The Nature of Lipids
The lipid class was conceived long ago as a separate group of biochemicals that remained in the organic layer when macerated living material was partitioned between water and an organic solvent such as ether, benzene, or chloroform. This led to a simple operational definition: lipids are more soluble in organic than in aqueous media. A more important theoretical definition has emanated from these experimental parameters. Since solubility depends on the amount and kind of polar functional groups, lipids must have relatively little capacity for hydrogen bonding or dipole-dipole interactions and a large capacity for van der Waals’ interaction. Consequently, lipids are now recognized simply as molecules in which the proportion of C atoms that possess no polar functionality (OH, NH2, etc.) is large regardless of the question of solubility. The molecules may be acyclic, linear, branched, mono- or polycyclic, and what polar functions there are may be quite varied. The great interest which has developed in these substances comes from the realization that the low degree of polarity inherent in their classification is the key to their biological function. Furthermore, it has become apparent that the nonpolar parts of the molecules, i.e., the hydrophobic moieties, frequently exert their biological significance in a nonmetabolic way, as in the membranous role of fatty acids and sterols. This is in sharp contrast to the previous trend of thought in biochemistry that was focused on molecular changes associated with processes such as energy production or biosynthesis.