Mitochondria

Figure I. Mitochondria work as the power plants of cells. Fats and sugars are broken down in the matrix and H⁺ ions are pumped out to the intermembrane space by the flow of electrons (e) in the Electron Transport Chain Complex (right), located in the inner membrane. The highly curved structure of the inner membrane (cristae) increases its surface area for the whole process; hence more energy can be produced within the compact volume. Mitochondria are surrounded by an additional outer membrane, so that H⁺ ions can be concentrated in the intermembrane space and their return to the matrix will be used for energy production.

One may wonder what all this has to do with the production of ATP. So far in the story, a mere one hundred enzymes and more than two hundred chemical reactions have taken place to move hydrogen ions (H⁺) – obtained from sugars and fats – across the inner membrane. ATP will be produced when H⁺ ions return to the mitochondria matrix in a highly elaborate mechanism, which will be explained below.