Carbohydrates such as starch and sugar are a vital source of fuel for both plants and animals. They are found in much greater concentrations in plants because they are also used in building cell walls. Bacteria also have carbohydrate-based cell walls. Carbohydrates perform a number of vital functions in animals, either alone or combined with proteins and other molecules.
The name carbohydrate indicates that these compounds are generally made up of hydrates of carbon (compounds in which carbon is bound to the hydrogen and oxygen of water). Most carbohydrates fit this definition, but some also contain other elements, such as nitrogen or sulfur. However, the simplicity of this definition disguises the tremendous variety of carbohydrates, as well as the subtle differences in structure that affect their properties and distribution in nature.
The simplest carbohydrates are the monosaccharides (saccharide comes from the Greek word for sugar). Most saccharides are single units made up of unbranched carbon chains between three and seven atoms long. The most common are the trioses (three carbons), pentoses (five), and hexoses (six). These single units can join together to form the other major carbohydrate groups: oligosaccharides, which can contain as few as two monosaccharide units (for example, sucrose), and polysaccharides, which may contain many thousands (for example, starch).
All simple carbohydrates contain a chain of carbon atoms, each joined to a hydroxyl group, which contains oxygen and hydrogen. One carbon atom is linked to an oxygen atom by a double covalent bond. This may be at the end of the molecule as in an aldehyde, in which case it is called an aldose, or it may be similar to a ketone, with the double bond in the middle of the chain, and is then known as a hetose. Glucose and fructose are both hex-ose sugars, but the former is an aldose and the latter a ketose.
The number of atoms alone does not determine the structure of the molecule. Apart from one of the simple triose sugars, all monosaccharides contain at least one asymmetrical, or chiral, carbon atom—a carbon atom bonded to four different atoms, or groups of atoms. A carbon attached to four different groups can be assembled in two different structures that are mirror images of each other. The two structures are known as stereoisomers. Each asymmetric carbon atom in a molecule can give rise to two different stereoisomers. Sugars can have several asymmetrical carbon atoms, and many different stereoisomers.
The structure of glucose is often simplified as a straight chain. In reality, the ends of the chain come together to form a ring. This accounts for two more important types of isomerism. (Isomerism is the phenomenon in which the same number and types of atoms join together in different ways, producing more than one distinct compound. This is why two compounds can have the same chemical makeup but different physical qualities.) For example, a glucose molecule can form either a six- or a five-membered ring. The form with five carbons and one oxygen atom in the ring is the commonest because it is more stable. The position of all groups attached to the cyclic ring (above or below the ring plane) is an important stereochemical feature that helps determine the properties of polysaccharides.
There are 16 compounds in the glucose family (six-carbon monosaccharides), but apart from glucose itself, only 2 of these (mannose and galactose) are common in nature. Glucose, the sugar that occurs naturally in our blood, is used by our tissues in releasing energy. Plants make glucose by photosynthesis, a process requiring water, carbon dioxide, and light. Animals cannot make glucose directly but obtain it by digesting plants or other animals.
Fructose (fruit sugar) is common in plants. It is joined to glucose to make the disaccharide sucrose—common table sugar or cane sugar. (A disaccharide consists of two linked monosaccharide units.) Sucrose is also found in honey. Sucrose is a temporary energy store for many plants, but 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, a simple combination of two glucose molecules, is found in germinating seeds, such as barley, and in the middle stages of the breakdown of more complex sugars. Lactose, the sugar found in milk, is formed from one molecule each of glucose and galactose.
As with the monosaccharides, the disaccharides are all sweet-tasting soluble solids. On the other hand, starch does not taste sweet It is the common storage product of green plants such as cereals, rice, and potatoes. Starch is a combination of two polysaccharides, the highly branched amylopectin and the straight-chained amylose molecule. Both anylopectin and amylose consist entirely of glucose units. The mixture of these polysaccharides forms microscopic granules in the storage tissues of green plants. (These granules stain blue in contact with iodine.;
Glycogen, another glucose storage molecule, is found in animals. It is so similar to starch in some respects that it is often called “animal starch.” These large molecules are useful in storage because they hold many glucose units together in one polymer unit Glycogen is found mainly in the muscles and liver of vertebrates—animals with backbones. Because the glycogen molecule can be broken down into its glucose units, the stored glucose can be quickly and easily utilized for energy.
Cellulose, the major constituent of the cell walls of higher plants, is made up of glucose units in long chains. Its fibrous quality makes it useful in the textile industry as cotton. Unfortunately, its structure makes it indigestible to most animals. Only animals such as cows and termites with special bacteria in their intestines can digest it. These bacteria contain enzymes that 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 in chitin 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.