Chemiosmotic Energy Coupling
Some of the energy initially captured by the photosynthetic light reactions is used to power the vectorial movement of protons from the stroma to the thylakoid lumen, thereby forming a transmembrane proton gradient (3). This proton translocation causes a small pH increase of the stroma, and a much larger pH decrease in the lumen due to its relatively small volume. This leads to the non-equilibrium condition of a pH difference of 3 to 4 units across the thylakoid membrane during the photosynthetic light reactions. The rate that protons escape from the lumen to the stroma is relatively fast (t(½) of decay of the trans-membrane proton gradient is ~1 sec). Consequently, the instability of the proton gradient makes it unsuitable for long-term energy storage. Instead, the proton gradient is used by the FₒF₁ ATP synthase as the energy source to drive ATP synthesis. In this manner, the photosynthetic light reactions are coupled to the synthesis of ATP via the proton gradient. Reagents like ammonia that can transport protons across the membrane more rapidly than the FₒF₁ ATP synthase will inhibit ATP synthesis because they collapse the proton gradient. These reagents that uncouple ATP synthesis from electron transfer reactions, called uncouplers, typically increase the rate of the latter reactions by relieving back-pressure from the proton gradient.
Energy coupling occurs because Fₒ serves as an efficient conduit to move protons across the thylakoid membrane and back toward equilibrium with the stroma. Protons move to the stroma in response to the energy gradient that is derived from the concentration difference across the membrane (ΔpH). Because each proton also carries a positive charge, the charge difference across the membrane (ΔΨ) also contributes to this proton energy gradient. A proton concentration difference of about 1000 fold across the membrane (ΔpH = 3) provides sufficient energy for FₒF₁ to drive ATP synthesis. Even though FₒF₁ transports only protons across the membrane, nonequilibrium concentration gradients of other ions like K⁺ can contribute to the energy of the proton gradient by changing the ΔΨ if the ion is permeable to the membrane.
In this chemiosmotic coupling process the magnitude of the energy gradient, designated the proton-motive force (pmf or μ(H⁺)), is related to the trans-membrane concentration and charge differences in mV at 30°C by equation 1. The protonmotive force translates into more conventional energy terms because it is the sum of the free energy derived from the trans-membrane concentration difference (equation 2), and the electrical potential gradient generated by the trans-membrane concentration gradient of charged species (equation 3) where n is the charge on the ion (+1 for protons). This relationship simplifies to equation 4.