Which statement about the stroma is correct?

Which statement about the stroma is correct?

• A. It is the site of the light-dependent reactions
• B. It is outside the chloroplast
• C. It is the double membrane around the chloroplast
• D. It is the fluid-filled interior of the chloroplast where the Calvin cycle occurs

Answer: (D)

The important idea here is how chloroplasts are organized and where different steps of photosynthesis take place. The stroma is the fluid-filled interior of the chloroplast. It is the site of the Calvin cycle, where carbon dioxide is fixed into sugars using ATP and NADPH produced by the light-dependent reactions. The stroma contains the enzymes of the cycle (including RuBisCO) and the substrates the cycle needs, all in the chloroplast’s interior. In contrast, the light-dependent reactions occur on the thylakoid membranes, not in the stroma, and the outside of the chloroplast is the cytosol. The double membrane that surrounds the chloroplast is the chloroplast envelope, not the stroma. So the statement that the stroma is the fluid-filled interior of the chloroplast where the Calvin cycle occurs is the correct description.

The important idea here is how chloroplasts are organized and where different steps of photosynthesis take place. The stroma is the fluid-filled interior of the chloroplast. It is the site of the Calvin cycle, where carbon dioxide is fixed into sugars using ATP and NADPH produced by the light-dependent reactions. The stroma contains the enzymes of the cycle (including RuBisCO) and the substrates the cycle needs, all in the chloroplast’s interior.

In contrast, the light-dependent reactions occur on the thylakoid membranes, not in the stroma, and the outside of the chloroplast is the cytosol. The double membrane that surrounds the chloroplast is the chloroplast envelope, not the stroma. So the statement that the stroma is the fluid-filled interior of the chloroplast where the Calvin cycle occurs is the correct description.