Figure 7.

A model of Ca²⁺ flux in the intact chloroplast. Dark-adapted chloroplasts contain micromolar levels of free Ca²⁺ in the stroma, and Mg²⁺ ions are sequestered in the thylakoid lumen (A). The light-stimulated release of Mg²⁺ from the thylakoid lumen reduces the trans-thylakoid membrane potential (B). The Ca²⁺/H⁺ antiporter pumps Ca²⁺ into the thylakoid lumen and prevents Ca²⁺-mediated inhibition of CO₂ fixation (C). Adaptation to the dark prompts the electroneutral exchange of Ca²⁺ and Mg²⁺ across the thylakoid membrane and results in Calvin-Benson-cycle inhibition by Ca²⁺ in the dark (D).