Membrane Permeability to Pyruvate
Pyruvate is negatively charged and quite polar, which makes it unfavourable to diffuse directly through any membrane. The outer mitochondrial membrane contains porins, which allow small molecules, like pyruvate, to passively diffuse through. Specifically, pyruvate uses voltage dependent anion channels. The inner mitochondrial membrane lacks such channels and depends on active transport by the long anticipated but only recently discovered mitochondrial pyruvate carrier (Herzig et al. and Bricker et al., 2012).