Electron Transport Chains

This diagram shows how the movement of protons across a membrane is coupled to the movement of electrons along the electron transport chain. Notice in this diagram of the chloroplast how two protons are passed with two electrons from P-680 to the protein labeled f that then releases the hydrogen but passes the electrons on to the next protein.

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Membrane of the chloroplast incorporates a light-driven system of pigments and other molecules that translocate protons inward. Two photons, or quanta of light, must be absorbed for each electron carried from water to the ultimate acceptor of electrons, NADP+. For each pair of electrons passing through the chain of carriers three protons are taken up outside the chloroplast and four appear inside. The first photon is absorbed by an array of chlorophyll molecules, the antenna complex associated with specialized chlorophyll designated P-680. Two electrons freed from P-680 molecules cross the membrane and are replaced by electrons taken from a molecule of water. The electrons from P-680, with two protons from outside the membrane, reduce plastoquinone (PQ) to PQH₂. The protons are released inside when the electrons are transferred to cytochrome f. The electrons then proceed through plastocyanin (PC) to a second photocenter, P-700. With the absorption of additional photons the electrons complete their journey through an iron-sulfur protein (FeS), ferredoxin (Fd) and flavin adenine dinucleotide (FADH₂) to NADP+. The synthesis of ATP in the CF₁-F₀ complex is the only process in chloroplasts known to require a proton gradient; three protons apparently cross the membrane for each ATP molecule formed.