In addition to being burnt for provision of energy, fats and oils enter, to some
extent, with the composition of photoplasm the very basis of life usually as
compounds more complex than the simple glycosides stored in animal or vegetable
tissues. So vital is the contribution of fats and oils to the life processes
that some animals which cannot synthesize certain of the unsaturated fatty acids
show symptoms of deficiency diseases and die prematurely if their food does not
contain these acids.
The fats and oils may be regarded in a way, as synonymous terms as the differ
once between the two is the difference in their melting points. Thus fats have
the same relation to oils as ice has to water.
The fats and oils contained in the tissues of plants and animals very
considerably in composition not only in different animals and plants but also
according to the part of the organism in which they occur. The number of fats
occurring in nature is therefore very large.