Lesson 7 of 12 · 6 min
Chemiosmotic hypothesis of ATP synthesis
NCERT §11.6.3
The lumen inside Kavya's rice-leaf thylakoids is, for a few minutes after sunrise, measurably more acidic than the stroma around it — and that pH difference is what actually pays for ATP.
The lesson in notes
In short
ATP synthesis in chloroplasts is linked to a proton gradient across the thylakoid membrane, with protons accumulating inside the thylakoid lumen rather than in the stroma.
Three things build the gradient: water splitting on the lumen side releases H⁺ into the lumen; a proton carrier in the membrane moves H⁺ from stroma to lumen as electrons pass through the photosystems; and NADP reductase on the stroma side takes up H⁺ from the stroma while reducing NADP⁺.
The result is fewer protons in the stroma and more in the lumen, so pH falls in the lumen.
The membrane is impermeable to protons, so they can return to the stroma only through the channel of ATP synthase.
ATP synthase has two parts: CF₀, embedded in the thylakoid membrane, forms the transmembrane channel for protons; CF₁ protrudes on the stroma-facing surface.
Proton flow through CF₀ causes a conformational change in CF₁ that makes ATP; energy from the gradient is used for this.
So chemiosmosis requires four things: ATP synthase, a proton pump, a proton gradient and a membrane. In the stroma, the ATP and NADPH produced are used straight away to fix CO₂.