20.6: The Z-Scheme of Electron Transport in Photosynthesis
The light reactions of photosynthesis assume a linear flow of electrons from water to NADP+. During this process, light energy drives the splitting of water molecules to produce oxygen. However, oxidation of water molecules is a thermodynamically unfavorable reaction and requires a strong oxidizing agent. This is accomplished by the first product of light reactions: oxidized P680 (or P680+), the most powerful oxidizing agent known in biology. The oxidized P680 that acquires an electron from the water-splitting process has a high positive redox potential. Similarly, the light reaction’s final product, NADPH, is a strong reducing agent having a high negative redox potential. Therefore, the movement of electrons from P680+ to NADPH is downhill in terms of their redox potential. When all the electron carriers between P680+ to NADPH are arranged in a sequence along a redox potential scale, a characteristic pattern is generated- called the Z-scheme.
Z-scheme describes the oxidation and reduction changes during two-light reactions of photosynthesis. The two light reactions were experimentally discovered by Robert Emerson in 1957 and later by Robin Hill and Fay Bendall in 1960, who published the theoretical Z-scheme of photosynthesis. The Z-scheme has inspired many studies that led to the development of clean, renewable, and low-cost energy systems. Analogous to the Z-scheme in natural photosynthesis, artificial photosynthesis has been developed to produce solar fuels such as hydrogen gas.
Artificial photosynthesis involves a photon-absorbing center and a catalytic center, with an electron transfer pathway joining the two centers. The photon-absorbing center triggers photolysis of water, generating molecular oxygen and protons. The protons thus formed are then reduced by the catalytic center to produce hydrogen gas. For instance, ‘artificial leaves’ are silicon-based devices that work on the Z-scheme principle, producing hydrogen energy in a clean way.
