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Fats And Lipids

12 Temmuz 2007

Fats and Lipids

Lipids are a group of organic compounds that are insoluble in water. They include fats, phospholipids, waxes, steroids, and certain vitamins. Lipids have two particularly important functions in living organisms: they are used to make the selectively permeable membranes that surround cells and cell organelles; and they are used to store energy.

Fats

Fats (triglycerides) are used to store energy, lubricate the skin, insulate against the cold, and to cushion or protect certain organs. While carbohydrates and proteins each provide 16–17 kilojoules of energy per gram, 1 gram of fat provides 37 kilojoules. This means that more energy can be stored in the body as fat than by the same weight of carbohydrate or protein.

Structure

Fat molecules are made of the elements carbon, hydrogen, and oxygen. Each consists of a glycerol (C3H8O3) subunit joined to three fatty acid subunits. The glycerol unit is the same in all fats, but the fatty acids can vary, resulting in fats with different properties.

Fatty acids contain a carboxyl group (COOH) attached to a long chain of carbon and hydrogen atoms (a hydrocarbon chain). The fatty acids are joined to glycerol molecules by a type of reaction called condensation, which results in the production of a three molecules of water (H2O). The reverse reaction, called hydrolysis, occurs when fats are broken down during digestion.

Diagram 1

This condensation reaction shows how three fatty acids and one glycerol molecule can join together to make a single molecule of fat.

Saturated and Unsaturated Fats

In Diagram 1, the carbon atoms in the fatty acids are joined by single bonds, and each is joined to the maximum possible number of hydrogen atoms. The resulting fat molecule is described as saturated. In some fats the fatty acid subunits contain some double bonds between carbon atoms, which reduces the number of hydrogen atoms present. Such fats are called unsaturated. The double bonds in unsaturated fats produce kinks in some of the fatty acid side-chains, which reduces the tendency of the fat to solidify. Such fats are usually oils, and are liquid at room temperature.

Human food contains three types of fat: saturated fats (found mostly in meat and dairy products); monounsaturated fats (found in olive oil and avocados); and polyunsaturated fats (found in fish and vegetable oils). Monounsaturated fatty acids contain a single double bond between carbon atoms, and polyunsaturated fatty acids have two or more double bonds. Unsaturated fats can be converted to saturated fats by an chemical process called hydrogenation, which adds hydrogen atoms to the carbon chain in place of any double bonds. This causes vegetable oils to solidify, and is used in the manufacture of margarine.

Phospholipids

Phospholipids are similar to fats, but one of the fatty acid side-chains is replaced by a phosphate group (PO4). The phosphate group is polar (hydrophilic), which means that it can dissolve in water. In contrast, the fatty acid chains are nonpolar (hydrophobic), and are repelled by water. This property enables phospholipids to form the bilayer structure found in cell membranes. This consists of two parallel sheets of phospholipids arranged so that all the polar groups are on the outside, and all the nonpolar groups in the middle.

Diagram 2: An Unsaturated PhospholipidMolecule

This diagram shows the molecular structure of a phospholipid molecule from a cell membrane. Note the double bond in one of the fatty acid chains, which produces a kink in the molecule. The fatty acids are shown here shortened; in reality, they are about three times longer.

Other Lipids

Lipids have other uses besides being membrane components and providing a form of energy storage. Waxes, for example, are large lipid molecules that provide a waterproof covering on skin, fur, leaves, fruits, and the cuticle (outer coat) of insects. Steroids are fat-soluble compounds made of carbon rings. The best-known steroids are the sex hormones testosterone, oestrogen, and progesterone, and their precursor cholesterol. Cholesterol also plays an important role in cell membranes, where it helps to maintain fluidity. Related to the steroids are another group of lipids, called terpenes, which are slightly smaller. These include the fat-soluble vitamins A, E, and K.

Copyright © 1994, 1997 Dorling Kindersley

Kategori: Biyoloji