Introduction

Lipids include a range of chemical compounds that have the common physical properties of being insoluble in water but soluble in non-polar organic solvents, such as chloroform, hydrocarbons or alcohols. Their most common chemical feature is that of ester bonds, in which the acid component is one or more aliphatic carboxylic acid. They range in complexity from the simple triglycerides to lipids containing other groupings, such as phosphate in phospholipids, carbohydrates in glycolipids and gangliosides, and proteins in lipoproteins. A further group are the sterols, which may exist as free alcohols or in esterified forms. The molecular structures of the major lipid groups are shown in Fig. 4.1. The lipids present in largest amounts in vertebrates, including birds, are the triglycerides, where their principle role is as an energy reserve. Phospholipids have a largely structural role in the cell membranes. The sterols are also present as an integral part of the membranes, but certain sterols are also hormones, such as the sex hormones and the adrenocortical steroids (see Chapter 10). The glycolipids are particularly important as structural entities in the central and peripheral nervous system. Lipoproteins are important in the transport of lipids between tissues.

This chapter is primarily concerned with the structure and metabolism of the triglycerides and their transport between tissues in the form of lipoproteins. The constant structural feature of all triglycerides is the glycerol moiety esterified by three carboxylic acids.