20.7:
The Z-Scheme of Electron Transport in Photosynthesis
During photosynthesis, the electron transport sequence from water to NADP+ follows a Z-shaped trajectory and is therefore called the Z-scheme.
It begins with water hydrolysis that supplies electrons to the oxidized P680 or PSII reaction center.
After reduction, P680 absorbs photons and transfers an excited electron to PSII's primary electron acceptor–pheophytin.
The reduced pheophytin transfers electrons across a series of acceptor molecules between PSII and PSI, starting from an electron carrier– plastoquinone, followed by a cytochrome b6f complex, and a mobile electron carrier– plastocyanin.
When plastocyanin is reduced, it transfers an electron to the oxidized P700 or PSI reaction center.
The reduced P700 then absorbs light and relays an excited electron to PSI's unique electron acceptor– ferredoxin, followed by ferredoxin-NADP+ reductase.
Finally, ferredoxin-NADP+ reductase utilizes the electrons to convert NADP+ to NADPH.
Overall, the redox changes during the Z-scheme generate an electrochemical proton gradient that drives ATP synthase to produce ATP.
20.7:
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.
Suggested Reading
- Shevela, Dmitriy, Govindjee, and Lars Olof Björn. "Evolution of the Z-scheme of photosynthesis: a perspective." Photosynthesis Research 133, no. 1 (2017): 5-15.