Gattermann Figure 4 (in Greenberg et al). Schematic representation of the heme biosynthetic pathway and its connection with the electron transport chain of the inner mitochondrial membrane.

The mitochondrial respiratory chain provides electrons needed to convert Fe³⁺ into Fe²⁺. Only Fe²⁺ can pass through the inner mitochondrial membrane. The respiratory chain creates the membrane potential needed to carry Fe²⁺ through the inner mitochondrial membrane. Iron must be provided in the right chemical form, because ferrochelatase can use only Fe²⁺ for heme synthesis.