The Chemiosmotic Hypothesis

Having established, in the preceding sections, that the light reaction pumps and generates protons on one specific side of the thylakoid membrane -- from the splitting of water directly, and from electron transport through the cytochrome b6f complex -- this section explains precisely how that proton gradient is converted into the chemical energy of ATP. The explanation is called the chemiosmotic hypothesis, proposed by the British biochemist Peter Mitchell, and it applies, with only minor variations, to ATP synthesis in mitochondria as well as chloroplasts -- a genuinely unifying insight across two very different-looking organelles.

Two separate processes together concentrate protons (H+) specifically within the thylakoid lumen, on the inner side of the thylakoid membrane, while simultaneously depleting protons from the surrounding stroma. The first is the splitting of water at Photosystem II, which, as already described, releases its protons directly into the lumen. The second is the passage of electrons through the cytochrome b6f complex, which actively pumps additional protons from the stroma into the lumen as electrons pass through it. Working in the opposite direction, NADP+ reductase, in reducing NADP+ to NADPH at Photosystem I, consumes protons drawn from the stromal side, further lowering the stromal H+ concentration. The net result of these processes acting together is a steep electrochemical gradient across the thylakoid membrane: a high concentration of H+ inside the lumen, and a comparatively low concentration of H+ in the surrounding stroma.

Because the thylakoid membrane is, like other biological membranes, generally impermeable to the free diffusion of ions such as H+, this gradient cannot simply dissipate on its own; the only route by which protons can flow back down their concentration gradient, from the lumen into the stroma, is through a specific channel provided by the enzyme ATP synthase. This enzyme is built from two functionally distinct parts: a membrane-embedded channel portion, called CF0, which spans the thylakoid membrane and provides the actual pore through which protons pass, and a bulbous, catalytic headpiece called CF1, which projects out from the membrane into the stroma. As protons flow down their gradient through CF0, this flow drives a physical, mechanical rotation of part of the enzyme complex, and this rotation is directly coupled to conformational changes in CF1 that catalyse the addition of an inorganic phosphate group to ADP, forming ATP. …