In addition to the triglycerides and phospholipids, steroids occupy an important place in the lipids group. Steroids have a structure characterised by a four-ring system derived from perhydrocyclopentanophenanthrene (top left).
The link between the A and B rings can occur in both cis (middle) and trans (right) forms.
An example of a cis-steroid is cholic acid. An example of a trans-steroid is androsterone.

An important steroid that plays a health role is the steroid alcohol cholesterol (bottom of the Illustration). This plays an important role in membrane structure, but is also responsible for heart and arterial diseases. It is an amphipatic steroid with a rigid, flat structure.

The alcohol function (polar head) orientates itself within a membrane towards the aqueous phase. The skeletal ring (apolar tail) orientates itself towards the fatty acid tails (see Illustration CG 06).