H⁺ Ions

WHERE DO ALL THE H+ IONS COME FROM FOR TRANSPORT? Students often wonder where the H+ ions come from that are picked up by PQ (forming PQH2) and then transported across the thylakoid membrane. Are they just floating around in the stroma, or what? Remember that the cytosol is actually an aqueous solution, and that water dissociates into H+ and OH- ions to a slight extent (dissociation constant for water is 1 x 10¹⁴), so H+ ions are constantly being provided by the dissociation of water. As H+ ions are removed from the stroma and transported into the thylakoid interior by PQ, the concentration of H+ ions in the stroma decreases, and more water dissociates to provide more H+ ions, and so on. In addition, some H+ ions are returned to the stroma during the process of ATP synthesis. It takes about 3 H+ ions flowing through ATP synthase to produce one ATP. These H+ ions may be picked up by reduced NADP and used in the Calvin Cycle, or they may be returned to the stroma solution. So, there is a constant cycling of H+ ions across the membrane and back again.
As H+ ions are removed from the stroma and transported into the thylakoid interior by PQ, the concentration of H+ ions in the stroma decreases, and the concentration of OH- ions increases. The result is that the stroma becomes more basic, reaching a pH of about 8. At the same time, the thylakoid interior becomes more acidic (pH 4) due to the pumping of H+ ions across the membrane by PQ and the splitting of water. This creates the powerful electrochemical gradient across the membrane that is used to synthesize ATP.