 There are some variations in this process. So, to kind of summarize those variations, I want to come back to this original visual that we did, because this kind of, I'm going to draw our variations onto this thing. First of all, imagine that you do not have oxygen. Now, you're not going to live very long because you need a lot of energy, but there are critters that can survive for oxygen and aerobic respiration. They don't need oxygen in order to produce energy. So, I want you to think about that for a second. If you don't have oxygen as your final electronic scepter, and the whole thing backs up, the question is how far does it back up? It backs up all the way to here. And in fact, you can go through glycolysis. Without oxygen, you can go through glycolysis. So, you can end up with pyruvate, which means how many ATPs did you get? That's it. You get two ATPs if you don't have oxygen as your final electronic scepter. And in order for this process to continue, you have to, this is interesting, and this is a chemistry fact, you have to get rid of pyruvate. You have to use it up somehow. In humans, we turn that into lactic acid. Lactic acid is a byproduct of anaerobic respiration. Sprinting is an anaerobic exercise. Lactic acid is produced when you do that. In yeasties, I've already told you this cool fact, yeast in the process of anaerobic respiration, don't produce lactic acid. Dude, they produce ethanol. Ethanol, which is drinking alcohol. It's delicious. It's E-East-P. You do not produce, can you imagine if you were a sprinter in anaerobic respiration and instead of lactic acid, you produced ethanol? That'd be kind of awesome. I mean, why bother going to the grocery store for your party? Just go out and do some sprints and then you would be nicely inebriated. All right. Well, that's not a reality. That's only in yeast. But we take advantage of that and we gather their pee that they make and anaerobically respire, respirate, whatever. If you want to burn a fuel, if you want to produce energy from something other than glucose, let me tell you how this works. This is kind of interesting as well. Protein. Protein can be broken into little chains, little chunks of, basically, little masses can be broken into little carbon chunks that will replace pyruvate. They act like pyruvate. Protein equals pyruvate. And then that pyruvate, just like those little molecules can enter into the whole process and play the same role that pyruvate played. So we can get, as long as we have oxygen, we can totally metabolize protein from that point. Now, fat, maybe I should go up and do another. I want to stay here. So the way fat is done, it actually breaks down the fatty acids and glycerol. The glycerol feeds right into glycolysis. So glycerol is like rock star. You just enter glycolysis. Instead of having glucose, you put glycerol in and no problem. The whole thing totally rolls. The fatty acids get broken up into little pieces that act like acetyl-chol-kinnaha. Fatty acids enter here and act like acetyl-CoA and enter the citric acid cycle just like that. So you can metabolize fats. You can metabolize glycerol fats. You can metabolize proteins and get energy out of those substances just like you can with glucose. Let's do a giant review of this whole process.