Source of ATP for Carbon Fixation—Photophosphorylation Link Carbon fixation in the Calvin cycle requires ATP molecules. These ATP molecules are generated by which photosynthetic process?

Introduction / Context:
The Calvin cycle needs energy (ATP) and reducing power (NADPH) to convert CO2 into carbohydrate. In chloroplasts, ATP for carbon fixation is supplied by photophosphorylation during the light reactions, not by the Calvin cycle itself or by mitochondrial oxidative phosphorylation under typical photosynthetic conditions.

Given Data / Assumptions:

• Light reactions occur in thylakoid membranes and generate ATP and NADPH.
• The Calvin cycle operates in the stroma and consumes ATP and NADPH.
• Plant cells also possess mitochondria, but chloroplast ATP supports stromal reactions.

Concept / Approach:
Photon energy drives electron flow through PSII and PSI, establishing a proton gradient across the thylakoid membrane. ATP synthase uses this gradient to phosphorylate ADP to ATP (photophosphorylation). These ATP molecules are then available in the stroma to power the carboxylation and reduction steps of the Calvin cycle.

Step-by-Step Solution:

Verification / Alternative check:
Isolated chloroplast experiments show light-dependent ATP generation (no Calvin cycle required) and carbon fixation depends on externally supplied ATP/NADPH when light is absent.

Why Other Options Are Wrong:

• Glucose formation does not generate ATP; it consumes ATP and NADPH.
• Chlorophyll replenishment is unrelated to ATP supply for fixation.
• Oxidative phosphorylation in mitochondria primarily supplies cytosolic/mitochondrial ATP, not the stromal pool during active photosynthesis.
• “None of the above” is incorrect because the ETS answer is correct.

Common Pitfalls:
Assuming ATP made anywhere in the cell is interchangeable; compartmentation matters—chloroplast-generated ATP is key for the Calvin cycle.

Final Answer:
Electron transfer system (ETS) during the light reactions, via photophosphorylation