Lipids: Water-Insoluble Molecules

The lipidsare a chemically diverse group of hydrocarbons. The property they all share is insolubility in water which is due to the presence of many nonpolar covalent bonds. Nonpolar hydrocarbon molecules are hydrophobic and preferentially aggregate among themselves, away from water, which is polar. When these nonpolar molecules are sufficiently close together, weak but additive van der Waals forces hold them together. These huge macromolecular aggregations are not polymers in a strict chemical sense, since their units (lipid molecules) are not held together by covalent bonds, as are, for example, the amino acids in proteins. But they can be considered polymers of individual lipid units. In this section, we will describe different types of lipids.

Lipids have a number of roles in living organisms:

· Fats and oils store energy.

· Phospholipids play important structural roles in cell membranes.

· The carotenoids help plants capture light energy.

· Steroids and modified fatty acids play regulatory roles as hormones and vitamins.

· The fat in animal bodies serves as thermal insulation.

· A lipid coating around nerves acts as electrical insulation.

· Oil or wax on the surfaces of skin, fur and feathers repels water.

Fats and oils store energy

Chemically, fats and oils are triglycerides, also known as simple lipids. Triglycerides that are solid at room temperature (20°C) are called fats; those that are liquid at room temperature are called oils. Triglycerides are composed of two types of building blocks: fatty acids and glycerol. Glycerol is a small molecule with three hydroxyl (—OH) groups (an alcohol). A fatty acid is made up of a long nonpolar hydrocarbon chain and a polar carboxyl group (—COOH). A triglyceride contains three fatty acid molecules and one molecule of glycerol. The carboxyl group of a fatty acid can form a covalent bond with the hydroxyl group of glycerol, resulting a functional group called an ester and water. The three fatty acids in a triglyceride molecule need notall have the same hydrocarbon chain length or structure: In saturatedfatty acids, all the bonds between the carbon atoms in the hydrocarbon chain are single bonds— there are no double bonds. That is, all the bonds are saturated with hydrogen atoms. These fatty acid molecules are relatively rigid and straight and they pack together tightly, like pencils in a box.

In unsaturatedfatty acids, the hydrocarbon chain contains one or more double bonds. Oleic acid, for example, is a monounsaturated fatty acid that has one double bond near the middle of the hydrocarbon chain, which causes a kink in the molecule. Some fatty acids have more than one double bond—are polyunsaturated— and have multiple kinks. These kinks prevent the molecules from packing together tightly. The kinks in fatty acid molecules are important in determining the fluidity and melting point of a lipid. The triglycerides of animal fats tend to have many long-chain saturated fatty acids, packed tightly together; these fats are usually solids at room temperature and have a high melting point. The triglycerides of plants, such as corn oil, tend to have short or unsaturated fatty acids. Because of their kinks, these fatty acids pack together poorly and have a low melting point, and these triglycerides are usually liquids at room temperature. Fats and oils are marvelous storehouses for energy. When they take in excess food, many animal species deposit fat droplets in their cells as a means of storing energy. Some plant species, such as olives, avocados, sesame, castor beans and all nuts, have substantial amounts of lipids in their seeds or fruits that serve as energy reserves for the next generation. This energy can be tapped by people who eat these plant oils or use them for fuel. Indeed, the famous German engineer Rudolf Diesel used peanut oil to power one of his early automobile engines in 1900.

Phospholipids form the core of biological membranes

Because lipids and water do not interact, a mixture of water and lipids forms two distinct layers. Many biologically important substances—such as ions, sugars, and free amino acids—that are soluble in water are insoluble in lipids.

Like triglycerides, phospholipidscontain fatty acids bound to glycerol by ester linkages. In phospholipids, however, any one of several phosphate-containing compounds replaces one of the fatty acids. The phosphate functional group has a negative electric charge, so this portion of the molecule is hydrophilic, attracting polar water molecules. But the two fatty acids are hydrophobic, so they tend to aggregate away from water. In an aqueous environment, phospholipids line up in such a way that the nonpolar, hydrophobic “tails” pack tightly together and the phosphate-containing “heads” face outward, where they interact with water. The phospholipids thus form a bilayer, a sheet two molecules thick, with water excluded from the core. Biological membranes have this kind of phospholipid bilayer structure.