1. FATTY ACID OCCURRENCE AND DISTRIBUTION
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which are major constituents of fish phospholipids, are not present in
plant phospholipids. On the other hand, the highly unsaturated C 20
and C 22 acids are found in considerable amounts in the phospholipids
of the higher land mammals, so that the differences in fatty acid composition between the higher land mammals and fish are not so apparent
as in the glycerides.
The examination of individual phospholipids shows a striking variation in fatty acid composition with each kind of phospholipid.
Even more outstanding is the presence of distinctive high molecular
weight fatty acids (not present in the glycerophospholipids) in the
related sphingomyeHn and cerebrosides. As sphingoHpids have not been
found in plant tissues, these compounds, together with their unusual
fatty acids, afford a sharp distinction in lipid composition between the
vegetable and animal kingdoms.
VI. Variations in Types and Distribution of Fatty Acids
in Different Parts of the Organism
Variations in the nature of the lipids of different parts of the organism may be quite extensive. Not only are there variations in fatty acid
composition and types of fatty acids, but also the kinds and proportions of the lipids themselves may change. The depot fats of mammals
consist largely of triglycerides. In brain tissues the fatty acids are combined as phospholipids and cerebrosides, glycerides being almost entirely
absent. The fatty acids are of relatively high molecular weight and include polyunsaturated C 20 and C 22 acids, in contrast to palmitic, stearic,
and oleic acids, which form the main fatty acid constituents of the depot
fats. In nonadipose tissues, such as liver, muscle, and blood plasma,
phospholipids are often more abundant than triglycerides and, in addition, cholesterol esters are present (303).
The lipids from skin and hair of some mammals, including man,
guinea pig, mouse, rabbit, and rat, appear to be largely nonglyceridic,
and the fatty acids are either present in the free form or combined as
sterol or wax esters (304, 305). In fish, the liver not infrequently forms
the main site of fat storage, fatty acids being present as triglycerides.
Under these circumstances, the body lipids may contain, as in the cod,
a small amount of triglycerides, such lipids as are present (less than
1% of the fresh tissue) consisting largely of phospholipids (306, 307).
In plants, the fatty acid composition of the fruit-coat fat bears little
relationship to that of the seed fat. The lipids of the fruit-coat fats and
the seed fats are largely triglycerides, but in the leaf the lipids contain mainly galactolipids in place of triglycerides, the remainder consisting of unsaponifiable matter and phospholipids. Moreover, the com-
63
which are major constituents of fish phospholipids, are not present in
plant phospholipids. On the other hand, the highly unsaturated C 20
and C 22 acids are found in considerable amounts in the phospholipids
of the higher land mammals, so that the differences in fatty acid composition between the higher land mammals and fish are not so apparent
as in the glycerides.
The examination of individual phospholipids shows a striking variation in fatty acid composition with each kind of phospholipid.
Even more outstanding is the presence of distinctive high molecular
weight fatty acids (not present in the glycerophospholipids) in the
related sphingomyeHn and cerebrosides. As sphingoHpids have not been
found in plant tissues, these compounds, together with their unusual
fatty acids, afford a sharp distinction in lipid composition between the
vegetable and animal kingdoms.
VI. Variations in Types and Distribution of Fatty Acids
in Different Parts of the Organism
Variations in the nature of the lipids of different parts of the organism may be quite extensive. Not only are there variations in fatty acid
composition and types of fatty acids, but also the kinds and proportions of the lipids themselves may change. The depot fats of mammals
consist largely of triglycerides. In brain tissues the fatty acids are combined as phospholipids and cerebrosides, glycerides being almost entirely
absent. The fatty acids are of relatively high molecular weight and include polyunsaturated C 20 and C 22 acids, in contrast to palmitic, stearic,
and oleic acids, which form the main fatty acid constituents of the depot
fats. In nonadipose tissues, such as liver, muscle, and blood plasma,
phospholipids are often more abundant than triglycerides and, in addition, cholesterol esters are present (303).
The lipids from skin and hair of some mammals, including man,
guinea pig, mouse, rabbit, and rat, appear to be largely nonglyceridic,
and the fatty acids are either present in the free form or combined as
sterol or wax esters (304, 305). In fish, the liver not infrequently forms
the main site of fat storage, fatty acids being present as triglycerides.
Under these circumstances, the body lipids may contain, as in the cod,
a small amount of triglycerides, such lipids as are present (less than
1% of the fresh tissue) consisting largely of phospholipids (306, 307).
In plants, the fatty acid composition of the fruit-coat fat bears little
relationship to that of the seed fat. The lipids of the fruit-coat fats and
the seed fats are largely triglycerides, but in the leaf the lipids contain mainly galactolipids in place of triglycerides, the remainder consisting of unsaponifiable matter and phospholipids. Moreover, the com-