<P> Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis . When monosaccharides are not immediately needed by many cells they are often converted to more space - efficient forms, often polysaccharides . In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells . In plants, starch is used for the same purpose . The most abundant carbohydrate, cellulose, is a structural component of the cell wall of plants and many forms of algae . Ribose is a component of RNA . Deoxyribose is a component of DNA . Lyxose is a component of lyxoflavin found in the human heart . Ribulose and xylulose occur in the pentose phosphate pathway . Galactose, a component of milk sugar lactose, is found in galactolipids in plant cell membranes and in glycoproteins in many tissues . Mannose occurs in human metabolism, especially in the glycosylation of certain proteins . Fructose, or fruit sugar, is found in many plants and in humans, it is metabolized in the liver, absorbed directly into the intestines during digestion, and found in semen . Trehalose, a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight . </P> <P> Two joined monosaccharides are called a disaccharide and these are the simplest polysaccharides . Examples include sucrose and lactose . They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other . The formula of unmodified disaccharides is C H O. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable . </P> <P> Sucrose, pictured to the right, is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants . It is composed of one D - glucose molecule and one D - fructose molecule . The systematic name for sucrose, O - α - D - glucopyranosyl - (1 → 2) - D - fructofuranoside, indicates four things: </P> <Ul> <Li> Its monosaccharides: glucose and fructose </Li> <Li> Their ring types: glucose is a pyranose and fructose is a furanose </Li> <Li> How they are linked together: the oxygen on carbon number 1 (C1) of α - D - glucose is linked to the C2 of D - fructose . </Li> <Li> The - oside suffix indicates that the anomeric carbon of both monosaccharides participates in the glycosidic bond . </Li> </Ul>

Form in which carbohydrates are transported in plants