Sterols

The   sterol   cholesterol is a main   constituent   of animal   plasma membranes but is absent from prokaryotes.  Fused ring system of cholesterol means, which it is more rigid than other membrane lipids.  As well as being important elements of membranes and cholesterol is the metabolic precursor of the steroid hormones.  Plants contain little cholesterol but have instead a number of other sterols, mostly β-sitosterol and stigmasterol that differ from cholesterol only in their aliphatic side-chains.