Fermentation & Anaerobic Respiration - Online Tutor, Practice Problems & Exam Prep

Fermentation occurs when aerobic organisms lack oxygen, leading to the conversion of pyruvate into lactic acid or ethanol, while regenerating NAD⁺ to sustain glycolysis. Lactic acid fermentation produces 2 ATP and occurs in human muscle cells during intense exercise, while alcohol fermentation is utilized in brewing and winemaking. Anaerobic respiration, using alternative electron acceptors like nitrate or sulfate, generates more ATP than fermentation but less than aerobic respiration. Understanding these processes is crucial for grasping cellular energy production in varying oxygen conditions.

What Happens. to Aerobic Organisms if There's No Oxygen?

In this video, we're going to begin our lesson on fermentation and anaerobic respiration. And so up until this point in our course, we've really been focusing on aerobic cellular respiration in the presence of oxygen. But here in this video, we're going to address, well, what happens if aerobic organisms don't have any oxygen around? Well, without oxygen, aerobic cellular respiration, as we've discussed in our previous lesson videos, cannot occur. So aerobic cellular respiration can only occur if oxygen is present. But without oxygen as the final electron acceptor, the electron transport chain is going to get backed up like a traffic jam. And ultimately the amount of NADH is going to increase, whereas the amount of NAD⁺ is going to decrease significantly down to dangerously low levels.

And so if we take a look at our image down below the top half of this image, notice that we have glycolysis here as the very first step of cellular respiration. And once again, if oxygen is present, then cellular respiration would occur as we've discussed in our previous lesson videos where pyruvate oxidation would occur, then the Krebs Cycle, then the Electron Transport Chain and Chemiosmosis. But once again these stages here are only going to occur if oxygen is present. If there's no oxygen present, then these stages are not going to occur. And instead, if there's no oxygen, then fermentation is going to take place. And so the process of fermentation is a process that's going to use the electrons from these NADHs that have increased to reduce pyruvate and generate alternative molecules that end up regenerating NAD⁺s that, the NAD⁺s have gotten dangerously low, they've decreased really really low.

So one of the big takeaways of fermentation is that it's going to help regenerate those NAD⁺s that have gotten dangerously low. Now, depending on the specific type of organism, the pyruvate that gets reduced can be reduced to either lactic acid or it could be reduced to alcohol. And so later in our course, we'll discuss lactic acid fermentation and alcohol fermentation as well. Now, fermentation ultimately is going to make very little amounts of ATP. And so really only some unicellular organisms can survive on just fermentation alone, but multicellular organisms, they cannot survive on just fermentation because it makes so little ATP that it's not enough to drive the energy processes that are needed by multicellular organisms. But fermentation is advantageous because it will allow for the regeneration of NAD⁺ as we've already indicated. And that regeneration of NAD⁺ is really critical to allow glycolysis to continue even in the absence of oxygen. And so even when there is no oxygen, glycolysis is able to continue and produce the small amount of ATP that it does because fermentation regenerates the NAD⁺ that it needs.

