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1. The effect of visible light on respiratory activity was studied in two strains of Neurospora crassa, one a wild-type strain able to synthesize carotenoid and the other an albino mutant lacking carotenogenic activity. Light had no effect on growth under the conditions studied, but inhibited respiration of hyphal suspensions, the degree of inhibition being dependent on the carotenoid content of the hyphae. 2. In studies of respiration of isolated mitochondria, three types of photosensitive site were detected. These were the flavoprotein dehydrogenases themselves, a site separate from the latter also associated with the dehydrogenase but re-activatable by treatment with a thiol reagent, and the respiratory quinone, ubiquinone. Cytochrome oxidase, previously reported as photosensitive from many sources, was not appreciably affected by light in these preparations. 3. The degree of inactivation of the respiratory quinone was dependent on the amount of carotenoid in the preparation, high concentrations of the pigment in the mitochondrial ridmbranes providing substantial protection against the effect of light. 4. Separation of the inner and outer membranes of mitochondria showed that under conditions where carotenoid appears to protect quinone, significant amounts are found in the inner mitochondrial membrane, otherwise carotenoid is restricted to the outer membrane.

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