Fermentation - Online Tutor, Practice Problems & Exam Prep

Fermentation is a crucial process in anaerobic respiration, breaking down sugar to produce ATP without oxygen. It involves glycolysis, which generates NADH and pyruvate, and fermentation, which regenerates NAD⁺ to sustain glycolysis. There are two main types: alcoholic fermentation, producing ethanol and CO₂, primarily in yeast, and lactic acid fermentation, producing lactate, used by muscles during intense activity. While anaerobic respiration yields only about 4 ATP, aerobic respiration can produce 32-34 ATP, highlighting the inefficiency of anaerobic pathways.

Fermentation

Hello, everyone. In this lesson, we are going to be learning about anaerobic respiration and fermentation. So let's talk about what fermentation actually is. Fermentation is going to be one of the major steps in anaerobic respiration or respiration without oxygen. So, fermentation is the process that breaks down sugar to produce ATP in the absence of oxygen. That is the key point there. Oxygen is not present for fermentation. That's why it's anaerobic respiration. Now, whenever we're talking about fermentation, we are going to have 2 main steps. We are going to have glycolysis, which does not require oxygen. The later stages of aerobic cellular respiration like the citric acid cycle and the electron transport chain do require oxygen. But glycolysis does not. So, glycolysis is going to happen in anaerobic organisms. But fermentation is needed for other steps. So glycolysis needs a supply, a constant supply of NAD⁺, which is an electron carrier that is able to accept that electron that is created or released via glycolysis. This is an electron carrier. Now, glycolysis cannot create its own NAD⁺. It only makes NADH, which is the form that is already carrying the electron. Now, in aerobic cellular respiration, you are going to see that NAD⁺ is going to be remade in the process of the Citric Acid Cycle and the Electron Transport Chain, and then it's given back to glycolysis. Now, what is going to be important here is that fermentation is going to be utilized to recreate the NAD⁺ that is needed so that glycolysis can continue. So, without oxygen, NADH cannot be oxidized to NAD⁺, so it cannot be reused. So glycolysis cannot continue. This is where the process of fermentation is going to step in. Fermentation regenerates NAD⁺, so it can be reused in glycolysis. So these organisms, even without the presence of oxygen, are able to continually make ATP. Okay? So electrons are transferred from the NADH created in glycolysis to the pyruvate molecules that are also created in glycolysis to generate the NAD⁺, and this is going to be the main process of fermentation.

So this is going to be an example of that happening in this diagram right here. So this example here is going to be a diagram that is representing anaerobic respiration. And as you guys can see, anaerobic respiration doesn't have that many steps, as compared to aerobic respiration. There are actually going to be those 2 main steps, glycolysis and fermentation. Okay. So, we know that it is going to start up here because we have glucose. And glucose is taken in as a nutrient, and it is going to be broken down. And, in fact, this particular stage in the process of anaerobic respiration is going to be glycolysis. Right here. Because we are breaking down glucose. And, as you guys can see, 2 ATP have been created, NADH has been created, and 2 pyruvate. This is going to be alcohol fermentation. And I knew that because you guys can see that 2 ethanol molecules were created. Now, there's also another type of fermentation, which is important to know, and that's going to be lactic acid fermentation. So this is alcohol fermentation, and our 2 pyruvate molecules are going to lose 2 molecules of CO₂, and they're going to turn into acetaldehyde. And then this is going to go through the process of fermentation. So, NADH is going to be turned into NAD⁺, and this is going to create the 2 ethanol molecules, and then that NAD⁺ can go back to glucose and be used again. This is alcohol fermentation. It's important to know that there is also lactic acid fermentation. So what's going to happen is you are going to have these 2 pyruvate molecules that are made via glycolysis. The process of glycolysis is exactly the same. You are going to have glucose, you are going to create 2 ATP, you are going to create NADH, and 2 pyruvate molecules. And then if you go down the path of lactic acid, something else is going to happen. So, we are going to have these 2 pyruvate molecules, and we are going to create 2 lactate molecules or lactic acid. Okay? So these 2 pyruvate molecules turn into lactic acid, and from this process, you are going to turn NADH into NAD⁺, and then that NAD⁺ can go on to be utilized in glycolysis again. So this one right here that I drew for you guys is lactic acid fermentation. It's just 2 different ways to do the same thing. Alcohol fermentation is commonly going to be utilized by things like yeast, yeast cells. This is how we are going to get many of the alcohols that we drink. And lactic acid fermentation is going to be used by your muscles whenever they run out of oxygen because you are working out so hard. Lactic acid fermentation is what our muscles are going to use.

