Sterol

Sterols, or steroid alcohols are a subgroup of steroids with a hydroxyl group in the 3-position of the A-ring. They are amphipathic lipids synthetised from acetyl-coenzyme A. The overall molecule is quite flat. The hydroxyl group on the A ring is polar. The rest of the aliphatic chain is non-polar.

Sterols of plants are called phytosterols and sterols of animals are called zoosterols. The most important zoosterols are cholesterol and some steroid hormones; the most important phytosterols are campesterol, sitosterol, and stigmasterol.

Sterols play essential roles in the physiology of eukaryotic organisms. For example cholesterol forms part of the cellular membrane where its presence affects the cell membrane's fluidity and serves as secondary messenger in developmental signaling.

Plant sterols are also known to block cholesterol absorption sites in the human intestine thus helping to reduce cholesterol in humans.

In humans sterols act to provide important signals and metabolic communications eg. circadian rhythms, blood clotting.