There are 16 compounds in the glucose family. Apart from glucose itself, only two of these (mannose and galactose) are common in nature. Glucose is the sugar that occurs naturally in the blood. It is used by the tissues in releasing energy. In plants, glucose is made by photosynthesis. This is a process that makes the glucose from water and carbon dioxide in the presence of light. Animals are not able to make glucose directly. They obtain their glucose by digesting plants or other animals.
Fructose (fruit sugar) is common in plants. It is combined with glucose to make the disac-charide sucrose. A disaccharide consists of two monosaccharide units. Sucrose is also found in honey. Sucrose, or cane sugar, is a temporary energy store for many plants. It must be broken down to its individual monosaccharide residues before it can be absorbed by animals. Maltose and lactose are two other important disaccharides. Maltose is a simple combination of two glucose molecules, it is found in germinating seeds, such as barley, and in the middle stages of the breakdown of more complex sugars. Lactose is the sugar found in milk. It is formed from one molecule each of glucose and galactose.
As with the monosaccharides, the disaccharides are all sweet-tasting soluble solids. Being soluble means that they can be dissolved in a liquid. On the other hand, starch does not taste sweet. It is the common storage product of green plants such as cereals and potatoes. Starch is a combination of two polysaccharides, the highly branched amylopectin and the straight-chained amylose molecule. Each polysaccharide consists entirely of glucose. The mixture of these polysaccharides forms microscopic granules in the storage tissues of green plants. These granules stain blue in contact with iodine.
Glycogen, a glucose storage molecule, is found in animals. In some ways, it is so similar to starch that it is often called "animal starch." These large molecules are useful in storage because their molecular concentration is low. In other words, they do not take up much space. Glycogen is found mainly in the muscles and liver of vertebrates—animals having backbones. Because the glycogen molecule has many branches, the stored glucose can be quickly and easily utilized whenever the tissues need these for energy.
Cellulose is the major constituent of the cell walls of higher plants. It is made up of glucose units in long chains. The fibrous quality of cellulose makes it useful in the textile industry as cotton. Unfortunately, its structure makes it indigestible to most animals. Those that can digest it (such as cows and termites) possess special bacteria in their intestines. These bacteria contain the enzymes needed to break up the long chains of glucose units. Chitin is a similar insoluble compound found in the shells of insects and crustaceans. The basic
building unit here is a nitrogen-containing derivative of glucose. Some carbohydrates occur in combination with different types of molecules. For example, they combine with proteins to form glycoproteins or with fats to form glycolipids.
Honeycontains the monosaccharide fructose, which bees produce from plant nectar. Honey is stored as an energy source in the comb cells of the hive.
Silkwormsconvert the polysaccharide cellulose into silk, a protein with which they make their cocoons. Cellulose is contained in the mulberry leaves on which silkworms feed.
CH.O.COR I 2 CHO.COR
Triglyceride (a lipid)
Lipids are formed by condensation reactions involving glycerol and fatty acids. In the example above, glycerol and fatty acids react to form a triglyceride. This is shown in the downward path (red). The reverse reaction, hydrolysis, converts the triglyceride (or any lipid) to fatty acids and glycerol, as shown in the upward path (blue).
Lipids are a large and very diverse group of compounds that occur naturally—for example, as fats and oils. 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, with or without a group of atoms at one end. This chain makes them relatively insoluble (unable to dissolve) in water. Lipids, however, 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. They are esters made up 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. The fatty acids are usually long-chain acids that are 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 different triglycerides. The presence of large numbers of unsaturated fatty acids lowers the melting point of the triglyceride. Unsaturated molecules have "space" for atoms or groups of atoms to attach to them. They are not "full." Thus, only a low melting point is needed, because the molecules are "ready" to take on other atoms or groups of atoms. The presence of a high pro-
portion of short-chain acids also lowers the melting point. Margarine is made solid by artificially saturating vegetable oils, which are high in 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 of these is in forming cell membranes (the walls of cells). 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. If spread onto water, a phospholipid forms a single-molecule layer on the surface. The top of each molecule (composed of fatty acids) does not absorb water. The bottom of each molecule (composed of the phosphoric acid) does absorb water.
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 not so simple, having a number of different substances incorporated into them.
Insoluble (hydrophobic) fatty acid chains
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Phospholipids are lipids in which one fatty acid has been replaced by a phosphate group and then reacted with an alcohol. This creates a complex molecule (A). A soluble (hydrophilic) polar end is linked to insoluble (hydrophobic) fatty acid chains. When a phospholipid is spread on water, the molecules align themselves with the polar end in the water and the fatty acids outside the water. In theory, a simple membrane could be formed by two such monolayers (B). The fatty acids would form the membrane, the barrier between the water on each side. However, in practice, the membrane is unstable. Stability may be achieved by the combination of protein molecules with a phospholipid bilayer. But theories differ regarding the actual structure. The proteins are either lined up on either side of the phospholipid layer (C) or interspersed within the layer (D).
-------- Glycerol residue
Soluble (hydrophilic) phosphate ester
C Phospholipid molecule
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Biochemistry: Lipids 109
For example, membranes contain minute pores lined with protein. These pores allow substances to pass through selectively. Other proteins on the inside or outside of the dual skin layer perform other specialized functions.
Other types of lipids are also important in forming membranes. The second largest group are called sphingolipids. These 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 amino alcohol. There are three main types of sphingolipids. The sphingomyelins are the simplest and most common. They occur in the myelin layers that surround nerve cells.
The second type, cerebrosides, have no phosphorus. Instead, they contain 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 are called gangliosides. They possess a very large end containing several carbohydrate units and 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. Two lipids—linoleic acid and linolenic acid—cannot be synthesized in the body. But they are necessary for the maintenance of health and are, therefore, called essential fatty acids. A deficiency of them can result in kidney failure and retarded growth. Such conditions are rare in the developed countries of the West because these acids occur in seed oils and fish. These foodstuffs are generally included in a balanced diet. Triglycerides make up most (around 98 per cent) of the lipids in food.
The average Western diet is high in fats. This factor is now known to be one of the causes of the high incidence of heart disease, strokes, and other cardiovascular disorders in developed countries. Cardiovascular disorders affect the heart and the blood vessels. Animal fats, such as those in dairy products and red meat, are high in saturated fats. They may be partly responsible for these disorders because they cause a build-up of abnormal fatty patches on the inner lining of the walls of arteries. This condition—called atherosclerosis-can eventually lead to the blocking of vital arteries. If the blockage happens to occur in the coronary artery—the artery that gives the heart its blood supply—a heart attack may follow.
Several other factors, such as smoking and high blood pressure, are also involved. And 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, probably protects against atherosclerosis. On the other hand, increased levels of another type, LDLP, along with the fatlike compound cholesterol, encourage atherosclerosis.
Palm nutsare a source of vegetable oil that has provided food and fuel to humans from early times. Other common sources of vegetable oils include olives, sunflower seeds, and nuts such as groundnuts and walnuts.
Most nerve fibersconsist of axons surrounded by fat-rich medullary sheaths. The photomicrograph shows nerves in cross section. The lipids making up the medullary sheaths are stained to appear as dark rings around the transparent axons.
Cholesterol,a steroid, has certain properties in common with lipids. Its carbon structure, however, is based on a ring system rather than a system of chains.