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How does a proton gradient power ATP synthesis in chloroplasts?
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photosynthesis
Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
In chloroplasts, the synthesis of ATP is powered by a proton gradient established across the thylakoid membrane during the light reactions of photosynthesis.12
The process functions as follows:
- Gradient Formation: Electron flow through the thylakoid membrane leads to the accumulation of hydrogen ions (protons) inside the thylakoid space. This concentration is increased by the oxidation of water and the activity of plastoquinone, which releases protons into the interior.12
- Potential Energy: Because the thylakoid membrane is impermeable to protons, this accumulation creates a significant proton concentration gradient and an electrical charge across the membrane, storing potential energy.1
- ATP Synthesis: An enzyme complex known as ATP synthase—or the coupling factor—is embedded in the thylakoid membrane. It contains a channel (formed by hydrophobic polypeptides) that allows protons to flow down their concentration gradient from the thylakoid space back into the stroma.12
- Energy Conversion: As protons pass through this channel, the enzyme complex uses the released energy to catalyze the phosphorylation of adenosine diphosphate (ADP) and inorganic phosphate into ATP.123