Photoelectron Transport from Energized Reaction Center Chlorophyll a Produces a Charge Separation

The absorption of a photon of light of wavelength «680 nm by chlorophyll a increases its energy by 42 kcal/mol (the first excited state). Such an energized chlorophyll a molecule in a plant reaction center rapidly donates an electron to an intermediate acceptor, and the electron is rapidly passed on to the primary electron acceptor, quinone Q, on the stromal surface of the thylakoid membrane. This light-driven electron transfer, called photoelectron transport, depends on the unique environment of both the chlorophylls and the acceptor within the reaction center. Photoelectron transport, which occurs nearly every time a photon is absorbed, leaves a positive charge on the chlorophyll a close to the luminal surface and generates a reduced, negatively charged acceptor (Q) near the stromal surface (Figure 8-33).

The Q" produced by photoelectron transport is a powerful reducing agent with a strong tendency to transfer an electron to another molecule, ultimately to NADP+. The positively charged chlorophyll a+, a strong oxidizing agent, attracts an electron from an electron donor on the luminal surface to regenerate the original chlorophyll a. In plants, the oxidizing power of four chlorophyll a+ molecules is used, by way of intermediates, to remove four electrons from 2 H2O molecules bound to a site on the luminal surface to form O2:

2 H2O " 4 chlorophyll a"

These potent biological reductants and oxidants provide all the energy needed to drive all subsequent reactions of photosynthesis: electron transport, ATP synthesis, and CO2 fixation.

Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 8-32). Such absorption raises

Primary electron Light Reaction acceptor

Primary electron Light Reaction acceptor

Chlorophyll a

Strong reducing agent (electron donor)

Stroma

Stroma

Charge separation

Thylakoid membrane

Lumen

Charge separation

Thylakoid membrane

Chlorophyll a

Lumen

Strong oxidizing agent (electron acceptor)

▲ FIGURE 8-33 Photoelectron transport, the primary event in photosynthesis. After absorption of a photon of light, one of the excited special pair of chlorophyll a molecules in the reaction center (left) donates an electron to a loosely bound acceptor molecule, the quinone Q, on the stromal surface of the thylakoid membrane, creating an essentially irreversible charge separation across the membrane (right). The electron cannot easily return through the reaction center to neutralize the positively charged chlorophyll a.

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