One last bit about mitochondria (we promise!). In addition to making ATP so that our cells can function, mitochondria also let off a lot of heat. Most of the energy from that molecule of glucose—60% in fact—is lost as heat. Before you start thinking, "What a waste of energy!" (we caught you thinking it, didn’t we?), keep in mind that most combustion engines are only 18-20% efficient, meaning 80-82% of the energy from every molecule of gasoline is lost as heat!10 Yeah, what is that you were saying about your mitochondria wasting energy?
In reality, the fact that mitochondria are only 40% energy-efficient is a really good thing. In fact, it’s vital to life. We find ourselves saying that a lot. The heat produced by mitochondria during cellular respiration is actually harnessed by the cells of our bodies and used to regulate our internal temperatures. The ability to harness heat from cellular respiration is a trademark characteristic of endothermic animals. The term "warm-blooded" is often used to describe endothermic organisms, but it is an inaccurate term. Blood cells do not have mitochondria and cannot, therefore, produce heat. They should be called "warm-mitochondriad" animals instead. As if that will happen.
During infection, our body’s temperature regulation system captures more heat than usual, and we experience the sensation of a fever. If too much heat is retained so that the core body temperature rises above 108 °F, the regulatory system breaks down, and the heat from our mitochondria quickly kills our cells. Eek. When this happens, we experience the uncomfortable sensation of…death. On the flip side, if the environment outside our cells gets too cold, especially in freezing water, our mitochondria have a hard time producing enough heat to bring our core temperature back up to our normal 98.6 °F. If this condition lasts for too long, we experience hypothermia and, eventually…death. Apologies for the bluntness. Therefore, in addition to providing our cells with the energy they need to grow and reproduce, mitochondria play a vital role in maintaining the internal temperature our cells need to survive.