Question

The photochemical reaction of photosynthesis occurs in which part of chloroplast?

• A. Outer membrane
• B. Inner membrane
• C. Gra
• D. Stroma

Hint:

Gra Thylakoids = Light Reaction (Photochemical phase: produces ATP, DPH, and $\text{O}_2$).
Stroma = Dark Reaction (Calvin Cycle: consumes ATP and DPH to fix $\text{CO}_2$ into sugar).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the structural compartment within the chloroplast where the light-dependent (photochemical) reactions of photosynthesis take place.

Step 2: Key Formula or Approach:
Photosynthesis is divided into two major phases occurring in distinct structural regions of the chloroplast:
• Photochemical Phase (Light Reactions): Occurs in the thylakoid membranes gra.

• Biochemical Phase (Dark Reactions Calvin Cycle): Occurs in the fluid matrix stroma.

Step 3: Detailed Explation:

• Structure of Gra: Gra (singular: granum) are stacks of disc-like membrane-bound structures called thylakoids suspended within the chloroplast stroma.

• Localization of Photochemical Machinery: The thylakoid membranes within gra harbor light-harvesting complex proteins, Photosystem I (PSI), Photosystem II (PSII), cytochromes $b_6f$, and ATP synthase.

• Light Reactions Process: During the photochemical phase, light absorption excites chlorophyll molecules, driving photolysis of water ($\text{H}_2\text{O} \to 2\text{H}^+ + \frac{1}{2}\text{O}_2 + 2e^-$), non-cyclic/cyclic electron transport, proton pumping, and generation of $\text{ATP}$ and $\text{DPH}$.

• Stroma Function: The stroma houses soluble enzymes like RuBisCO required for $\text{CO}_2$ fixation during the Calvin cycle.

Step 4: Fil Answer:
The photochemical reactions of photosynthesis occur within the gra (thylakoid membranes) of the chloroplast.