Making ATP from a proton gradient.

Count protons on either side of the thylakoid membrane as water splitting, the PQ shuttle and NADP reductase move them; the lumen pH falls and protons can only return through CF0, turning CF1 to make ATP on the stroma side.

Drive lumen pH below 5 and make 30 ATP, then make the membrane leaky and explain why ATP stops even in full light.

Protons accumulate in the stroma; CF1 faces the lumen.

• 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.

Teaching this idea? Put the simulation on your own page or LMS with this code.