Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Membrane Lipids
Fatty Acids

Historically, Lipids (from the Greek λίπος, lipos — fat) are defined as naturally occurring compounds obtained from plant or animal Tissues by extraction with non-polar Solvents (such as ether, benzene, or chloroform) that are insoluble in Water.

However, rather than providing a clear characterization of a Class of chemical compounds, this definition merely describes their physical properties. Today, a considerable number of compounds are known that are either insoluble in non-polar solvents or, conversely, highly soluble in water, yet are still classified as lipids.

In modern organic chemistry, the definition of lipids is based on the biosynthetic relationship of these compounds, with Fatty acids and their derivatives being classified as lipids.

Fatty acids serve as the foundation of Phospholipids, which form Plasma Membranes, and consist of a hydrocarbon chain and a carboxyl group COOH (Figure 14).

Figure 14 - Diagrams of certain C18 fatty acids:

a - stearic; b - oleic; c - linoleic; d - α-linolenic

Fatty acids vary in chain length. In Cells, fatty acids predominantly have an even number of carbon atoms. In plants and animals, the majority of fatty acid molecules contain 16 or 18 carbon atoms.

Fatty acids are designated as Cx:y, where x is the number of carbon atoms in the chain and y is the number of double bonds. Fatty acids lacking C=C double bonds are termed saturated, whereas those containing at least one double bond are called unsaturated.

The principal biological fatty acids are listed in Table 1.

Table 1 - Principal biological fatty acids (LipidBank http://www.lipidbank.jp/)