Chemiosmotic Theory

The chemiosmotic theory, proposed by Peter Mitchell in 1966, explains the crucial missing link in the light reaction's energy story: how does moving electrons through a membrane-bound carrier chain actually get converted into ATP? According to this theory, as electrons are transported along the thylakoid membrane through PS I and PS II, connected in between by the cytochrome b6-f complex, the resulting flow of electrical current arises specifically from a difference in proton (H+) electrochemical potential that builds up across the membrane, rather than from the electron flow doing any chemical work directly on ADP. Because water photolysis happens on the lumenal side of the thylakoid membrane, and the chain's primary electron acceptors are positioned on the outer, stromal-facing side, protons accumulate steadily inside the thylakoid lumen as electron transport proceeds - the theory notes that H+ concentration inside the lumen can rise 1000 to 2000-fold above the surrounding stroma. Since the stroma correspondingly ends up relatively proton-poor by comparison, a steep proton concentration gradient (also described as a proton motive force) is established across the thylakoid membrane. This gradient is not simply an inert difference in concentration - it represents stored potential energy, and that energy is only released, and only converted into ATP, when protons are allowed to flow back down their concentration gradient from the lumen into the stroma through the CF0 channel portion of the ATP synthase enzyme; this proton flow drives the CF1 head of the same enzyme to phosphorylate ADP and inorganic phosphate (Pi) into ATP. This is exactly wh …