The effect of different temperatures on the biochemical activity and morphology of insect flight muscle mitochondria was examined. It was found that respiration and phosphorylation have the same thermal response at temperatures of 25°C. and below. The energy of activation for both systems is approximately 12,300 calories. Oxidation and phosphorylation can be uncoupled effectively by temperature, for at temperatures above 25°C. there is more rapid heat inactivation of phosphorylation. This is evident from reduced P/O values as well as from morphological deterioration in the mitochondrial population. The thermal response of both this sarcosomal enzyme system and the respiration in the living fly are quantitatively similar.

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