Final answer:
Humans cannot digest cellulose because we lack the enzyme cellulase required to break down its beta-glycosidic linkages, whereas we can digest starch due to the activity of a-glucosidases on its alpha-glycosidic linkages. Some animals can digest cellulose with the help of microorganisms in their digestive tracts that produce cellulase. Cellulose in human diets aids in gut mobility and waste absorption.
Step-by-step explanation:
Humans can digest food that contains starch because we have specific enzymes, called a-glucosidases, that can breakdown the alpha-glycosidic linkages in starch.
However, we are unable to digest cellulose because our digestive system lacks beta-glucosidase, or cellulase, the enzyme necessary to break down the beta-glycosidic linkages found in cellulose.
While starch and cellulose are both polysaccharides made of glucose molecules, the types of glycosidic linkages in them are different, contributing to their differing digestibilities.
Animals like cows, horses, and sheep, as well as termites, can digest cellulose because they host microorganisms in their digestive tracts that produce cellulase.
Including cellulose in human diets increases the bulk of the food, which aids in maintaining the mobility of the food through the gastrointestinal tract and supports the stretch reflex leading to defecation.
Cellulose is also beneficial as it absorbs waste in the large intestine, promoting digestive health.