Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A. N. Ogurtsov 2011

Photosynthetic Systems
Charge Separation in the Photosystem Reaction Center

When a chlorophyll a molecule located within the "special pair"—a chlorophyll dimer near The surface of the reaction center facing the thylakoid lumen—absorbs light at its absorption maximum of 680 nm, it transitions from the ground state to the first excited state, and its energy increases by 42 kcal/mol. The reaction center is structured in such a way that, through quantum-mechanical tunneling, the excited electron leaves the chlorophyll molecule even before it can relax back to the ground state.

The tunneling process within the reaction center along a chain of intermediate electron acceptors terminates at the stromal surface of the thylakoid membrane on the primary electron acceptor, the plastoquinone Q molecule (Figure 180(6)), which is structurally similar to the ubiquinone molecule (Figure 180(a)).

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Figure 180 - Lipophilic electron carriers: (a) ubiquinone in the mitochondrial Respiratory Chain; (b) plastoquinone in the photosynthetic Electron Transport Chain of the chloroplast

Such photon-driven electron transfer along the chain of intermediate acceptors is referred to as photoelectron transport.

The Specific features of photoelectron transport depend on the protein environment surrounding both the chlorophyll and the electron acceptors within the reaction center. As a result of photoelectron transport, a positively charged chlorophyll a molecule (chlorophyll a⁺) remains near the luminal surface of the reaction center, while a negatively charged acceptor (Q^-) is formed near the stromal surface (Figure 181). Thus, photosynthetic charge Separation is achieved within the reaction center.

Figure 181 - Photoelectron transport in the reaction center

Plastoquinone Q is a powerful reducing agent that efficiently transfers the excess electron to other molecules, most notably NADP.

The positively charged chlorophyll a⁺ is a strong oxidizing agent that captures an electron at the luminal surface to regenerate the initial ground state of chlorophyll a.

In plants, this electron abstraction by four chlorophyll a⁺ molecules ultimately results in the removal of four electrons from two Water molecules bound to the luminal surface of the reaction center, yielding molecular oxygen:

These two powerful biological reducing and oxidizing agents serve as the energy source for all subsequent photosynthetic reactions—electron transport, ATP synthesis, and CO₂ fixation.

Chlorophyll a also absorbs photons with wavelengths shorter than 680 nm (higher-energy photons) (Figure 38). As a result of this absorption, the molecule is promoted to one of the higher excited states. However, these higher excited states rapidly relax to the lowest (first) excited state within an extremely short time of about one picosecond (10^-12 s), with the excess energy being dissipated as heat.

Since photoelectron transport processes leading to charge separation invariably originate only from the first excited state of the reaction center chlorophyll a, the quantum yield—the number of photosynthetic events per absorbed photon—is independent of the wavelength of light (for wavelengths shorter, and consequently energies higher, than 680 nm).