Monosaccharides are the simplest form of a large and diverse group of organic compounds called carbohydrates. They are water soluble substances that contain more than one hydroxyl group (OH) and a carbonyl group (where a carbon and oxygen atom are linked via a double bond). Monsaccharides are either polyhydroxy aldehydes or ketones depending on whether the carbonyl group is formed from a terminal carbon (aldoses), or not (ketoses). They are grouped by the number of carbon atoms they contain; trioses have three, tetroses four, pentoses five, and hexoses six. The smallest monosaccharides are the two forms with a three-carbon skeleton: glyceraldehyde and dihydroxyacetone. Pentoses and hexoses are most common, but monosaccharides may contain as many as nine carbons. They are named by combining a prefix that may designate the number of carbons in the molecule or some other descriptive feature, and a generic suffix -ose.
Important pentoses include xylose, and arabinose, both of which are components of plant cell walls, and ribose and deoxyribose, which are found in ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively. Common hexoses include glucose, galactose, (both aldoses) and fructose (a ketose).
Glucose is found in many fruits and is the sugar that circulates in the blood of higher animals. It is also the basic sugar unit in important polysaccharides such as starch, glycogen and cellulose, which may contain many thousands of glucose units. Fructose is the building block for the polysaccharide, inulin, found in the roots and tuber of dahlias and Jerusalem artichoke. Inulin serves as a commercial source for fructose, which because it is the sweetest of all monosaccharides, is valued as an ingredient in many soft drinks. Honey, composed of approximately 80% fructose and glucose, was almost the only source of sugar available in ancient times. Sucrose, or table sugar, which has replaced honey as the most used sweetener, is a disaccharide made up of glucose and fructose. Lactose, the disaccharide that makes up 2-8% of mammalian milk, is composed of galactose and glucose.
Cane sugar (sucrose) was introduced to Europe by the Moors about 700 A.D., although it had been used in parts of the orient long before that. Cane production for European use was most prevalent in North Africa, the Canary Islands, and Madeira. Sugar beets were not exploited as a source of sucrose until about 1800. The development of sugar beets as a commercial crop was encouraged by Napoleon when the French were not able to import cane sugar because Britain controlled the seas. The use of sucrose has expanded enormously in the last hundred years. Today, the world production, almost entirely from sugar cane and sugar beets, is in excess of ten million tons annually.
When humans ingest sucrose it is hydrolyzed to glucose and fructose by sucrase enzymes in the intestinal mucosa. The monosaccharides are then further metabolized to provide needed energy.
Important derivatives of monosaccharides include ascorbic acid (vitamin C), a derivative of glucose, and sorbitol and mannitol, sweetening agents derived from glucose and mannose, respectively.