Briefly describe the chemistry of fatty acids, including the number and location of double bonds.

Question

What will be an ideal response?

Answer 1

Like carbohydrates and proteins, lipids are composed of carbon (C), hydrogen (H), and oxygen (O). Because lipids have many more carbons and hydrogens in proportion to their oxygens, they can supply more energy per gram than carbohydrates can. All fatty acids have the same basic structure—a chain of carbon and hydrogen atoms with an acid group (COOH) at one end and a methyl group (CH3) at the other end. Fatty acids may differ from one another, however, in the length of their carbon chains and in the number and location of their double bonds.
Most naturally occurring fatty acids contain even numbers of carbons in their chains—up to 24 carbons in length. The 18-carbon fatty acids are abundant in our food supply. Stearic acid is the simplest of the 18-carbon fatty acids; the bonds between its carbons are all alike.
The long-chain (more than 12 carbons) fatty acids of meats, seafood, and vegetable oils are most common in the diet. Smaller amounts of medium-chain (eight to 12 carbons) and short-chain (up to six carbons) fatty acids also occur, primarily in dairy products.
Stearic acid is a saturated fatty acid, which is fully loaded with all its hydrogen atoms and contains only single bonds between its carbon atoms. A fatty acid with hydrogens missing and at least one double bond is an unsaturated fatty acid. A polyunsaturated fatty acid has two or more carbon-to-carbon double bonds. Linoleic acid, the 18-carbon fatty acid common in vegetable oils, lacks four hydrogens and has two double bonds. A fourth 18-carbon fatty acid is linolenic acid, which has three double bonds.
Fatty acids differ not only in the length of their chains and their degree of saturation, but also in the locations of their double bonds. Chemists identify polyunsaturated fatty acids by the position of the double bond closest to the methyl (CH3) end of the carbon chain, which is described by an omega number. A polyunsaturated fatty acid with its closest double bond three carbons away from the methyl end is an omega-3 fatty acid. Similarly, an omega-6 fatty acid is a polyunsaturated fatty acid with its closest double bond six carbons away from the methyl end.