A monosaccharide is a simple sugar whereas a polysaccharide consists of chains of monosaccharides or disaccharides bonded together. Both types of molecules are sugars that can be used by organisms as sources of energy.
Monosaccharides include sugars like the well-known glucose; fructose, a sugar found in fruits; and galactose, found in milk. In animals, the better-known polysaccharide is glycogen, whereas in plants, the more common polymers are starch and cellulose. Glycogen and starch are the forms in which the monosaccharide glucose is stored. Cellulose plays a structural role in the walls of plant cells.
Monosaccharides differ from one another in their configurations or the number of carbon atoms they contain. Glucose contains six carbon atoms while deoxyribose, the sugar in DNA, has five carbon atoms. Fructose, glucose and galactose are isomers of one another, meaning they have the same number and types of atoms, but their forms vary.
With oxygen present, monosaccharides can be broken down to release energy for use in cells. Joining chains of the monosaccharide glucose can yield either starch or glycogen. The liver stores glycogen until it is needed; starch can be found in the roots of plants. Glycogen consists of glucose connected in a branching manner; starch is found as a linear chain of glucose molecules. When energy is needed and glucose is in short supply in the cells, a signal is sent out to mobilize glycogen or starch, depending on the type of organism. These two polysaccharides are broken down to release glucose for cells to use.