(A) The “old” and (B) the “new” model of the inner structure of mitochondria. Instead of the large openings connecting the intercristae space to the intermembrane space present in the old model (A), narrow tubular openings (crista junctions) connect these spaces in the new model (B) thereby creating a third aqueous intramitochondrial compartment. Panel (B) also shows the presence of two out of several protein complexes which control the formation and remodeling of the cristae junctions. This schematic illustration is based on figures in [22].