So in order to get a better understanding of this, let's take a look at this image that we have down below. And so recall that once again, the electron carriers, NADH and FADH₂s, they can be represented as these electron taxi cabs. And so notice here we have these electron taxi cabs and all of these other electron carriers here that we're showing as these other vehicles. And so notice that what we're showing you here in this image is that there is no oxygen acting as the final final electron acceptor. And so when you take a look at this sign here, notice that it says specifically that the electron transport chain is backed up because there's no final electron acceptor or no oxygen gas, to act as the final electron acceptor. And so when there's no oxygen, what happens is the amount of NADHs, are going to increase significantly, and so the electron transport chain is gonna get backed up like a traffic jam. And so notice here what we have is a traffic jam because there's no final electron acceptor, and there's no oxidative phosphorylation, which means there's not a lot of ATP being generated when there's no oxygen. However, even when there's no oxygen, fermentation can take place. And so notice over here we have this fermentation plant that has a sign that says, hey, we'll empty your taxi to help glycolysis and make a little bit of ATP just from glycolysis. And so this electron carrier here, this electron taxicab is basically saying, let's take this exit so that we can help out glycolysis and help glycolysis make a little bit of ATP. And so the fermentation plant is able to take the NADHs that are being built up, and it is basically able to take those electrons and it is able to reduce pyruvate to generate either lactic acid in some organisms or ethanol or alcohol in some other organisms. And so this is, fermentation is going to regenerate the NAD⁺ or the empty taxi cab, and the empty taxi cab or NAD⁺ is needed in order to allow glycolysis to continue forward. And so the empty taxicab is going to allow for glycolysis to take place and glycolysis is going to be able to produce a little bit of ATP even when there's no oxygen gas and the NADHs are, backed up in this traffic jam. And so notice that this here is a loop that can continuously happen so that glycolysis is able to continuously run even in the absence of oxygen. But once again glycolysis only produces a small amount of ATP, just 2 ATP molecules, and so, glycolysis, the amount of ATP that it produces is not enough to allow multicellular organisms like ourselves to survive in the absence of oxygen. And so, this here really just shows how fermentation is critical to allowing glycolysis to continue in the absence of oxygen. And so we'll get to talk even more about fermentation moving forward in our course when we talk about lactic acid fermentation and alcohol fermentation. But for now, this here concludes our introduction to what happens to aerobic organisms if there's no oxygen and, how fermentation is going to take place when there's no oxygen. And so we'll be able to get some practice applying these concepts as we move forward in our course, so I'll see you all in our next video.

Fermentation allows a cell to:

a) Recycle NADH to NAD⁺ for glycolysis.

b) Use NADH as a terminal electron acceptor.

c) Reduce NAD⁺ to NADH for glycolysis.

d) Synthesize ATP via ATP synthase.

• A. Recycle NADH to NAD⁺ for glycolysis.
• B. Use NADH as a terminal electron acceptor.
• C. Reduce NAD⁺ to NADH for glycolysis.
• D. Synthesize ATP via ATP synthase.

Lactic Acid Fermentation

In this video, we're going to briefly introduce lactic acid fermentation. Lactic acid fermentation is when pyruvate is reduced by NADH to form lactic acid or lactate along with NAD⁺ that's ultimately going to help drive glycolysis. Notice in our image below that we're showing you lactic acid fermentation. Notice that when there's absolutely no oxygen, the normal processes that we've talked about for aerobic cellular respiration such as pyruvate oxidation, the Krebs cycle, and the electron transport chain in chemiosmosis cannot occur. However, glycolysis is going to be able to occur and the reason for that is because lactic acid fermentation, represented by this red arrow right here, is going to ultimately generate lactic acid or lactate, and the pyruvate here is going to get reduced instead of being oxidized like it was normally with step 2 of aerobic cellular respiration. So the pyruvate is going to get reduced because it's going to be gaining electrons and ultimately it's gonna be converted to lactic acid. And through this lactic acid fermentation here, the big part is that it regenerates the NAD⁺ here, the empty electron taxicab. And this empty electron taxicab, the NAD⁺, is needed in order for glycolysis to continue forward. Ultimately, lactic acid fermentation is allowing for glycolysis to produce 2 ATP molecules. And a little bit of ATP is certainly better than no ATP molecules at all. Lactic acid fermentation will allow for a small amount of ATP to be made via glycolysis.

Lactic acid fermentation actually occurs in human muscle cells, but it can also occur in bacteria as well. When it does occur in bacteria, this is what gives yogurt its sour taste. Notice down below on the right-hand side of the image, we're showing you a human muscle to remind you that lactic acid fermentation will occur in our muscle cells during strenuous exercise when our muscles are low on oxygen. Lactic acid fermentation allows our muscles to still produce a little bit of energy via ATP. However, the small amount of ATP produced via glycolysis is not going to be enough to sustain our muscle cells for a long time. Then we'll have to stop performing the exercise so that we can get more oxygen and allow our normal aerobic cellular respiration to take place. Lactic acid fermentation can also occur in bacteria found in yogurt, which gives yogurt, once again, its sour taste. This concludes our brief introduction to lactic acid fermentation, and in our next video, we'll talk about alcohol fermentation. So I'll see you all there.

