MEMBRANES, TRANSPORT, AND SIGNALING

Lipids are defined as water-insoluble compounds extracted from living organisms by weakly polar or nonpolar solvents. This definition is based on a physical property, in contrast to the definitions of proteins, carbohydrates, and nucleic acids, which are based on chemical structure. Consequently, the term lipid covers a structurally diverse group of compounds, and there is no universally accepted scheme for classifying lipids. 

Consider the following compounds, most of which are lipids  

Compounds 1, 3, and 5 to 9 are lipids because they are biological in origin and are soluble in organic solvents. The latter property arises because they contain a high proportion of carbon and hydrogen and are therefore insoluble in water. Compound 4 is not a lipid because it does not occur free in living organisms. Compound 2 is water-soluble, but because it is a member of the same series of compounds as compound 1, it is usually considered to be a lipid. 

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Caveolin is an integral membrane protein that associates with the inner leaflet of the plasma membrane, forcing it to curve inward to form caveolae, probably involved in membrane transport and signaling. 

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