The Calvin cycle takes place in which part of the chloroplast?

• A. Outer membrane
• B. Stroma
• C. Thylakoid membranes
• D. Thylakoid lumen

Answer: (B)

The main concept here is where the Calvin cycle operates inside the chloroplast. It happens in the stroma, the fluid-filled space that surrounds the stacks of thylakoids. In the stroma, the enzymes that fix carbon dioxide, like RuBisCO, use energy from ATP and reducing power from NADPH—both produced by the light-dependent reactions in the thylakoid membranes—to build sugars. The outer membrane is just a boundary, the thylakoid membranes host the light reactions, and the thylakoid lumen is the internal space of the thylakoids, not where the Calvin cycle runs. So, the Calvin cycle takes place in the stroma.