For glucose in solution, equilibrium favors the ring conformation because it is more stable than the straight-chain conformation. There are two ring conformations of glucose, based on the spatial orientation of the hydroxyl group attached to the number 1 carbon. The location of the hydroxyl group either below or above the plane of the ring produces two types of glycosidic linkage. One is an alpha linkage; the other is a beta linkage. These linkages determine the three-dimensional structure of polysaccharides made from glucose. Starch is an energy-storage molecule in plants. It is composed of glucose monomers in the alpha conformation. Starch comes in two forms: amylose and amylopectin. The glucose units in amylose are linked by alpha-1,4-glycosidic bonds, giving amylose a distinctive spiral structure. The glucose units in amylopectin are also linked by alpha-1,4-glycosidic bonds, but they can branch off the main chain through alpha-1,6-glycosidic bonds. Plant starch is a mixture of 25% amylose and 75% amylopectin. Cellulose, the primary structural component of plant cell walls, is made from glucose molecules linked by beta-1,4-glycosidic bonds. These linkages orient each glucose molecule upside down relative to adjacent monomers, producing straight, unbranched molecules and allowing for extensive hydrogen bonding among hydroxyl groups of parallel cellulose molecules. Few organisms have enzymes that can break the beta-1,4-glycosidic bonds in cellulose. Grass-eating ruminants such as cows and deer have bacteria in their guts that can break these bonds, and so they can derive energy from cellulose molecules. Glycogen, a polymer of glucose, is the primary energy-storage molecule in animals. Its highly branched structure consists of chains of glucose molecules linked by alpha-1,4-glycosidic bonds and connected to one another by alpha-1,6-glycosidic bonds. Glycogen is much more extensively branched than amylopectin. Glycogen accounts for up to 10% of the liver mass and 2% of the muscle mass in mammals.