Lipids are diverse in their structure and properties. Lipids include the lipid-soluble vitamins, steroid hormones, prostaglandins, fatty acids, triacylglycerols (fats), glycerophospholipids, sphingolipids and derivatives of these compounds, as well.

Though lipids as a group are more nonpolar than other biological molecules, most of them have some amphiphilic character. Fatty acids, for example, illustrate the amphiphilic nature of many lipids. (see here, Figure 10.1). Fatty acids have a polar end containing the carboxyl group and a nonpolar tail, allowing it to readily form micelles in water. Fatty acids may be saturated (no double bonds) or unsaturated (contain one or more double bonds). Virtually all biologically produced unsaturated fatty acids contain cis double bonds, which induce a bend in the molecules.

When three fatty acids are esterified to glycerol, the resulting molecule is a fat (if solid at room temperature) or an oil (if liquid at room temperature). Fats which are rich in unsaturated fatty acids are typically oils. Esterification of the fatty acids to make fats greatly diminishes the hydrophilic nature of the polar end of the original fatty acid. Consequently, fats are very nonpolar and do not form micelles readily. Fats are used to store energy.

Some lipids, such as glycerophospholipids and some sphingolipids, have a very nonpolar end containing a phosphate. These molecules readily form lipid bilayers and are important in forming membranes surrounding cells (Figure 10.5). In this case, the polar portions face outwards, towards water, and the nonpolar moieties associate with each other inside the bilayer.

Another class of lipids, called steroids, is a large group of molecules that includes cholesterol and is only weakly amphiphilic due to few polar groups (Figure 10.9). Cholesterol is a prominent component of lipid bilayers, but its bulky shape tends to disrupt the regularity of the membrane.