The thylakoid membrane is asymmetrically organized, or "sided," like the mito-chondrial membrane. It also shares the property of being a barrier to the passive diffusion of H+ ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H+ ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic.
A proton-motive force of approximately —250 mV is needed to achieve ATP synthesis. This proton-motive force, ap, is composed of a membrane potential, a^, and a pH gradient, apH (Chapter 21). The proton-motive force is defined as the free energy difference, AG, divided by 9, Faraday's constant:
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