For instance, red blood cells don't have mitochondria, and so they cannot actually undergo aerobic cell respiration. So the majority of the high energy ATP molecules generated in breaking down glucose are produced via aerobic respiration, which must take place in the presence of oxygen and inside the mitochondria of our cell. So let's take a look at the following diagram. So, notice that when glucose is broken down to pyruvates, we produce not only ATP molecules, but we also actually collect electrons. So we abstract four electrons when a single glucose molecule is broken down into two Pyruvate molecules. Now, those two electrons aren't actually by themselves.