Question

The following question consist of two statements, assertion and reason. While answering the question, you are required to choose any one of the following five responses.

Assertion: Each molecule of ribulose-1,5-bisphosphate fixes one molecule of $C{O}_2$.

Reason: Three molecules of NADPH and two ATP are required for fixation of one molecule of $C{O}_2$.

• A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
• B. If assertion is true but the reason is false.
• C. If both assertion and reason are true but the reason is not the correct explanation of the assertion.
• D. If both assertion and reason are false.
• E. If assertion is false but the reason is true.

Answer 1

The correct option is B If assertion is true but the reason is false.

Assertion-Reason relation

In $C_3$ plants, fixation of carbon dioxide involves a molecule of ribulose-1,5-bisphosphate. It combines with $C{O}_2$ with the help of enzyme RuBP carboxylase. This forms two molecules of 3-phosphoglycerate (3C).

Hence, the given assertion statement- “Each molecule of ribulose-1,5-bisphosphate fixes one molecule of $C{O}_2$” is true.

Assertion-Reason relation

Calvin cycle, also known as dark reaction uses energy molecules like ATP and NADPH to produce glucose by carbon dioxide fixation.

In order to fix a single molecule of carbon dioxide, three molecules of ATP and two molecules of NADPH are required.

Hence, the given reason statement- “Three molecules of NADPH and two ATP are required for fixation of one molecule of $C{O}_2$” is false.

Final answer

(C) If assertion is true but the reason is false.