Okay. So now let's go on and talk about the rest of this information. Okay. So, what's going to happen is this is going to be the rest of the information, which I already talked to you guys about up there. You are going to use the pyruvate as an electron acceptor, and then this is going to actually begin to limit the other metabolic pathways that you have. You guys can see up here that this pyruvate that was created was used as an electron acceptor. It took the electrons from NADH and generated NAD⁺. And once it takes those electrons, it's going to turn into a different molecule, either ethanol or lactic acid. And once it turns into those other molecules, it's going to limit the use of other metabolic pathways. Ethanol and lactic acid are difficult for organisms to use. So in the presence of oxygen, it generally goes through other metabolic pathways. Whenever glycolysis creates pyruvate, it then will travel into the citric acid cycle, and then those products will travel to the electron transport chain. And the average amount of energy that is made in aerobic cellular respiration is vastly higher than that which is created in anaerobic respiration. So without oxygen, many byproducts are made, and those byproducts are things like ethanol and lactic acid, and they do not go on to create ATP. So you lose a lot of that potential energy because lactic acid and ethanol are not utilized by the rest of the system. So, the next sentence here says, pyruvate conversion results in numerous byproducts. And the problem with these byproducts is we don't utilize these byproducts to make more energy. They're simply waste products. So again, you can have lactate fermentation which produces lactate or lactic acid. And this is commonly found in things like cheese and milk, but your muscles use this process as well. Whenever they run out of oxygen and they turn to anaerobic respiration, they are going to use lactic acid fermentation or lactate fermentation. And then, alcohol fermentation is the one in the example above, and this is going to produce ethanol and CO₂. Yeast do this. They make ethanol. They also make CO₂ gas, which is going to make things like bread rise for baking. And the ethanol is going to be used for brewing and the creation of beer. So we have harnessed these methods, these pathways for our own good, but we utilize these biological pathways as well. We commonly do aerobic respiration simply because it makes vastly more ATP. I believe it makes around 32 to 34 ATP. Well, the process of glycolysis and fermentation in anaerobic respiration make about 4 ATP, I think. So it's really inefficient. It's not good. And it has all of these byproducts. And lactic acid, when it pools up in your muscle cells, can actually make you sore. So it's not a good product for us to make. We want to have that oxygen so we can get the most energy out of it. But organisms that do not possess oxygen or live in environments without oxygen are going to use anaerobic respiration and that includes glycolysis and fermentation.

Okay, everyone. Let's go on to our next lesson.

Which of the following is not associated with fermentation?

• A. It occurs in the absence of oxygen
• B. It results in a variety of byproducts
• C. It regenerates NAD+
• D. It uses ATP

The purpose of fermentation is to produce what?

• A. ATP
• B. NADH
• C. NAD+
• D. H₂O

True or False:The creation of pyruvate in the absence of oxygen is more energetically favorable than the creation of pyruvate in the presence of oxygen.

• A. True
• B. False

Here’s what students ask on this topic:

What is fermentation in cellular respiration?

Fermentation is a metabolic process that breaks down sugar to produce ATP in the absence of oxygen. It is a key step in anaerobic respiration, which occurs when oxygen is not available. The process involves glycolysis, which generates NADH and pyruvate. Fermentation then regenerates NAD⁺ from NADH, allowing glycolysis to continue producing ATP. There are two main types of fermentation: alcoholic fermentation, which produces ethanol and CO₂, and lactic acid fermentation, which produces lactate. While anaerobic respiration yields only about 4 ATP, aerobic respiration can produce 32-34 ATP, making anaerobic pathways less efficient.

What are the main types of fermentation?

The two main types of fermentation are alcoholic fermentation and lactic acid fermentation. Alcoholic fermentation occurs primarily in yeast and some bacteria, producing ethanol and CO₂. This process is used in brewing beer and making bread rise. Lactic acid fermentation occurs in muscle cells during intense activity when oxygen is scarce, producing lactate. This type of fermentation is also used in the production of dairy products like cheese and yogurt. Both types of fermentation regenerate NAD⁺ from NADH, allowing glycolysis to continue in the absence of oxygen.

How does fermentation regenerate NAD⁺?

Fermentation regenerates NAD⁺ by transferring electrons from NADH to pyruvate, the end product of glycolysis. In alcoholic fermentation, pyruvate is first converted to acetaldehyde, which then accepts electrons from NADH to form ethanol and CO₂. In lactic acid fermentation, pyruvate directly accepts electrons from NADH to form lactate. This regeneration of NAD⁺ is crucial because it allows glycolysis to continue producing ATP in the absence of oxygen.

Why is anaerobic respiration less efficient than aerobic respiration?

Anaerobic respiration is less efficient than aerobic respiration because it produces significantly less ATP. Anaerobic pathways, including glycolysis and fermentation, yield only about 4 ATP per glucose molecule. In contrast, aerobic respiration, which includes glycolysis, the citric acid cycle, and the electron transport chain, can produce 32-34 ATP per glucose molecule. Additionally, anaerobic respiration results in byproducts like ethanol and lactic acid, which are not further utilized for energy production, making the process less efficient overall.

What role does glycolysis play in fermentation?

Glycolysis is the initial step in both aerobic and anaerobic respiration, including fermentation. It breaks down one glucose molecule into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH. In the absence of oxygen, fermentation follows glycolysis to regenerate NAD⁺ from NADH, allowing glycolysis to continue. Without this regeneration, glycolysis would halt, and ATP production would stop. Thus, glycolysis is essential for providing the substrates and initial energy yield that fermentation processes rely on.