholipids are the main lipid components of
cellular membranes and are also found in cells in
soluble transport forms. Non-structurally bound
phospholipids are found in the vertebrates in
plasma lipoproteins, in the bile and in the lining
of the inner wall of lung alveoli (lung surfactant).
In crustaceans, such as the shore crab Carcinus
meanas, the lobster Homarus american us and the
crayfish Orconectes virilis, the protein-bound
transported lipid of the haemolymph is not diacylglycerol, as in the insects, but consists of twothirds phospholipids, with more phosphatidylcholine (66-88 %) than phosphatidylethanolamine
(12-20 %) [37]. The spermatozoa of the sea
urchin Hemicentrotus pulcherrimus utilize the
fatty acids of endogenous phosphatidylcholine as
an energy source for swimming [176].
In the classification of the phospholipids a distinction is made, according to the nature of the
backbone, between glycerophospholipids and
sphingophospholipids (sphingomyelins) (Table 15.1). The glycerophospholipids are further
subdivided on the basis of their head groups; the
basic structure common to all, 1,2-diacyl-snglycero-3-phosphate (Fig. 15.20), is known as
phosphatidic acid. The different glycerophospholipids are derived by substitution of the phosphate
residue with choline, ethanolamine, L-serine,
inositol or glycerol; in cardiolipin two phosphatidic acids are bound by glycerol (Fig. 15.20 g).
Phosphatidylcholine and phosphatidylethanolamine are the predominant phospholipids of cellular membranes. In the case of these two compounds, the negative charge of the phosphate
residue is compensated by the positive charge of
the nitrogen under physiological conditions. The
other phospholipids, however, generate a negative surface charge on the membrane, and this
may be the reason why they are present in lower
amounts. Phosphatidylcholine is usually more
prevalent than phosphatidylethanolamine, e.g. in
the vertebrates, crustaceans and molluscs (Tables 15.14 and 15.16). The situation in the insects is
more confusing. Phosphatidylcholine predominates in the Hymenoptera, Lepidoptera and
Orthoptera, and phosphatidylethanolamine in
the aphids and Diptera. Finally, in the Coleoptera
the ratio of the two phospholipid classes is
approximately 1: 1 [37, 55, 63, 89], and a similar
situation pertains in the ciliate Tetrahymena (Table 15.15). The temperature dependence of the
phospholipid spectrum must also be taken into
account: in cold-adapted cells, the proportion of
were even higher in the gangliosides [6].
As position sn-2 of the glycerophospholipids is
in any case mostly occupied by an unsaturated
fatty acid, this must be exchanged for an even
more unsaturated fatty acid; in position sn-1,
there is an increase in the proportion of monoenic
acids. Desaturation often coincides with an
increase in chain length; this occurs, for example,
in Tetrahymena and the rainbow trout [92, 124].
Two mechanisms are proposed for the adaptive
desaturation of fatty acids in membrane lipids.
The direct stimulation of desaturases by a colddependent decrease in fluidity has been postulated for bacteria and Tetrahymena but has not yet
been demonstrated. In contrast, the induction of
desaturases as a result of a decrease in temperature has been clearly shown in bacteria, Tetrahymena and the carp [66, 122,287]. The cells ofthe
homeothermic mammals are generally not
exposed to large fluctuations in temperature;
they become dysfunctional at temperatures below
10-15 °C. During hibernation, the body temperature may fall to a few degrees above zero, and
adaptations of cell membranes are to be expected
in these conditions. In fact, only minor changes
that are indicative of homeoviscous adaptation of
the membranes of hibernating animals are discernible; the issue undoubtedly requires further
study [2].
Because cooling lowers the pH of the body
fluids (~ pH per °C ranges from -0.015 to -0.013)
there are interacting effects of pH and temperature on membrane fluidity [93]. Compared with
the multitude of investigations on the effects of
temperature, the reactions of cell membranes to
other environmental changes have received little
attention. In deep-sea fish, the desaturation of
fatty acids in phosphatidycholine and phosphatidylethanolamine of the liver increases with depth
under water, presumably to compensate for the
increasing pressure [51]. It has been shown in
marine crustaceans that dilution of the aqueous
environment brings about an increase both in the
contents of cardiolipin, phosphatidylethanolamine and phosphatidylserine in the posterior
osmoregulation-active gills, and in the degree of
desaturation of phospholipids. However, in the
absence of investigations on isolated membranes
or cell organelles, these results are difficult to
interpret [37].
15.4.1 Phospholipids
599
15.4.1 Phospholipids
Phospholipids are the main lipid components of
cellular membranes and are also found in cells in
soluble transport forms. Non-structurally bound
phospholipids are found in the vertebrates in
plasma lipoproteins, in the bile and in the lining
of the inner wall of lung alveoli (lung surfactant).
In crustaceans, such as the shore crab Carcinus
meanas, the lobster Homarus american us and the
crayfish Orconectes virilis, the protein-bound
transported lipid of the haemolymph is not diacylglycerol, as in the insects, but consists of twothirds phospholipids, with more phosphatidylcholine (66-88 %) than phosphatidylethanolamine
(12-20 %) [37]. The spermatozoa of the sea
urchin Hemicentrotus pulcherrimus utilize the
fatty acids of endogenous phosphatidylcholine as
an energy source for swimming [176].
In the classification of the phospholipids a distinction is made, according to the nature of the
backbone, between glycerophospholipids and
sphingophospholipids (sphingomyelins) (Table 15.1). The glycerophospholipids are further
subdivided on the basis of their head groups; the
basic structure common to all, 1,2-diacyl-snglycero-3-phosphate (Fig. 15.20), is known as
phosphatidic acid. The different glycerophospholipids are derived by substitution of the phosphate
residue with choline, ethanolamine, L-serine,
inositol or glycerol; in cardiolipin two phosphatidic acids are bound by glycerol (Fig. 15.20 g).
Phosphatidylcholine and phosphatidylethanolamine are the predominant phospholipids of cellular membranes. In the case of these two compounds, the negative charge of the phosphate
residue is compensated by the positive charge of
the nitrogen under physiological conditions. The
other phospholipids, however, generate a negative surface charge on the membrane, and this
may be the reason why they are present in lower
amounts. Phosphatidylcholine is usually more
prevalent than phosphatidylethanolamine, e.g. in
the vertebrates, crustaceans and molluscs (Tables 15.14 and 15.16). The situation in the insects is
more confusing. Phosphatidylcholine predominates in the Hymenoptera, Lepidoptera and
Orthoptera, and phosphatidylethanolamine in
the aphids and Diptera. Finally, in the Coleoptera
the ratio of the two phospholipid classes is
approximately 1: 1 [37, 55, 63, 89], and a similar
situation pertains in the ciliate Tetrahymena (Table 15.15). The temperature dependence of the
phospholipid spectrum must also be taken into
account: in cold-adapted cells, the proportion of