Earth Science: Carbon – Lipids and Fatty Acids
Lipids and fatty acids are essential for the nutrition and health of all living organisms. Butter or margarine, candle wax, and olive oil consist mostly of these water-insoluble substances.
Plants and animals produce these water-insoluble or hydrophobic (derived from the Greek word meaning “water repelling”) substances that serve as structural elements in cell membranes, energy reservoirs, or as messenger substances. Lipids are divided into various categories: fatty acids, triacylglycerides (fats and oils), waxes, phospholipids, sphingolipids, lipopolysaccharides, and isoprenoids (such as steroids and carotenoids).
Fatty acids are un-branched hydrocarbons that have carbonic acid at their end. The simplest fatty acid, butanoic acid, contains a chain of four carbon atoms and is a monocarbonic acid. The carbon atoms are connected by simple or multiple bonds. The former are said to be saturated, while those with at least one double bond are called unsaturated fatty acids. Fats, such as butter, contain triacylglycerides, the largest group of dietary lipids.
These consist of a sugar alcohol, glycerin, and three molecules of fatty acids. The principal components of animal membranes are phospholipids. They are distinguished from triacyl- glycerides in that the glycerin molecule is esterized with two long fatty acids only. There is a phosphoric acid molecule at the third carbon atom of glycerin, which forms the hydrophilic (derived from the Greek word meaning “water loving”) or polar head of the phospholipid.
Yet, the fatty acids are hydrophobic and form the nonpolar tail. In contrast, steroids such as cholesterol are structured differently. These lipids have four rings and one hydrocarbon chain. Cholesterins are located in outer membranes and assure that these become more “rigid”—they increase the melting point of a membrane. Therefore fish have very little cholesterol, otherwise their mem- branes would become rigid and they would die.
Apart from these simple lipids, there are also complex lipids in nature. These are made up—just like lipoproteins—of several components. They consist of several lipids and a single protein and transport cholesterols to and from individual cells.
BASICS
LIPIDS function as: energy suppliers energy storage membrane building blocks signal molecules hormones fat-soluble vitamins pigments