Atp synthesis in chloroplasts is very similar to that in mitochondria: electron transport is coupled to the formation of a proton (h+) gradient across a membrane. The energy in this proton gradient is then used to power atp synthesis. Two types of processes that contribute to the formation of the proton gradient are: processes that release h+ from compounds that contain hydrogen, and processes that transport h+ across the thylakoid membrane.
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Explanation:
principles by which ATP synthesis takes place in chloroplasts are nearly identical with those for oxidative phosphorylation. We have seen how light induces electron transfer through photosystems II and I and the cytochrome bf complex. At various stages in this process, protons are released into the thylakoid lumen or taken up from the stroma, generating a proton gradient. Such a gradient can be maintained because the thylakoid membrane is essentially impermeable to protons. The thylakoid space becomes markedly acidic, with the pH approaching 4. The light-induced transmembrane proton gradient is about 3.5 pH units. As discussed in Section 18.4, energy inherent in the proton gradient, called the proton-motive force (Δp), is described as the sum of two components: a charge gradient and a chemical gradient. In chloroplasts, nearly all of Δp arises from the pH gradient, whereas, in mitochondria, the contribution from the membrane potential is larger. The reason for this difference is that the thylakoid membrane is quite permeable to Cl- and Mg2+. The light-induced transfer of H+ into the thylakoid space is accompanied by the transfer of either Cl- in the same direction or Mg2+ (1 Mg2+ per 2 H+) in the opposite direction. Consequently, electrical neutrality is maintained and no membrane potential is generated. A pH gradient of 3.5 units across the thylakoid membrane corresponds to a proton-motive force of 0.20 V or a ΔG of -4.8 kcal mol-1