A mitochondrion contains two main membranes composed of phosopholipid bilayers and proteins -
The smooth outer membrane – This encloses the entire organelle and contains porin protein molecules that serve as diffusion channels for minute protein molecules across the membrane. Larger molecules can enter only if their signaling sequence can bind to a large translocase protein in the outer membrane.
The folded inner membrane – This contains about 1/5th of the mitochondrion protein, but has no porin proteins, and includes several hundred polypeptides. The inner membrane is impermeable and ions and molecules require special membrane transporters to pass through it. The inner membrane folds are known as cristae.
The cristae – With their folds, the cristae increase the total surface area of the inner membrane. The cristae create two sections in the mitochondrion -
The intermembrane space – This lies between the outer and inner membranes.
The matrix – This is the inner space enclosed by the inner member. The matrix contains about 2/3rd of the total mitochondrion protein. It also contains several hundred enzymes, transfer RNA (tRNA) and copies of the mitochondrial DNA genome.