Fermentation - Online Tutor, Practice Problems & Exam Prep

The fate of pyruvate depends on cellular conditions. Under anaerobic conditions, humans produce lactate, while yeast generates ethanol and carbon dioxide, crucial for fermentation. In aerobic conditions, pyruvate converts to Acetyl CoA for cellular respiration. Fermentation recycles NAD⁺ from NADH, allowing glycolysis to continue. Alcohol fermentation involves decarboxylating pyruvate to acetaldehyde, which is then reduced to ethanol. Lactic acid fermentation directly reduces pyruvate to lactate, regenerating NAD⁺. This process is vital for cells like red blood cells that rely solely on glycolysis.

Fermentation

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.

Here’s what students ask on this topic:

What is the main purpose of fermentation in cells?

The main purpose of fermentation in cells is to regenerate NAD⁺ from NADH, allowing glycolysis to continue in the absence of oxygen. During glycolysis, glucose is broken down into pyruvate, producing a net gain of 2 ATP and 2 NADH. Without oxygen, cells cannot use NADH in oxidative phosphorylation, leading to a shortage of NAD⁺. Fermentation solves this by converting NADH back to NAD⁺, ensuring a continuous supply for glycolysis. This process is crucial for cells like red blood cells, which rely solely on glycolysis for energy production.

How does alcohol fermentation differ from lactic acid fermentation?

Alcohol fermentation and lactic acid fermentation differ in their end products and the organisms that perform them. In alcohol fermentation, pyruvate is decarboxylated to form acetaldehyde, which is then reduced by NADH to produce ethanol and CO₂. This process is common in yeast and some bacteria. In contrast, lactic acid fermentation involves the direct reduction of pyruvate by NADH to form lactate. This process occurs in human muscle cells under anaerobic conditions and in certain bacteria. Both types of fermentation regenerate NAD⁺ to sustain glycolysis.

Why is NAD⁺ regeneration important in fermentation?

NAD⁺ regeneration is crucial in fermentation because it allows glycolysis to continue producing ATP in the absence of oxygen. During glycolysis, NAD⁺ is reduced to NADH. Without a mechanism to oxidize NADH back to NAD⁺, glycolysis would halt due to a lack of NAD⁺. Fermentation provides this mechanism by converting NADH to NAD⁺, either through the production of ethanol and CO₂ in alcohol fermentation or lactate in lactic acid fermentation. This ensures a continuous supply of NAD⁺ for glycolysis, enabling cells to produce energy anaerobically.

What are the end products of alcohol fermentation?

The end products of alcohol fermentation are ethanol and carbon dioxide (CO₂). During this process, pyruvate is first decarboxylated to form acetaldehyde, releasing CO₂. Acetaldehyde is then reduced by NADH to produce ethanol. This type of fermentation is commonly carried out by yeast and some bacteria. The primary purpose of alcohol fermentation is to regenerate NAD⁺ from NADH, allowing glycolysis to continue in the absence of oxygen.

What is the Cori cycle and its significance in lactic acid fermentation?

The Cori cycle is a metabolic pathway that involves the conversion of lactate produced in muscles during anaerobic glycolysis back to glucose in the liver. During intense exercise, muscles produce lactate from pyruvate via lactic acid fermentation to regenerate NAD⁺ and sustain glycolysis. Lactate is then transported to the liver, where it is converted back to glucose through gluconeogenesis. This glucose can be released into the bloodstream and taken up by muscles for energy. The Cori cycle is significant because it helps maintain blood glucose levels and provides a way to recycle lactate, preventing its accumulation in muscles.