What are carbohydrates?
The carbohydrates are a large and diverse group of organic compounds that includes sugars and starches. Carbohydrates perform a number of major functions in living systems. For instance, one type of sugar (deoxyribose) is a building block of deoxyribonucleic acid (DNA), the molecule that carries hereditary information.
Other sugars are needed for the cell walls. Simple carbohydrates are used in the synthesis of amino acids and fats or fatlike substances, which are used to build cell membranes and other structures. Macromolecular carbohydrates function as food reserves. The principal function of carbohydrates, however, is to fuel cell activities with a ready source of energy.
Carbohydrates are made up of carbon, hydrogen, and oxygen atoms. The ratio of hydrogen to oxygen atoms is always 2:1 in simple carbohydrates. This ratio can be seen in the formulas for the carbohydrates ribose (C5H10O5), glucose (C6H12O6), and sucrose (C12H22O11). Although there are exceptions, the general formula for carbohydrates is (CH2O)n, where n indicates that there are three or more CH2O units.
Types or Classification of Carbohydrates
Carbohydrates can be classified into three major groups on the basis of size: monosaccharides, disaccharides, and polysaccharides.
Simple sugars are called monosaccharides (sacchar = sugar); each molecule contains three to seven carbon atoms. The number of carbon atoms in the molecule of a simple sugar is indicated by the prefix in its name.
For example, simple sugars with three carbons are called trioses. There are also tetroses (four-carbon sugars), pentoses (five-carbon sugars), hexoses (six-carbon sugars), and heptoses (seven-carbon sugars). Pentoses and hexoses are extremely important to living organisms. Deoxyribose is a pentose found in DNA. Glucose, a very common hexose, is the main energy-supplying molecule of living cells.
Disaccharides (di = two) are formed when two monosaccharides bond in a dehydration synthesis reaction. For example, molecules of two monosaccharides, glucose, and fructose, combine to form a molecule of the disaccharide sucrose (table sugar) and a molecule of water.
Similarly, the dehydration synthesis of the monosaccharides glucose and galactose forms the disaccharide lactose (milk sugar). It may seem odd that glucose and fructose have the same chemical formula, even though they are different monosaccharides. The positions of the oxygens and carbons differ in the two different molecules; consequently, the molecules have different physical and chemical properties.
Two molecules with the same chemical formula but different structures and properties are called isomers (iso = same). Disaccharides can be broken down into smaller, simpler molecules when water is added. This chemical reaction, the reverse of dehydration synthesis, is called hydrolysis (hydro = water; lysis = to loosen).
A molecule of sucrose, for example, may be hydrolyzed (digested) into its components of glucose and fructose by reacting with the H+ and OH– of water. The cell walls of bacterial cells are composed of disaccharides and proteins, which together are called peptidoglycan.
Carbohydrates in the third major group, the polysaccharides, consist of tens or hundreds of monosaccharides joined through dehydration synthesis. Polysaccharides often have side chains
branching off the main structure and are classified as macromolecules. Like disaccharides, polysaccharides can be split apart into their constituent sugars through hydrolysis. Unlike monosaccharides and disaccharides, however, they usually lack the characteristic sweetness of sugars such as fructose and sucrose and usually are not soluble in water.
One important polysaccharide is glycogen, which is composed of glucose subunits and is synthesized as a storage material by animals and some bacteria. Cellulose, another important glucose polymer, is the main component of the cell walls of plants and most algae.
Although cellulose is the most abundant carbohydrate on Earth, it can be digested by only a few organisms that have the appropriate enzyme. The polysaccharide dextran, which is produced as a sugary slime by certain bacteria, is used in a blood plasma substitute.
Chitin is a polysaccharide that makes up part of the cell wall of most fungi and the exoskeletons of lobsters, crabs, and insects. Starch is a polymer of glucose produced by plants and used as food by humans. Digestion of starch by intestinal bacteria is important to human health. Many animals, including humans, produce enzymes called amylases that can break the bonds between the glucose molecules in glycogen.
However, this enzyme cannot break the bonds in cellulose. Bacteria and fungi that produce enzymes called cellulases can digest cellulose. Cellulases from the fungus Trichoderma are used for a variety of industrial purposes. One of the more unusual uses is producing stone-washed or distressed denim. Because washing the fabric with rocks would damage washing machines, cellulase is used to digest, and therefore soften, the cotton.