Question

Assertion: Dark reaction occurs only at night in the stroma of chloroplast.
Reason: CO₂ fixation occurs only during C₃ cycle.

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

Answer 1

Answer: (D)

Dark reaction is also known as lightindependent 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. $CO_2$ fixation occurs in both $C_3$ and $C_4$ cycle. In $C_3$ cycle, $CO_2$ 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 $CO_2$ fixation (takes place in mesophyll) is a 4-carbon compound, oxaloacetic acid. It is seen in some tropical plants.