Glycerolipids are fatty acid esters of glycerol. Triacylglycerols (also called triglycerides) consist of a glycerol molecule that is esterified with three fatty acids. In contrast to animals, in plants triacylglycerols do not serve as an energy store but mainly as a carbon store in seeds, and they are used as vegetable oils. In polar glycerolipids, the glycerol is esterified with only two fatty acids and a hydrophilic group is linked to the third –OH group. These polar lipids are the main constituent of membranes. The polar glycerolipids are amphiphilic molecules, consisting of a hydrophilic head and hydrophobic tail. This property enables them to form lipid bilayers, in which the hydrocarbon tails are held together by hydrophobic interactions and the hydrophilic heads protrude into the aqueous phase, thus forming the basic structure of a membrane. Since the middle C atom of the glycerol in a polar glycerolipid is asymmetric, a distinction can be made between the two esterified groups of glycerol at the C-1 and C-3 positions. Plants often contain unusual fatty acids in their storage lipids (e.g., with conjugated double bonds, carbon-carbon triple bonds, or hydroxyl-, ketoor epoxy groups). By now more than 500 of these unusual fatty acids are known. There are also glycerolipids in which the carbon chain is connected with the glycerol via an ether linkage.
