Are size and mitochondrial power of cells inter-determined?

Mitochondria are the central hub of ATP production in most eukaryotic cells. Cellular power (energy per unit time), which is primarily generated in these organelles, is crucial to our understanding of cell function in health and disease. We investigated the relation between a mitochondrion's power (metabolic rate) and host cell size by combining metabolic theory with the analysis of two recent databases, one covering 109 protists and the other 63 species including protists, metazoans, microalgae, and vascular plants. We uncovered an interesting statistical regularity: in well-fed protists, relatively elevated values of mitochondrion power cluster around the smallest cell sizes and the medium-large cell sizes. In contrast, in starved protists and metazoans, the relation between mitochondrion power and cell size is inconclusive, and in microalgae and plants, mitochondrion power seems to increase from smaller cells to larger ones (where this investigation includes plant cells of volume up to ca. 2.18 × 105 µm3). Using these results, estimates are provided of the number of active ATP synthase molecules and basal uncouplers.

ATP synthase: a moonlighting enzyme with unprecedented functions.

ATP synthase's intrinsic molecular electrostatic potential (MESP) adds constructively to, and hence reinforces, the chemiosmotic voltage. This ATP synthase voltage represents a new free energy term that appears to have been overlooked. This term is at least roughly equal in order of magnitude and opposite in sign to the energy needed to be dissipated as a Maxwell's demon (Landauer principle).

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) fromca.100µm3to 1 millionµm3, are estimated to exhibit a power per mitochondrion ranging fromca.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.