FIGURE 12-41 Photoelectron transport, the primary event in photosynthesis. After absorption of a photon of light, one of the excited special-pair chlorophyll a molecules in the reaction center (left) donates, via several intermediates (not shown), 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). Subsequent transfers of this electron release energy that is used to generate ATP and NADPH (see Figures 12-43 and 12-44). The positively charged chlorophyll a⁺ generated when the light-excited electron moves to Q is eventually neutralized by the transfer to the chlorophyll a⁺ of another electron. In plants, the oxidation of H₂O to O₂ provides this neutralizing electron and takes place in a multiprotein complex called photosystem II (see Figure 12-44). Photosystem I uses a similar photoelectron transport pathway, but instead of oxidizing water, it receives an electron from a protein carrier called plastocyanin to neutralize the positive charge on chlorophyll a⁺ (see Figure 12-44).