Light regulation of the Calvin cycle. The light reactions of photosynthesis transfer electrons out of the thylakoid lumen into the stroma and transfer protons 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. The stromal pH is also increased (the concentration of H⁺ is lowered) in the light as a result of proton pumping from the stroma to the thylakoid lumen. Each of these concentration changes helps couple the Calvin cycle reactions to the light reactions.