Photosynthetic Phosphorylation

PHOTOSYNTHETIC PHOSPHORYLATION in chloroplasts derives the energy needed for making ATP from light. As in oxidative phosphorylation, hydrogen ions are transported across the membrane to create a proton gradient, and ATP is synthesized as the protons flow back across the membrane down the gradient. In chloroplasts, however, the direction of proton flow is reversed; the light driven movement of electrons pumps protons inward, making the interior acidic, and phosphorylation is driven by an outward flow. Moreover, stoichiometry, or ratio of reactant molecules and ions, is different from that in mitochondria. Each two electrons cross the membrane only twice, translocating only four protons, and for each molecule of ATP formed three protons must pass through the enzyme complex, which is designated CF₁-F₀.

(click on image for larger version)