For those that missed it the first time or just plain forgot, here is a mitochondrion:
As you may, and should, recall from the section on mitochondria, these organelles are chiefly responsible for converting the chemical energy in macromolecules, like glucose, into molecules of ATP that can be used by the cell for energy. The ability of a mitochondrion to convert glucose and ADP, adenosine triphosphate, into ATP is intricately connected to the structure of the mitochondrial membranes. In a process called glycolysis, which occurs in the cytoplasm just outside the mitochondrion, electrons are stripped from glucose and passed through the outer mitochondrial membrane into the intermembrane space. Here, the electrons are passed to a series of special proteins embedded in the IMM. As the electrons move from one membrane protein to the next, energy is released and protons (hydrogen ions, or H+) in the matrix are pumped across the IMM and into the intermembrane space. Fairly quickly, a large number of protons accumulate in the intermembrane space, and, like water behind a dam, exert great pressure on the IMM.
Luckily for them, and for life as we know it, there is a special protein complex embedded in the IMM that allows protons to flow back into the matrix. The special part about this channel protein complex is that it is capable of harnessing the enormous energy produced by the rush of protons. Exactly like a turbine in a dam, ATP synthase, as this protein complex is named—and yes, it is an enzyme (-ase )—has a rotor that spins when protons push past.
Can't visualize what we mean? Here's a picture:
The energy generated by the turning of the rotor is converted into ATP, just like the energy made by turning a water turbine is converted into electricity in a dam. In this way, one molecule of glucose can be converted into about 38 molecules of ATP. A pretty good investment, if you ask us. This awesome process is called cellular respiration, and it is all made possible by the mitochondrial membranes!
The mitochondrial membrane is the site of ATP synthesis, and the ATP made is actually inside the mitochondrion. How do you think it gets to the cytoplasm so that it can be used? Naturally, it gets where it needs to go by using those transmembrane channels we discussed earlier.