Lipids are chiefly fatty acid esters, and are the basic building blocks of biological membranes. Another biological role is energy storage (e.g., triglycerides).

Most lipids consist of a polar or hydrophilic head and one to three nonpolar or hydrophobic fatty acid tails, and therefore they are amphiphilic.

Fatty acids consist of unbranched chains of carbon atoms that are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.

Biological membranes

For lipids present in biological membranes, the hydrophilic head is from one of three classes:

 glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide residues.
 phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
 sterols, whose heads contain a planar steroid ring, for example, cholesterol.

By cellular localization

 intranuclear lipids

By types

 triglycerides (fatty acids)
 steroids
 glyoclipids
 phospholipids
 polyisoprenoids
 ether lipids
 inositol lipids

Features

 lipid homeostasis
 lipid biosynthesis
 lipid catabolism