Sugar, term applied loosely to any of a number of chemical compounds in the carbohydrate group that are readily soluble in water; are colorless, odorless, and usually crystallizable; and are more or less sweet in taste. In general, all monosaccharides, disaccharides, and trisaccharides (see Carbohydrate) are termed sugars, as distinct from polysaccharides such as starch, cellulose, and glycogen. Sugars, which are widely distributed in nature, are manufactured by plants during the process of photosynthesis and are found in many animal tissues (see Sugar Metabolism). Ribose, a monosaccharide sugar containing five carbon atoms in its molecule, is a constituent of the nuclei of all animal cells; five-carbon sugars are known as pentoses. Trioses (three-carbon sugars), tetroses (four-carbon sugars), heptoses (seven-carbon sugars), octoses (eight-carbon sugars), and nonoses (nine-carbon sugars) are also found in nature, but the most widespread of the sugars are the hexose sugars, characterized by the presence of six carbon atoms in the molecule. The various hexoses, having the same empirical formula and molecular weight, are structural isomers of each other. Each hexose is known in a dextrorotatory and a levorotatory form; in solution a dextrorotatory form will rotate the plane of a beam of polarized light to the right and a levorotatory form will rotate it to the left, but all hexoses taken into the animal body are converted into dextrorotatory forms. The most important of the hexose sugars are glucose and galactose, which are aldehydes, and fructose, which is a ketone, similar to but less reactive than an aldehyde.