Figure 23.11 Light regulation of the Calvin cycle. The light reactions of photosynthesis transfer electrons out of the thylakoid lumen into the stroma, and they transfer H⁺ from the stroma into the thylakoid lumen. As a consequence of these processes, the concentrations of NADPH, reduced ferredoxin (Fd), and Mg²⁺ in the stroma are higher in the light than in the dark, whereas the concentration of H⁺ is lower in the light. Each of these concentration changes helps couple the Calvin-cycle reactions to the light reactions.