Lipid Distribution and Function

The term lipid describes a group of biological compounds that are insoluble in water but are relatively soluble in many organic solvent.

Lipid can be classified in three subgroups based on chemical composition; hydrocarbon, simple lipids and complex lipids contain C, H, and O. Complex lipid contain one or more additional elements, such as phosphorous (P), nitrogen (N), or sulfur (S).

Simple lipid can be aggregated into structural types, which are fatty acid is a long chain monocarboxylic acid. A wax is the ester of a long chain alcohol and a fatty acid. A triglyceride is the ester of a glycerol that contains tree FA molecules. Sterols are a special class of alcohols; they may combine with a fatty acid to form sterol esters.

Among the complex lipids, important structural types are phosphoglyceridas, phosphosphingolipids, and glycolipids. The parent phosphoglyceride, phosphatidic acid (PA), is similar in structure to triglyceride except that the 3-hydroxyl group of the glycerol moiety is esterified to phosphoric acid rather than to FA. Further esterification of the phosphoric acid of PA with a variety of small, hydroxyl containing molecules leads to a series of derived phosphoglycerides, including phospphatidyl choline (PC), commonly known as lecithin, and phosphatidyl ethanolamine (PE).

The phosphosphingolipids are derived from sphingosine. The formation of an amide with an FA with the 2-position yield ceramide. Esterification of the 1-hydroxyl of ceramide with phosphosphingolipid. If the 1-hydroxyl group of ceramide is linked instead to a simple sugar, a cerebroside glycolipid is formed. The further addition of several amino sugars yields more complex glycolipids, the gangliosides.

Lipid Distribution and Function
Lipids are found in all organism as structural components of the cell membrane. In most animals the major membrane lipids are lecithin, phosphatidyl ethanolamine, phosphatidyl serine, and a sterol, cholesterol. Cell membranes of the central nervous system contain, in additional to the above, shingmyelin, cerebrosides, and gangliosides. In higher plant membranes, lecithin and PE predominate, although phosphatidyl glycerol (PG) and phosphatidyl inositol (PI), know as phytosterols, are commonly present.

Bacterial membranes are unique in that lecithin is rarely present and sterols are completely absent; PE and PG are usually the major lipids.

Although triglycerides are not important membrane lipids, they are stored in most animals and plants as a metabolic energy reserve. In vertebrate TG is located in adipose (fat) tissue. In insect TG is concentrated in a specific fat body that fuction both as a depot and as a center of triglyceride metabolism. In higher plant TG is found in the seeds of most plants and is the source of most vegetable oils. In a few plant such as avocado, the palm and the olive, the fruit also contains large amount of triglycerides.

Lipids have a number of specialized functions. In mammals subcutaneous fat retards loss of body heat. Hydrocarbons and waxes on insect cuticle, as well as on plant leaves and fruit, aid in water retention. Certain cyclic FA, the prostaglandins, are involve in blood clotting and hormonal responses in mammals, and a veriety of other FA derivatives serve as sex attractants and growth regulators in insects. Sex hormones and the adrenal corticoids of higher animals are lipids derived from cholesterol. Essential dietery lipids includes certain polyunsaturated fatty acids and the vitamins A, D, E and K.