Questions: Question 12 Humans cannot digest cellulose because they - have intestinal flora which use up B-glycosides - lack the necessary enzymes to digest B-glycosides - are poisoned by B-glycosides - cannot digest chlorophyll - are allergic to B-glycosides

The answer is the second one: lack the necessary enzymes to digest β-glycosides.

Explanation for each option:

1. Have intestinal flora which use up β-glycosides: This is incorrect. While humans do have intestinal flora that can break down some components of food, they do not significantly break down cellulose into digestible components for humans.

2. Lack the necessary enzymes to digest β-glycosides: This is correct. Humans lack the enzyme cellulase, which is necessary to break down cellulose, a β-glycoside, into glucose units that can be absorbed.

3. Are poisoned by β-glycosides: This is incorrect. Humans are not poisoned by β-glycosides; they simply cannot digest them.

4. Cannot digest chlorophyll: This is incorrect. While humans cannot digest chlorophyll, this is not the reason they cannot digest cellulose.

5. Are allergic to β-glycosides: This is incorrect. Humans are not generally allergic to β-glycosides; they just lack the enzyme to digest them.

In summary, humans cannot digest cellulose because they lack the necessary enzyme, cellulase, to break down β-glycosides.