Monosaccharides
Sources
The most common naturally occurring monosaccharides are D-glucose, D-mannose, D-fructose, and D-galactose among the hexoses and D-xylose and L-arabinose among the pentoses. In a special sense, D-ribose and 2-deoxy-D-ribose are ubiquitous because they form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively; these sugars are present in all cells as components of nucleic acids.
| Some naturally occurring monosaccharides | |
|---|---|
| sugar | sources |
| L-arabinose | mesquite gum, wheat bran |
| D-ribose | all living cells; as component of ribonucleic acid |
| D-xylose | corncobs, seed hulls, straw |
| D-ribulose | as an intermediate in photosynthesis |
| 2-deoxy-D-ribose | as constituent of deoxyribonucleic acid |
| D-galactose | lactose, agar, gum arabic, brain glycolipids |
| D-glucose | sucrose, cellulose, starch, glycogen |
| D-mannose | seeds, ivory nut |
| D-fructose | sucrose, artichokes, honey |
| L-fucose | marine algae, seaweed |
| L-rhamnose | poison-ivy blossom, oak bark |
| D-mannoheptulose | avocado |
| D-altroheptulose | numerous plants |
D-Xylose, found in most plants in the form of a polysaccharide called xylan, is prepared from corncobs, cottonseed hulls, or straw by chemical breakdown of xylan. D-Galactose, a common constituent of both oligosaccharides and polysaccharides, also occurs in carbohydrate-containing lipids, called glycolipids, which are found in the brain and other nervous tissues of most animals. Galactose is generally prepared by acid hydrolysis (breakdown involving water) of lactose, which is composed of galactose and glucose. Since the biosynthesis of galactose in animals occurs through intermediate compounds derived directly from glucose, animals do not require galactose in the diet. In fact, in most human populations the majority of people do not retain the ability to manufacture the enzyme necessary to metabolize galactose after they reach the age of four, and many individuals possess a hereditary defect known as galactosemia and never have the ability to metabolize galactose.
D-Glucose (from the Greek word glykys, meaning “sweet”), the naturally occurring form, is found in fruits, honey, blood, and, under abnormal conditions, in urine. It is also a constituent of the two most common naturally found disaccharides, sucrose and lactose, as well as the exclusive structural unit of the polysaccharides cellulose, starch, and glycogen. Generally, D-glucose is prepared from either potato starch or cornstarch.
D-Fructose, a ketohexose, is one of the constituents of the disaccharide sucrose and is also found in uncombined form in honey, apples, and tomatoes. Fructose, generally considered the sweetest monosaccharide, is prepared by sucrose hydrolysis and is metabolized by humans.
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