Question

Dark reactions of photosynthesis occur in
A. Granal thylakoid membranes
B. Stromal lamella membranes
C. Stroma outside photosynthetic lamellae
D. Periplastidial space

Answer 1

Hint: Dark reactions utilize the product of light reactions. The stromal enzymes do not require light but require ATP and NADPH produced in the light reaction.

Complete answer
The dark reactions of photosynthesis occur in the stroma of the chloroplast where they utilize the products of the light reaction. In the dark reaction, plants use carbon dioxide with ATP and NADPH from the light reactions to produce glucose. Dark reactions occur during the day but since the reaction does not use the light directly hence it is known as the dark reaction. Food is actually synthesized in dark reactions.

Additional information:

Light reactions	Dark reactions
It takes place only in the presence of light.	It can take place in the presence or absence of sunlight.
It is a photochemical phase.	It is a biochemical phase.
It takes place in the grana of the chloroplast.	It occurs in the stroma of the chloroplast.
NADP utilizes H+ ions to form NADPH.	The hydrogen of NADPH combines with CO2
The end products are ATP and NADPH.	Glucose is the end product. ATP and NADPH help in the formation of glucose.
The water molecules split into hydrogen and oxygen.	Glucose is produced. CO₂ is utilized in the dark reaction.
Photolysis occurs in PS-II.	Photolysis does not occur.

So, the correct answer is ‘Stroma outside photosynthetic lamellae’.

Note:
- Dark reaction is also called carbon-fixing reaction.
- The dark reaction occurs both in the presence and absence of sunlight.
- Calvin, Benson and Bassham are the three scientists behind the discovery of dark reactions, so this cycle is also called the Calvin- Benson- Bassham cycle.