Final answer:
Glycogen is a storage form of glucose in animals, starch is a plant glucose storage form, and cellulose is a structural component in plants. Humans digest starch but cannot digest cellulose due to its different glycosidic linkages. Cellulose acts as dietary fiber, whereas glycogen and starch are broken down for energy.
Step-by-step explanation:
Metabolic Fates of Glycogen, Cellulose, and Starch
Glycogen, starch, and cellulose are three polysaccharides composed of glucose units, but they have different structures and functions. Glycogen is a highly branched storage form of glucose found in animals, including humans; it can be quickly mobilized to release glucose when energy is needed. Starch, which includes amylose (linear) and amylopectin (branched), is a storage form in plants, efficiently packing glucose for energy when the plant requires it.
Cellulose, on the other hand, is a linear polymer with glucose units linked through ß-1,4-glycosidic bonds. It serves as a rigid structural component in plant cell walls. Because of the different linkages (ß for cellulose and α for starch and glycogen), cellulose cannot be digested by humans; we lack the enzyme capable of breaking down ß-1,4-glycosidic linkage, which renders it as dietary fiber for us.
The metabolic fate of glycogen involves its hydrolysis to release glucose in response to the body's energy needs. Starch is digested by humans through salivary and pancreatic α-amylase, breaking down into maltose and then further into glucose, which can be used as an energy source or stored. Cellulose, however, is not digested but aids in the digestive process by helping to push food through the gut.