A mitochondria is enclosed by a double membrane envelope composed of lipid and protein. The two membranes are separated by a narrow fluid –filled space called the outer compartment. The outer membrane is smooth, it is more permeable to small molecules, contains some enzymes but is poorer in proteins. The inner membrane surrounds a central cavity or matrix (inner compartment) filled with a fluid. Folds of inner wall of mitochondria are called cristae.