Question

Cows can digest cellulose, but humans can't. Why not?

Short answer

Humans cannot digest cellulose because they lack both a specialized chamber (rumen) in their digestive system and the enzyme cellulase, which cows possess. Although human gut bacteria can partially ferment cellulose in the large intestine, the resulting glucose is not efficiently absorbed, providing little energy contribution.

Step-by-step solution

Understanding Cellulose

Cellulose is a complex carbohydrate found in plant cell walls. It is made of glucose units connected by beta-glycosidic linkages. Cellulose provides structural support and rigidity to plants. However, breaking down cellulose is crucial for digestive processes in herbivores because it releases the glucose needed for energy.

Digestive System of Cows

Cows, being herbivores, have evolved a specialized digestive system called the rumen to digest cellulose efficiently. The rumen is a large fermentation chamber filled with microorganisms (bacteria and protozoa), which produce an enzyme called cellulase. Cellulase breaks down the cellulose into glucose through a fermentation process. After the cellulose is broken down, cows can absorb glucose and use it for their energy needs.

Digestive System of Humans

Unlike cows, humans have a monogastric (single-chambered) digestive system, which lacks a specialized chamber like the rumen. Humans are omnivores and primarily rely on easily digestible carbohydrates in their diet, such as starch and sugars. Our digestive system does not produce cellulase and, consequently, does not efficiently break down cellulose.

Role of Gut Bacteria

Even though our digestive system is not equipped to break down cellulose, humans have gut bacteria that can partially ferment cellulose. However, the fermentation takes place in the large intestine, and the resulting glucose from cellulose is not efficiently absorbed into the bloodstream. This limited fermentation contributes little to our energy needs and cannot be considered true digestion of cellulose.

Conclusion

Humans cannot digest cellulose because we lack the necessary specialized chamber (rumen) and enzyme (cellulase) in our digestive system to break down cellulose into glucose. While small amounts of cellulose may get broken down by the gut bacteria, this does not provide us with a significant source of energy like it does for cows and other herbivores.