Lipids are a large and diverse group of compounds that occur naturally—as fats and oils, for example. The only difference between a fat and an oil is the melting point An oil is simply fat that is liquid at room temperature. Lipids contain long chains, or ring systems, of carbon atoms, often with a characteristic group of atoms (such as a carboxylic acid) at one end. While this chain makes them relatively insoluble in water, lipids are soluble in organic solvents such as ether, trichloromethane (chloroform), and benzene. This solubility accounts for many of their biological properties.
The simplest and most common lipids are fats and oils called triglycerides, which make up about 98 per cent of the lipids in food. Triglycerides are esters composed of glycerol (a type of alcohol) and residues of three fatty acids. (Esters are a class of compounds produced by reaction between acids and alcohols with the elimination of water. Fatty acids are usually long-chain acids, found in most, but not all, of those compounds classified as lipids.) Many natural fats and oils, such as butter and olive oil, are mixtures of several triglycerides. The presence of large numbers of unsaturated fatty acids lowers the melting point of a triglyceride. (Unsaturated molecules contain carbon-carbon double bonds.) The presence of a high proportion of short-chain acids also helps lower the melting point Margarine is made solid by artificially saturating vegetable oils, which have a large proportion of unsaturated chains.
Along with carbohydrates, lipids are the main sources of energy in the diet. But they also have a number of other important functions, the most important being the formation of cell membranes. All cells, plant and animal, take advantage of the hydrophobic (water-avoiding) properties of the fatty-acid chain. Substituting one of the fatty acids in a triglyceride with a phosphoric acid molecule gives the complex a hydrophilic (water-attracting) end that mixes easily with water. If this phosphate residue is chemically combined with an alcohol, the product is a phospholipid. Spread on water, a phospholipid forms a single-molecule layer. The top of each molecule (composed of fatty acids) does not absorb water, whereas the bottom (composed of the phosphoric acid) does.
In principle, a cell membrane could consist of two such monolayers. The hydrophobic fatty-acid chains, which do not absorb water would be oriented toward each other. The molecules of phosphoric acid, which mix easily with water, would face the water on either side of this “sandwich.” Such a cell membrane would form a skin less than one-millionth of an inch thick. In practice, however, cell membranes are more complicated, having a number of different substances incorporated into them.
Other types of lipids are also important in forming membranes. The second largest group, called sphingolipids, are similar to phospholipids in that they both contain a phosphate. Sphingolipids are made up of a fatty acid running parallel to and linked to sphingosine, a long-chain molecule that contains both an amino and an alcohol group.
There are three main types of sphingolipids. The sphingomyelins, the simplest and most common, occur in the myelin layers around nerve cells. The second type, cerebrosides, have no phosphorus, containing instead a carbohydrate molecule, usually a sugar. Galactose is a sugar residue in cerebrosides found in the brain. Glucose is found in the cerebrosides of non-nerve tissue. The third group of sphingolipids, called gangliosides, possess a very large end containing several carbohydrate units. Sphingolipids are found in highest concentration in the gray matter of the brain.
Lipids in the diet
Most lipids can be made in the body from carbohydrates or proteins, but two essential fatty acids—linoleic acid and linolenic acid—cannot be synthesized in the body, though they are necessary for good health. A deficiency of these fatty acids can result in kidney failure and retarded growth, but these conditions are rare in developed countries where diets include seed oils and fish, both rich in these acids.
The average Western diet is high in fats, which helps explain the high incidence of heart disease, strokes, and other cardiovascular disorders in developed countries. Cardiovascular disorders affect the heart and blood vessels. Animal fats, such as those in dairy products and red meat, are high in saturated fats. They contribute to these disorders by causing atherosclerosis, a build-up of abnormal fatty patches on the inner lining of the walls of arteries, which may eventually lead to the blocking of vital arteries. A blockage of the coronary artery (the artery that supplies blood to the heart) may cause a heart attack.
Diets rich in animal fats are known to increase the levels of lipoproteins in the blood Lipoproteins are complexes of lipid and protein that allow lipids to be carried from the liver to the tissues. One type of lipoprotein, called HDLP, seems to protect against atherosclerosis. On the other hand, increased levels of another type of lipoprotein, LDLP, along with the fatlike compound cholesterol, promote atherosclerosis.