The fate of pyruvate is then determined by what type of conditions our cell is performing glycolysis under and also what type of cell we have. If we have anaerobic conditions in our cells as humans, we are going to generate lactate. If we have anaerobic or hypoxic, meaning not a lot of oxygen conditions, or if we are dealing with yeast, for example, we're actually going to make ethanol and carbon dioxide. This is actually the basis for making things like beer, a very important process in human history, fermentation of alcohol. It's actually one of the reasons that some anthropologists believe humans organized into societies, but that's a whole other story. Lastly, if we have aerobic conditions, our pyruvate is going to be turned into Acetyl CoA, and that's what we're going to talk about in the next unit when we get into aerobic cellular respiration. For now, let's talk about fermentation. So, alcohol fermentation, let's talk about that first because, well, it's a subject near and dear to my heart. As someone who has brewed beer in the past, it's a very fun, super awesome process that relies on organisms to actually carry out these reactions. Super cool. Alright. So basically, what we have on top is the short version of glycolysis, right? We start out with our 6 carbon glucose. We make our 3 carbon pyruvate. And in the process, we're going to generate a net of 2 ATP and 2 NADH. The whole point of fermentation is that in the absence of oxygen, we can't use that NADH to fuel oxidative phosphorylation. So, we have to do something with it because we need more NAD+ to keep glycolysis going. There's not just infinite NAD+ in a cell, so the cell has to take that NADH, do something with it, and regenerate NAD+. It needs to oxidize NADH and so fermentation is the way our cells accomplish that. And again, the whole point is to sustain glycolysis in the absence of oxygen, and it's worth noting that actually some cells only do fermentation. Our red blood cells, for example, only carry out glycolysis. So they rely on fermentation to keep glycolysis going. Now, of course, our red blood cells don't do alcohol fermentation. Otherwise, we'd all get drunk when we go for a run, and obviously, that doesn't happen. We do lactic acid fermentation, which we're going to get to in a second. So, in alcohol fermentation, here's our pyruvate. What's going to happen is we are going to decarboxylate our pyruvate, and remember, our carbon numbering here is 3 or 4, 2, 5, and 1 or 6. We're going to decarboxylate this group right here. So the CO2 that's leaving, that carbon is either carbon 3 or carbon 4 from glucose. What we're left with is a molecule called acetaldehyde. Acetaldehyde is going to contain carbon 2 or 5 and carbon 1 or 6, and this molecule is currently somewhat oxidized, with a carbonyl group. It's going to get reduced by NADH. And remember, of course, we have 2 NADH and we have 2 of these molecules for every one glucose we put in. It's going to get reduced to ethanol, and just to be crystal clear, that's carbon 2 or 5 and carbon 1 or 6. So that's alcohol fermentation and the whole point is to recycle NAD+ so that we can continue glycolysis. Now, let's take a look at lactic acid fermentation or what's known as the Cori cycle, named after the scientists who discovered it. Here, pyruvate is actually going to be directly reduced by NADH to form lactate and NAD+. So again, we have our 1 glycolysis, I'm sorry, 1 glucose go through glycolysis, make a net two ATP, two NADH and we're going to have 2 pyruvates. Numbering is the same as above. Here, NADH is going to directly reduce this molecule to lactate. This is lactate, and that's going to regenerate NAD+, and you can see we're reducing this carbonyl here, and the numbering stays the same. We have 3, 4, 2, 5, and 1, 6. And again, the whole point of fermentation is just to keep glycolysis going by regenerating NAD+. Alright. With that, let's turn the page.
Review 2: Biosignaling, Glycolysis, Gluconeogenesis, & PP-Pathway
Fermentation