On the power per mitochondrion and the number of associated active ATP synthases

Recent experiments and thermodynamic arguments suggest that mitochondrial temperatures are higher than those of the cytoplasm. A hot mitochondrion calls for a closer examination of the energy balance that endows it with these claimed elevated temperatures. As a first step in this effort, we present here a semi-quantitative bookkeeping whereby, in one stroke, a formula is proposed that yields the rate of heat production in a typical mitochondrion and a formula for estimating the number of active ATP synthase molecules per mitochondrion. The number of active ATP synthase molecules is the equivalent number of ATP synthases operating at 100% capacity to maintain the rate of mitochondrial heat generation. Scaling laws are shown to determine the number of active ATP synthase molecules in a mitochondrion and mitochondrial rate of heat production, whereby both appear to scale with cell volume. Four heterotrophic protozoan cell types are considered in this study. The studied cells, selected to cover a wide range of sizes (volumes) from ca. 100 mu m(3) to 1 million mu m(3), are estimated to exhibit a power per mitochondrion ranging from ca. 1 pW to 0.03 pW. In these cells, the corresponding number of active ATP synthases per mitochondrion ranges from 5000 to just about a hundred. The absolute total number of ATP synthase molecules per mitochondrion, regardless of their activity status, can be up to two orders of magnitudes higher.

Automated summary

Recent experiments and thermodynamic arguments suggest that mitochondrial temperatures are higher than those of the cytoplasm. A formula is proposed to calculate the rate of heat production in mitochondria and estimate the number of active ATP synthase molecules per mitochondrion, showing that both scale with cell volume. The study on four heterotrophic protozoan cell types reveals a range in mitochondrial power and active ATP synthase molecules, indicating significant differences based on cell size.