Why can humans digest starch, but not cellulose?
The only difference from linear starch is beta-1-4 links instead of alpha-1-4 links, but it makes a world of difference in properties. Starch is a polysaccharide and the main ingredient used by plants to store glucose that can then be made into energy. When people consume starch, an enzyme in their bodies breaks the bonds between the glucose molecules, allowing the glucose to be absorbed easily into the bloodstream. The human body then sends the glucose or sugar to whatever parts of the body need energy or stores it. The sugar is stored as a special polymer called glycogen that can be used for energy when needed. Like starch, cellulose is a polysaccharide. In spite of this, cellulose differs chemically from starch in that instead of its being a chain of glucose molecules, it’s a two-dimensional structure with glucose and hydrogen units bonded together with polymers. This makes cellulose more stable.