How large is the drop in PO2 between cytosol and mitochondrion?

The subject of this brief review is the size of the local drops in PO2 around consuming mitochondria. We show that large drops (several Torr or more) are in conflict with the predictions of basic diffusion theory, when one uses accepted values for relevant parameters. In particular, oxygen diffusion coefficients must be reduced by at least a factor of 10 below measured values to reconcile Fick's law with large PO2 drops. Experimental evidence offered for large drops is often ambiguous because of system heterogeneities. In those cases where tractable models of heterogeneous systems can be developed, the experimental data are consistent with drops in PO2 on the order of a few hundredths of a Torr between cytosol and mitochondrion.