A sugar consisting of single molecules (e.g., glucose) is called a monosaccharide. Some sugars consist of two sugar molecules joined by glycosidic linages. They are called disaccharides.
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Examples of familiar disaccharides are lactose (a molecule of glucose linked with a molecule of galactose), sucrose or ordinary cane sugar (one molecule of glucose and one of fructose) and maltose, the sugar in malted barley (two molecules of glucose. Three-unit sugars are called trisaccharides or trioses. Trioses are of vital importance in glucose metabolism and in photosynthesis.
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In plants “higher” than fungi the carbohydrate polymers starch and cellulose are among the most important structural and nutritional substances. Starch consists of masses of long chains of alpha glucosyl residues.
By releasing glucose on hydrolysis starch functions as a reserve food, furnishing both energy and carbon. In the formation of cellulose, beta glucosyl residues are used instead of alpha residues.
The strength of wood depends greatly on the tensile strength of the covalent bonds in the chains of glycosyl units. These cellulose fibrils are bundled together lengthwise with a tough “glue,” a very complex substance called lignin.
In all of the “higher” plants the cell walls are cellulose. Masses of these, with lignin, constitute wood. In many bacteria the cell walls are heteropolymers made up of several amino acids and amino sugars.
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They are more fully discussed in the section on cell walls. Glycogen (animal starch), found mainly in animal livers and only rarely in plants, is also a homopolymer of glucose, but the polymer chains are branched.
Several polysaccharides of biological importance are hetero polymers. Some of these, conjugated with lipids (lipopolysaccharides) and other substances are major constituents of the cell walls of many species of bacteria. Most important of all, perhaps, are the polymers made up of phosphated molecules of the pentoses (sugars with five linked carbon atoms) ribose and deoxyribose.
Ribose and deoxyribose phosphate polymers form the very backbone of the chains of nucleotides that make up the nucleic acids RNA and DNA described farther on this chapter. These control and direct all heredity and all of the enzymic activities and resulting properties of very living cell.