What is the main difference between C3 and C4 carbon fixation?

C3 fixation occurs in mesophyll cells across much of the leaf. In C4 plants, carbon is first fixed in mesophyll cells and then delivered to bundle-sheath cells deeper in the leaf, so the steps are separated between cell types. Choice B describes this difference; opening stomata at night is characteristic of CAM plants, not C4 plants.

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