Final answer:
Humans cannot digest cellulose because they lack the enzyme cellulase which is required to break down the β1→4 glycosidic bonds in cellulose. Cellulose acts as dietary fiber in humans, aiding in the digestive process but not providing metabolic energy.
Step-by-step explanation:
Humans cannot obtain metabolic energy from cellulose because humans do not have an enzyme that can hydrolyze the β1→4 glycosidic links in cellulose polymers. This type of linkage gives cellulose its strength and rigidity which is essential to plant cells but makes it indigestible by human digestive enzymes.
While cellulose is a complex carbohydrate made of glucose units, these units are bound together in a way that is not accessible by human enzymes. Cellulase, the enzyme needed to break down these bonds, is not produced by the human body. However, certain herbivores and other organisms have symbiotic bacteria that produce cellulase, enabling them to use cellulose as a source of glucose for energy.
In humans, cellulose passes through the digestive system as dietary fiber, which aids in digestion by increasing the bulk of the food and promoting mobility through the gastrointestinal tract, but it does not yield any metabolic energy.
Humans cannot obtain metabolic energy from cellulose because they lack the enzyme cellulase which is needed to break down the ß-glycosidic linkages in cellulose polymers. While cellulose can be hydrolyzed into glucose by certain microorganisms, humans do not possess these microorganisms in their digestive tracts.