 Hey everybody, in this video we're probably going to talk about lactic acid, but so we see here we've taken our six carbon glucose, we've split it in half during glycolysis. So then what? Well that question is answered by the presence or absence of oxygen as you can see here at the bottom. So if we have oxygen, then we're going to fully metabolize this through our oxidative phosphorylation system or cellular respiration pathway, which means that pyruvate will become acetyl-CoA in what's called the intermediate step, it'll travel through the Krebs cycle where a bunch of electrons are peeled off of it and protons and then it'll go through the electron transfer system and generate bunches of energy, which is why a human fully oxidizes a glucose that can generate 36 ATP for bacteria can be 38. So that's going to happen in the presence of oxygen. We talk about that all of the time. Let's look at what happens if, because we'll do separate videos on the Krebs cycle, etc. Let's look at what happens if we don't have oxygen or we don't have enough oxygen. So here, if oxygen is limited, then pyruvate is going to have to follow this anaerobic pathway. And you're going to see here that you're not going to get a bunch of energy generated here and you also see at the bottom that lactate becomes the final electron acceptor instead of oxygen because we don't have oxygen. But during this process, the anaerobic process, pyruvate is going to become lactate or lactic acid. Now we used to think of this as always a negative thing, like if I ask students about lactic acid, they almost always talk about the burning sensation when you're exercising. And certainly, lactic acid build up and can lead to that, you know, feeling the burn. It can also lower the pH to a point where your muscles can no longer contract. So it can lead to muscle failure. Absolutely. A trained athlete needs to be able to clear lactate if they're going to continue to perform. And there will be a limit where you just cannot perform any longer because of lactate. But it doesn't make it a waste product. We always talk about it like it's always a waste product, but there's a couple of key things to note here. Number one, why did we do this? In the absence of oxygen, why did we turn pyruvate into lactate? Well, the key is that you're actually recycling NAD. And this is not a huge deal to me, but a big picture to understand that if you didn't do this, you see at the bottom there, it frees NAD so it can go back and oxides and other glucose. So if you didn't produce lactate, glycolysis would shut off and we wouldn't have any energy. So in a low oxygen environment, we make a little bit of energy during glycolysis and then we can spare those NADs so they can go back and make a little bit more if we still don't have enough oxygen. The hope is we're going to reach an environment. We're going to stop pushing really hard. We're going to reach an environment where oxygen returns and then we can use lactate. But even before that, lactate is a brain fuel. So again, we've always talked about like a waste product, but your brain isn't the only tissue that can use it, but lactate certainly can. At least 10% of the energy that's used by your brain today is going to be from astrocytes, taking glucose and turning it into lactate. So again, don't think of it just as a waste product. And then another way to look at it's kind of cool is once oxygen returns, you have this side to something called the corey cycle where lactate can go back and become pyruvate, which means it could then generate glucose in a process called gluconeogenesis or the creation of new glucose. So it can be used for fuel. It can regenerate glucose and so these are all good things, right? And this is also where this idea of the oxygen debt comes from. Like when you're done, if you push really hard, you're exercising, training really hard and you're done, you have built up some lactate. This is also why you don't just immediately, your metabolism doesn't immediately turn to normal. You don't just start breathing normally, right? When you're done, if you just done a bunch of wind sprints, you're going to be sucking air for a while because you're trying to repay the oxygen debt. What that means is you didn't have enough oxygen, which is where all the lactate came from. Now that you have oxygen, you're going to undergo this corey cycle, CORI, in the liver and it's going to turn that lactate. If it wasn't used for fuel already, it's going to be turned into something else. It's primarily turning it back into pyruvate or glucose. So it's kind of cool. So it's basically, if you think about it, it's a way to, in an environment where there's not enough oxygen, it's a way to temporarily borrow the energy you need to keep going, but you're going to have to repay that debt and that's what the corey cycle is all about. So okay, that's just a little information about what happens to pyruvate, especially in a low oxygen environment, well, the rest of our videos will be about what happens if there is plenty of oxygen because most of us, unless you train really, really hard, unless you're exercising hard, most of us are always in a situation where we have enough oxygen unless we're climbing a mountain or something like that. All right, I hope this helps. Have a wonderful day. Be blessed.