15 - Lipids are membrane constituents and function as carbon stores

Abstract

Lipids are subdivided into glycerolipids, shingolipids, and steroids. Glycerolipids are fatty acid esters of glycerol. Triacylglycerols consist of a glycerol molecule that is esterified with three fatty acids. Whereas in animals triacylglycerols serve primarily as an energy store, they function in plants mainly as a carbon store in seeds, which are used by humans 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 constituents of membranes. The polar glycerolipids are amphiphilic molecules, consisting of a hydrophilic head and a 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. Other membrane lipids in plants are sphingolipids, which are important constituents of plasma membranes. The sterols are also amphiphilic. The hydroxyl groups form the hydrophilic head and the sterane skeleton with the side chain serves as the hydrophobic tail. In addition to the sterols shown here, plants contain a large variety of other sterols as membrane constituents, many of them present in the outer membrane of mitochondria, in the membranes of the endoplasmatic reticulum, and in the plasma membrane. Sterols determine to a large extent the properties of these membranes.