The protein complex that carries out this reaction is called coupling factor or, more accurately, ATP synthase. ATP synthase is a complex of several proteins, shaped like a mushroom, with the cap on the stromal side of the thylakoid disc and the stalk going through the thy lakoid membrane. The actions of PSII and PSI pump protons into the lumen of the thylakoid. When chloroplasts are actively illuminated, the pH gradient produced by the two photosystems can be as much as four pH units, or a 10,000‐fold difference in hydrogen ion concentration. These are the relative acidities of vinegar and tap water.
The membrane potential in chloroplasts is almost entirely composed of the ΔpH component. The thylakoid membrane is permeable to Mg2+ and Cl − ions, so electrical neutrality is maintained. This differs from mitochondrial ATP synthesis where both a pH and an electrochemical potential exist. Because the chloroplast gradient is primarily ΔpH in nature, three protons must move across the membrane to synthesize an ATP, rather than the two that move during mitochondrial synthesis of a single ATP. The ATP and NADPH from synthesis are both formed on the stromal side of the thylakoid membrane. They are available for the fixation of CO 2, which occurs in the stroma. See Figure 1.