Charge neutralization Chlorophyll

The chlorophyll-protein complexes are oriented in the lamellar membranes in such a way that the electron transfer steps at the reaction centers lead to an outward movement of electrons. For instance, the electron donated by Photosystem II moves from the lumen side to the stromal side of a thylakoid (see Figs. 1-10 and 5-19). The electron that is donated back to the trap chi (Pgg0) comes from H20, leading to the evolution of 02 by Photosystem II (Eq. 5.8). The 02 and the H+ from this reaction are released inside the thylakoid (Fig. 5-19). Because 02 is a small neutral molecule, it readily diffuses out across the lamellar membranes into the chloroplast stroma. However, the proton (H+) carries a charge and hence has a low partition coefficient (Chapter 1, Section 1.4A) for the membrane, so it does not readily move out of the thylakoid lumen. 

Figure 19.20. Electron Flow Through Photosystem I to Ferredoxin. Light absorption induces electron transfer from P700 down an electron-transfer pathway that includes a chlorophyll molecule, a quinone molecule, and three 4Fe-4S clusters to reach ferredoxin. The positive charge left on P700 is neutralized by electron transfer from reduced plastocyanin.

A 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. 

Stable chlorophyll 7r-cation radicals may be generated by either chemical or electrolytic one-electron oxidation of the neutral species, and their properties have been extensively studied [46]. These cation radicals are of particular significance, as there is evidence that radicals of Chi or BChl dimers are formed during the primary charge separation processes in green plant and bacterial photosynthesis, respectively [47, 48]. 

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