Animation of the structure of a mitochondrion, the organelle that supplies much of the energy for a cell. Each mitochondrion has an outer membrane (yellow) enclosing an inner membrane (brown), which is organised into folds called cristae. These increase the surface area of the inner membrane. The inner membrane surface is where the energy-producing reactions occur, so these folds increase the efficiency of the mitochondrion. The inset shows part of the inner membrane studded with ATP synthase proteins, which make the chemical energy molecule ATP. This enzyme is driven by a flow of protons down a gradient between the outside and the inside of the inner membrane.