Which site within the chloroplast hosts the Calvin cycle?

• A. Stroma
• B. Thylakoids
• C. Grana
• D. Nucleus

Answer: (A)

The Calvin cycle runs in the stroma, the fluid inside the chloroplast surrounding the thylakoid membranes. This is where the enzymes needed for carbon fixation and sugar synthesis are located, using ATP and NADPH produced by the light-dependent reactions. In the stroma, RuBisCO fixes CO2 and a series of steps convert it into glyceraldehyde-3-phosphate, ultimately building sugars like glucose. The thylakoid membranes and grana host the light-dependent reactions that generate ATP and NADPH, so they’re involved in providing energy and reducing power but not in the Calvin cycle itself. The nucleus is a separate organelle and not part of chloroplast carbon fixation.