The physical properties of fish liver and rat liver mitochondria were compared as a function of temperature and osmotic pressure. The data indicate that fish mitochondria are more flexible and swell at a more rapid rate over a 0 to 30°C temperature range, whereas the rates of swelling at 30 to 40°C are comparable. The swelling rates of both fish and rat mitochondria vary with temperature and approximate the Arrhenius relationship. Apparent energies of activation for swelling averaged 26.5 kcal and 12.9 kcal for rat and fish, respectively. Fish mitochondria were less stable than rat mitochondria to osmotic variation, and the disparity in initial swelling rates became increasingly greater with lower osmotic pressure. The hypotonic swelling of both fish and rat mitochondria was readily reversed osmotically; however, there was a very rapid decay of reversal in fish mitochondria and only a very slow decay in the case of rat. All the data indicate that under comparable conditions the fish mitochondrial membranes are more flexible and presumably more permeable and labile than rat mitochondrial membranes. The findings are discussed in relation to the general metabolic implications and the possible contributions of the membrane constituents to membrane behavior.

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