Region between the inner and outer membranes. The second compartment, the mitochondrial matrix, is enclosed by the inner membrane. Some of the metabolic steps of cellular respiration occur in the matrix, where many different enzymes are concentrated. Other proteins that function in respiration, including the enzyme that makes ATP, are built into the inner membrane. The cristae give the inner mitochondrial membrane a large surface area that enhances the productivity of cellular respiration, another example of the correlation between structure and function.