Carbohydrates include sugars and polymers of sugars. These molecules are used as building materials and sources of energy. Single sugar molecules, called monosaccharides, and linked pairs of sugars, called disaccharides, are important as building blocks and cellular fuels. Polysaccharides are long chains of sugars. A polysaccharide called cellulose is a major component of plant cell walls. Plant cells store sugars in the form of a polysaccharide called starch. Animals store a polysaccharide called glycogen. Polysaccharides on cell membranes act as cell identification tags. Simple sugars, or monosaccharides, are the simplest carbohydrates. Monosaccharides vary in size; their carbon skeletons range from three to seven carbon atoms. They generally have a molecular formula that is some multiple of CH2O. The formula for glucose, for example, is C6H12O6. Monosaccharides are often drawn with linear carbon skeletons, but in water all monosaccharides that contain more than three carbon atoms bend around to form ring structures. The monosaccharide glucose is a major nutrient, central to cellular metabolism. It is broken down for energy in the process of cellular respiration. The carbon skeleton of glucose can also be used to build many other organic molecules, including amino acids and fatty acids. Glyceraldehyde is the monosaccharide that is the energy-storing molecule produced by photosynthesis. Two glyceraldehyde molecules combine to make glucose. The monosaccharide galactose combines with glucose to form lactose, the disaccharide in milk. Fructose is the monosaccharide that is sometimes called fruit sugar, the one that makes apples and berries sweet. Fructose and glucose combine to make the disaccharide sucrose or table sugar. Ribose is the monosaccharide that is an important component of RNA and ATP. A modified form is used in building DNA. Sometimes organisms link sugar molecules in pairs, forming disaccharides. Here are several examples. Plants make sucrose by joining glucose and fructose. Sucrose circulates in plant sap, and we obtain it from sugar cane and sugar beets and use it as table sugar. Lactose is formed by joining galactose and glucose. Lactose is the disaccharide that gives milk its sweet taste. Maltose consists of two linked glucose molecules. Digestion of starch, in a sprouting seed or in the intestine of an animal, produces maltose. Polysaccharides are polymers, long chains consisting of hundreds to thousands of linked monosaccharides. Starch represents a compact stockpile of glucose units linked together in a chain that can be stored by a plant for later use. Glycogen is the polysaccharide that stores glucose for energy in animal cells, especially those in the liver and muscle. Glycogen is the animal equivalent of plant starch, but its molecules are more branched than starch. Cellulose is the polysaccharide that plant cell walls are made of. It is the most abundant organic compound on Earth. Like starch, cellulose is made of glucose, but the chains are made from a different glucose isomer, so they have different properties.