Kinds of lipids. Lipids are classified as simple lipids or complex lipids, according to their structure.

Simple lipids contain only carbon, hydrogen, and oxygen. They consist of an alcohol in combination with certain organic acids containing a variable number of carbon atoms. A molecule of triglyceride (fat), the most common type of simple lipid, contains one molecule of an alcohol called glycerol and three molecules of fatty acid (a kind of organic acid). Fats include butter, lard (pig fat), tallow (beef or mutton fat), blubber (whale fat), castor oil, coconut oil, and olive oil. Waxes, another common group of simple lipids, contain an alcohol molecule that is larger than the glycerol molecule.

Complex lipids have a more complicated structure than simple lipids. They include phospholipids (lipids that contain phosphorus), steroids (lipids made up of four rings of carbon atoms joined together), and other compounds such as glycolipids (lipids with one or more sugar molecules), fat-soluble vitamins (vitamins A, D, E, and K), and terpenes (yellow pigments like carotene).

Phospholipids are found in all bacteria, and in the cells of all plants and animals. They are most plentiful in sperm, eggs, embryos, and brain cells. A molecule of phospholipid contains a molecule of glycerol, a phosphate ion, and two molecules of fatty acid. Most phospholipids also have a nitrogen compound. Some contain inositol, a substance found in vitamin B complex.

Steroids make up an important part of living things. Many animal hormones, including the sex hormones and those produced by the cortex (outer part) of the adrenal glands, are steroids. Cholesterol, a substance found in the membranes of animal cells, is a steroid. Yeasts and other fungi and the seeds of higher plants also contain steroids.

Quinton R. Rogers, Ph.D., Professor of Physiological Chemistry, University of California, Davis. -