Lipids are a very diverse group of organic substance which is classified together because they are all soluble in such fat solvents as ether and chloroform. As a rule, they are insoluble in water.
The lipids are generally classified into three main groups, as:
(1) simple lipids, (2) compound lipids and (3) steroids. In order to gain some insight into the nature of lipids, let us concentrate briefly upon one of the simpler types, the fats.
As is the case with so many of the biologically important organic compounds, fats are macromolecular complexes composed of smaller micro molecules, or building blocks. Specifically, a fat molecule is composed of the three- carbon macromolecule glycerol, to which are attached three fatty acids of varying length. The waxes, which we shall mention only in passing, are similar to the fats in that they are fatty acid esters.
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However, instead of being esters of glycerol, they are esters of long-chain alcohol. They are found as protective coatings on the skins and furs of animals and on the leaves and fruits of plants. The compound lipids are important structural compounds in living systems.
Chemical analysis and electron microscope studies strongly indicate that a double layer of phospholipids makes up a large part of the membranes of cells. In addition, they are thought to play a role in a number of metabolic processes including fat metabolism, respiration, and enzyme activation.
The glycolipids are quite common in certain nervous tissues of the more complex animals. The steroids are chemically related to the triglycerides and compound lipids, although their molecular structure is quite different from that of other lipids.
They are classified as lipids primarily because of their solubility in fat solvents. Steroids are very important molecules in the bodies of complex animals, where they are found primarily in the heart, blood vessels, and liver. In addition, certain vitamins and hormones which are produced in animal bodies are steroids.