The mitochondrion consists of four major sections – the outer membrane, the intermembrane space, the inner membrane, and the matrix.
The Outer Membrane
This membrane contains a great number of large transport proteins, which allows for large molecules to enter with ease. This membrane includes proteins that can convert lipid substrates into forms that can be used by the matrix.
The Intermembrane Space
This space contains enzymes that use ATP to phosphorylate other nucleotides.
The Inner Membrane
This membrane is highly convoluted, forming many folds called cristae. This serves to greatly increase the surface area, allowing more work to be done is a smaller space. It contains three major proteins - 1. the proteins that carry out the oxidation reactions of the respiratory chain, 2. an enzyme complex called ATP synthetase which makes ATP, and 3. transport proteins which regulate the transfer of molecules into and out of the matrix. This is where the oxidation phosphorylation takes place.
The Kreb Cycle takes place here. It also contains several copies of the mitochondrial DNA genome, special mitochondrial ribosomes, tRNAs, and various enzymes required for the expression of the mitochondrial gene.