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pheophytin The first electron acceptor in the light-dependent reactions of photosynthesis. It accepts an electron from the excited form of photosystem II (see photosystems I and II), and passes it on via another acceptor, QA, to plastoquinone. Pheophytin is a form of chlorophyll a in which the magnesium ion is replaced by two hydrogen ions. It participates in the crucial step of converting light energy to chemical energy.