The individual sacs formed by the inner membrane of mitochondria in prokaryotic and eukaryotic cells are called cristae. These are shelf-like structures that project into the mitochondrial matrix. Cristae increase the surface area of the inner membrane, which is essential for cellular respiration. The mitochondrial matrix is the space enclosed by the inner mitochondrial membrane. It contains mitochondrial DNA (mtDNA), ribosomes, enzymes, and other molecules necessary for mitochondrial function.