Humans cannot digest cellulose present in plant cell walls because they lack which essential component for its breakdown?

Introduction / Context:
This question explores why humans cannot digest cellulose, even though it is chemically similar to starch and is abundant in plant based foods. Understanding the role of specific digestive enzymes and the structure of cellulose helps explain why cellulose passes through the human gut largely unchanged and functions mainly as dietary fibre.

Given Data / Assumptions:
- Cellulose is a polysaccharide composed of glucose units linked by beta 1,4 glycosidic bonds.- Human digestive enzymes can digest some carbohydrates but not others.- The options include lack of specific enzymes, lack of general carbohydrate enzymes, lack of acid, poor chewing and lack of blood supply.- You must identify what is actually missing in humans for cellulose digestion.

Concept / Approach:
Humans produce enzymes such as amylase that break down starch, which has alpha 1,4 glycosidic bonds. However, humans do not produce cellulase, the enzyme needed to hydrolyse beta 1,4 bonds in cellulose. Some herbivores rely on symbiotic microbes that supply cellulase, allowing them to digest cellulose. Humans lack both endogenous cellulase and significant populations of cellulase producing microbes in the appropriate gut compartments. The correct option therefore refers specifically to the missing enzyme cellulase.

Step-by-Step Solution:
Step 1: Recall that cellulose is a structural polysaccharide in plant cell walls, composed of glucose units linked by beta 1,4 bonds.Step 2: Remember that humans can digest starch through amylase but cannot digest cellulose.Step 3: Recognise that digestion of cellulose would require the enzyme cellulase, which humans do not produce.Step 4: Examine the options and identify which one explicitly mentions the lack of cellulase and its role in breaking beta 1,4 bonds.Step 5: Select option A as the correct explanation.

Verification / Alternative check:
Biology textbooks explain that cellulose passes largely undigested through the human alimentary canal and contributes to dietary fibre.They also note that ruminant animals depend on cellulase producing microbes in their rumen to digest cellulose.These sources confirm that absence of cellulase is the main reason humans cannot digest cellulose.

Why Other Options Are Wrong:
Option B is wrong because humans clearly possess enzymes for digesting starch and other carbohydrates, so it is not a complete lack of carbohydrate enzymes.Option C is wrong because hydrochloric acid helps denature proteins and kill microbes but does not by itself digest cellulose.Option D is wrong because mechanical chewing, while helpful, cannot replace the chemical breakdown performed by specific enzymes.Option E is wrong because the intestine has a rich blood supply; the issue is chemical digestion, not blood flow.

Common Pitfalls:
Students sometimes think that strong stomach acid should be able to dissolve any food component, forgetting that enzymes are highly specific to bond types.Another pitfall is to assume that inability to digest cellulose arises from inadequate chewing rather than biochemical limitations.Remember that cellulose is resistant to human digestive enzymes because humans do not produce cellulase, making cellulose a beneficial but indigestible fibre.

Final Answer:
Humans cannot digest cellulose because they lack the specific enzyme cellulase needed to break the beta 1,4 glycosidic bonds in cellulose.