 Hey everybody, Dr. O. I know metabolism and cell respiration, I know it's complicated. In this video I'm going to try to make it as manageable as possible by just showing you where the energy comes from. We'll talk about the steps and the pathways more later. I just want to talk about what comes out of each of these four major areas, okay? So let's go ahead and start with glycolysis. So big picture, you're taking one six carbon glucose and splitting it in half. While you do that, you are going to generate what's called a net gain of two ATP. And that's because you had to spend two to make four, but for now it's no net gain of two ATP. This ATP has been produced, so in class I like to say, this is money that I have in my hands. But also during glycolysis, you've now made two NADHs. So imagine we're at a casino. These NADHs are the chips and each of these NADH chips is worth $3 or three ATP. So after glycolysis, I have a net gain. I've made two ATP, two that I can spend right now. But I also have the ability to make six more ATP when I take it to the cashier. And the cashier is going to be our electron transport system. All right, so then in the presence of oxygen, now we move on. We split glycosin half into those two, three carbon pyruvates. Now we take them, it calls them a transformation step. I like to call this their intermediate step, where you take your two, three carbon pyruvates and turn them into two, two carbon acetylcoase. So during this intermediate step, you have produced zero ATP. So we've still only made the $2, the two ATP in our pocket, but we made two more NADHs. So now we have two ATP and we now have four NADHs, which gives us the ability to make 12 more ATP. So we have, so just keep track of what's in your two pockets, two ATP, now we have four NADHs. Then we go to the Krebs cycle. The Krebs cycle is very complicated, doesn't generate much energy, it actually will just generate two ATP total. But this is where we get most of our electrons. This is where we do most of our harvesting and we make most of our chips that will cash in. So here at the Krebs cycle, also known as the citric acid cycle, TCA cycle, lots of names, but at the Krebs cycle, you have to go through it twice. That's why, so each run through the cycle only makes one ATP, but remember we split glucose in half. So you're going to go through it two times. So you're going to generate two ATP. But we're now going to generate six more NADHs and two FADHs too. So NADH is the electron carrier made from niacin and each one of those is worth three ATP. FADH2 is the electron carrier made from riboflavin and each is only worth two. So let's do our running totals now and see where we're at. So we're ready to walk up to the cashier. Remember the cashier in my example is the electron transport system. We have made four ATP potentially. We've made four dollars, actual money in our hands, ATP that can be used. But we now have 10 NADHs. We got two during glycolysis, two during the intermediate step, and six here during the Krebs cycle. So those are going to be worth a total of 30 ATP. And we also have two FADH2s that are each worth two ATP apiece for a total of four. So now with our electron transport system, yes, we've made a handful of energy. But here's where we're going to make 34 more ATP. Remember why? 10 NADHs are going to be worth a total of 30. Two FADH2s are going to be worth a total of four. So we made our two ATP in glycolysis. And we made zero in the intermediate step, and we made two during the Krebs cycle. This is what gives you a maximum ATP production of 38. So we're talking about prokaryotes. We're talking about bacteria. It would be 38. If they can fully oxidize glucose, they would make 38 ATP. When you look at the screen here, you see that the electron transport system or the whole system, the cell respiration pathways, only leads to 36 ATP per glucose in humans. And here's why. Notice that minus two for the NADH transport cost. Here's how I like to think about that. We make a net gain of two ATP during glycolysis just like bacteria do. We have to spend two ATP to get these building blocks into the mitochondria. Remember the mitochondria, the powerhouse of the human cell, the site of 95% of ATP production. It's a very important organelle. But it costs money to use it. So we make 38 ATP just like bacteria do. We have to spend two that they don't. And that's why if I ask you, depending on the class you're in, if you fully oxidize glucose in a human, you are going to get 36 ATP in a bacteria. It's going to be 38. And that's why. And this image here just kind of shows the same. This shows it here in bacteria without subtracting those two ATP. The maximum amount of ATP you can get by fully metabolizing glucose would be 38 and all the same numbers apply there. Now we say theoretical yield because this is way messier than this. Some cells are going to produce less. Some are going to produce a lot less. But this is the text, for example, of how much energy can be produced when you fully metabolize and fully oxidize glucose. All right. So that's all the key players. And that's where the energy comes from in those four steps. Glycolysis, the intermediate step, the Krebs cycle. And then at the end, the electron transport system. I hope this helps. We'll dive deeper in other videos. Have a wonderful day. Thank you.