Question

Assertion :Dark reaction occurs only at night in the stroma of chloroplast. Reason: $C{O}_2$ fixation occurs only during $C_3$ cycle.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
• C. Assertion is correct but Reason is incorrect.
• D. Both Assertion and Reason are incorrect.

Answer 1

The correct option is D Both Assertion and Reason are incorrect.

Dark reaction is also known as light-independent phase. Unlike, light reaction, it does not require light as an essential factor. Thus, can takes place both in the presence or absence of light. The term dark reaction does not mean that it takes place only in dark period or at night. $C{O}_2$ fixation occurs in both $C_3$ and $C_4$ cycle. In $C_3$ cycle, $C{O}_2$ is added by the enzyme, RuBisco to a $5$ carbon compound RuBP that is converted to $2$ molecules of $3$-carbon PGA. In $C_4$ cycle the first product of $C{O}_2$ fixation(takes place in mesophyll) is a $4$-carbon compound, oxaloacetic acid. It is seen in some tropical plants.