Question

Assertion (A): Dark phase reactions take place at night.
Reason (R): Dark phase is independent of light and hence is called the light-independent phase.
Options:-
a) Both A and R are correct and R is the correct explanation of A.
b) Both A and R are correct but R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.
e) Both A and R are false.

Answer 1

Explanation:

Correct option is

D

Both Assertion and Reason are incorrect.

Dark reaction is also known as the light-independent phase. Unlike, light reaction, it does not require light as an essential factor, thus, can take 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 PGA. In C

4

cycle the first product of CO

2

fixation (takes place in mesophyll) is a 4 - carbon compound, oxalo acetic acid. It is seen in some tropical plants. Thus, the correct answer is option D.

Answer 2

The correct option is- d) A is false but R is true.

• Dark phase reactions take place in the stroma of chloroplast and involve the conversion of water and carbon dioxide into sugars. By convention, they are called dark reactions but they do not occur at night.
• They are also called light-independent reactions as they do not require sunlight for the reactions to happen. Rather, they depend upon the products of light reactions such as ATP and NADPH.