A water-soluble hydrocarbon that can be extracted from cells and tissues by nonpolar solvents. Two major classes of lipid can be recognized. Complex or saponifiable lipids comprise all those lipids that yield soaps on alkaline hydrolysis. These include *acylglycerols
, and *sphingolipids
. Simple lipids are a heterogeneous group of compounds, including the sterols, carotenes, and xanthophylls. Although they are called simple lipids they can be very complex structurally; natural rubber is a simple lipid but it is an extremely large and complex polymer.
The functions of lipids within the plant are varied. Storage triacylglycerols provide an energy store, especially in oil-rich seeds such as the castor bean. The phosphoglycerides are important structural components of membranes, while waxes and cutin, components of the cuticle, help control water loss from the plant.