Simulation · Biology · Class 11
Making ATP from a proton gradient
From the lesson Chemiosmotic hypothesis of ATP synthesis in Photosynthesis in Higher Plants. Change the values and watch what happens.
Making ATP from a proton gradientBiology · Class 11
The idea behind it
NCERT §11.6.3
- 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.