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ATP Synthesis by Photophosphorylation:- The Approximate Stoichiometry of Photophosphorylation Has Been Established

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  P741

2026-06-24

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ATP Synthesis by Photophosphorylation:- The Approximate Stoichiometry of Photophosphorylation Has Been Established

As electrons move from water to NADP in plant chloroplasts, about 12 H move from the stroma to the thy lakoid lumen per four electrons passed (that is, per O2 formed). Four of these protons are moved by the oxygen-evolving complex, and up to eight by the cytochrome b6 f complex. The measurable result is a 1,000-fold difference in proton concentration across the thylakoid membrane (ΔpH=3). Recall that the energy stored in a proton gradient (the electrochemical potential) has two components: a proton concentration difference (ΔpH) and an electrical potential (Δ Ψ) due to charge separation. In chloroplasts, pH is the dominant component; counterion movement apparently dissipates most of the electrical potential. In illuminated chloroplasts, the energy stored in the proton gradient per mole of protons is

so the movement of 12 mol of protons across the thylakoid membrane represents conservation of about 200 kJ of energy—enough energy to drive the synthesis of several moles of ATP (ΔG=30.5 kJ/mol). Experimental measurements yield values of about 3 ATP per O2 produced.

At least eight photons must be absorbed to drive four electrons from H₂O to NADPH (one photon per electron at each reaction center). The energy in eight photons of visible light is more than enough for the syn thesis of three molecules of ATP. ATP synthesis is not the only energy-conserving re action of photosynthesis in plants; the NADPH formed in the final electron transfer is (like its close analog NADH) also energetically rich. The overall equation for noncyclic photophosphorylation (a term explained be low) is