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3.1 Lipids
Figure 4: Examples of lipids embedded in the cell membrane.
Examples of the major membrane phospholipids and glycolipids: phosphatidylcholine
(PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns),
phosphatidylserine (PtdSer) are presented in figure 4.
The cell membrane contains three classes of amphipathic lipids: phospholipids, glycolipids,
and colesterol. The relative composition of each depends upon the type of cell, but in the
majority of cases phospholipids are the most abundant. In RBC studies, 30% of the plasma
membrane was shown to be made up by lipids. The fatty chains in phospholipids and
glycolipids usually contain an even number of carbon atoms, typically between 14 and 24.
The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or
unsaturated, with the configuration of the double bonds nearly always cis. The length and the
degree of unsaturation of fatty acids chains have a profound effect on membranes fluidity as
unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly,
thus decreasing the melting point (increasing the fluidity) of the membrane.
The morphology of phospholipids will be very different depending on the nature of the two
fatty acids included in the molecule. A phospholipids exclusively made of saturated fatty
acids will be very rigid and the assembly of such kind of phospholipids will result in a
membrane very “dense” which will not allow physiological exchanges. For these reasons
generally a phospholipids is constituted by one saturated fatty acid and by one
polyunsaturated fatty acid. In this case the membrane will be fluid and the exchanges will be
facilitated.
The fatty acid present in membrane phospholipids came from the diet; depending on the
chain length and of the degree of unsaturation of the fatty acids in the diet , e.g. a good
balance of n-6/n-3 fatty acids, membrane will result more or less fluid. In the case of
unbalanced ratio a certain number of pathologies can originate.
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