Carbohydrates are members of a very abundant and widespread class of natural organic substances that includes the sugars, starch, and cellulose. Many carbohydrates have the general formula Cx(H2O)x, but the class is so broad that no simple definition encompasses them all. One of the most common classification schemes for carbohydrates divides them into four major groups—monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Molecules of monosaccharides, or simple sugars, contain from three to nine carbon atoms, most commonly five or six. Three of the most important simple sugars are glucose (also known as dextrose, grape sugar, or corn sugar), fructose (fruit sugar), and galactose. Two simple-sugar molecules are linked to each other in a disaccharide, or double sugar. The disaccharide sucrose (table sugar) consists of one molecule of glucose and one molecule of fructose. Lactose (milk sugar) and maltose are also disaccharides. Oligosaccharides, which consist of three to six monosaccharide units, are rare. Polysaccharides are large molecules, such as cellulose, starch, and glycogen, in which as many as 10,000 monosaccharide units are linked together. They include most of the structural and storage carbohydrates found in nature.