Illustration depicting a cross section of a mitochondrion. A mitochondrion is composed of two membranes, an outer and an inner one. The outer membrane is smooth, while the inner membrane is characterised by numerous folds called cristae. These folds house ATP synthase (blue), a protein responsible for providing energy to biological systems. The number of mitochondria present in a cell is largely dependent on the energy demands of the cell. For instance, highly active human muscle cells may contain thousands of mitochondria while less active red blood cells may not contain any.