Polysaccharides break down to disaccharides, and then to monosaccharides (glucose units). This happens by means of hydrolysis reactions catalyzed by enzymes. The reverse process occurs in carbohydrate metabolism. Monosaccharides not broken down into carbon dioxide and water (to provide energy) combine in condensation reactions to produce polysaccharides. In animals, monosaccharides become glycogen, also called "animal starch," in which form polysaccharides can be stored. The polysaccharide starch consists of a chain of numerous alpha glucose units.
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. Although animals are less dependent on the structural properties of carbohydrates, they perform a number of vital functions, either alone or combined with proteins and other molecules.
The name carbohydrate refers to the fact that carbohydrates are generally made up of hydrates of carbon. Hydrates are substances united chemically with water. Thus, a hydrate of carbon consists of hydrogen and oxygen (the two elements in water) and carbon. Most carbohydrates fit this definition, but some do not. Some also contain other elements, such as nitrogen or sulfur. The simplicity of this definition disguises the fact that there is a tremendous variety of carbohydrates. There are also subtle differences in structure that affect their properties and distribution in nature.
The simplest of carbohydrates are the monosaccharides. The name saccharide comes from the word for sugar in Greek. More saccharides are single units made up of un-branched carbon chains between three and seven atoms long. The most common are the trioses (three carbons), pentoses (five), and hexoses (six), which are the best known. These single units can join together to form the other major carbohydrate groups. These are the oligosaccharides, which can contain as few as two monosaccharide units (for example, sucrose) and the 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. Alternatively, it may be similar to a ketone, with the double bond in the middle of the chain. It is then known as a hetose. Aldehydes and ketones are explained in a preceding article in the section on organic chemistry. Glucose and fructose are both hexose 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 carbon atom. This is a carbon atom uniquely attached to the molecule. There is no corresponding carbon atom on the opposite side to give a balanced arrangement. Because of its special shape, a carbon attached to four different groups can arrange the groups into two separate formations that are mirror images of each other. This phenomenon is known as stereoisomerism. Sugars can have several asymmetrical carbon atoms.
For simplicity, the structure of glucose is often shown 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 a phenomenon in which the same number and types of atoms join together in different ways, producing more than one distinct compound. Thus, 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.
Starch (polysaccharide) ■Hydrolysis
1 t 1
OH H,0 H
\l , l/H
c ^f c
Biochemistry: Polysaccharides and sugars 107
Also, the new hydroxy! group in the five-mem-bered ring can be either above (termed beta) or below (alpha) the plane of the ring. This difference between the beta and alpha forms is important in determining the properties of some polysaccharides.