Alcohol Fermentation

In this video, we're going to briefly introduce alcohol fermentation. Alcohol fermentation is really similar to lactic acid fermentation, and the difference is that the pyruvate in alcohol fermentation is going to be reduced by NADH to form ethanol instead of forming lactic acid, and ethanol is really a type of alcohol. Now also, alcohol fermentation is not only going to produce ethanol but it's also going to regenerate NAD⁺ allowing for glycolysis to continue even in the absence of oxygen. Taking a look at our image down below, notice that we're showing you alcohol fermentation, and notice that alcohol fermentation is basically going to be very similar to lactic acid fermentation. The only difference is that ethanol is going to be produced instead of lactic acid. And also, once again, the NAD⁺s are going to get regenerated through alcohol fermentation. When NAD⁺ gets regenerated, that can be utilized to help continuously drive glycolysis even in the absence of oxygen, and glycolysis can continue to produce its very small amount of ATP. Alcohol fermentation is really going to produce beer from barley, the beer that humans can drink, and also wine from grapes as well. Over here on the right-hand side, notice that we're just showing you an image of a brewery and ice-cold beer. Just to remind you that alcohol fermentation is what can drive and produce the alcohols that we drink. This here concludes our introduction to alcohol fermentation, and we'll be able to get some practice as we move forward in our course. So I'll see you all in our next video.

Which of the following describes a primary function of both lactic acid fermentation and alcohol fermentation?

a) Reduction of NAD⁺ to NADH.

b) Oxidation of NADH to NAD⁺.

c) Reduction of FAD to FADH₂.

d)  Hydrolysis of ATP to ADP.

• A. Reduction of NAD⁺ to NADH.
• B. Oxidation of NADH to NAD⁺.
• C. Reduction of FAD to FADH₂.
• D. Hydrolysis of ATP to ADP.

Anaerobic Respiration

In this video, we're going to introduce anaerobic respiration. Recall from our previous lesson videos when we introduced fermentation, that fermentation can only produce a very small amount of ATP by allowing glycolysis to continue. Again, glycolysis only produces a little bit of ATP, and so not a lot of ATP is made during fermentation. However, some unicellular organisms can actually survive and make a significant amount of ATP even without oxygen. This is exactly where anaerobic respiration comes into play. The term 'anaerobic' means without oxygen or in the absence of oxygen. Anaerobic respiration is going to use some other molecule other than oxygen gas as the final electron acceptor of the electron transport chain. Anaerobic respiration is practically going to be the same as aerobic respiration except for the fact that the final electron acceptor is not oxygen. Some alternative electron acceptors include nitrate or NO3^-, sulfate or SO42-, and even carbon dioxide can act as the final electron acceptor in anaerobic respiration.

The biggest difference between anaerobic respiration and fermentation is that anaerobic respiration is going to make a lot more ATP than fermentation. Even though they both occur without oxygen, anaerobic respiration will make more ATP than fermentation, which only makes a little bit of ATP. But anaerobic respiration is actually going to end up making a lot less ATP than aerobic cellular respiration. Aerobic cellular respiration, which uses oxygen, is going to make the most amount of ATP followed by anaerobic respiration, and then the least amount of ATP is going to be made by fermentation. If we take a look at our image down below here, notice that it's the same image of the electron transport chain from our previous lesson videos, but really, the only difference here is that there is some alternative final electron acceptor. The final electron acceptor is not oxygen. Instead, during anaerobic respiration, there's going to be some alternative final electron acceptor such as either the nitrate or the sulfate.

This concludes our introduction to anaerobic respiration, and we'll be able to get some practice as we move forward in our course. I'll see you all in our next video.

In which of the steps of aerobic and anaerobic cellular respiration does substrate-level phosphorylation occur?

a) In glycolysis only.

b) In the Krebs cycle only.

c) In the electron transport chain only.

d) In both glycolysis and the Krebs cycle.

e) In both the Krebs cycle and the electron transport chain.

• A. In glycolysis only.
• B. In the Krebs cycle only.
• C. In the electron transport chain only.
• D. In both glycolysis and the Krebs cycle.
• E. In both the Krebs cycle and the electron transport chain.

Which of the following statements about NAD⁺ is true?

a) NAD⁺ is reduced to NADH during glycolysis, pyruvate oxidation, and the Krebs cycle.

b) NAD⁺ has more chemical energy than NADH.

c) NAD⁺ donates high energy electrons to the electron transport chain.

d) In the absence of NAD⁺, glycolysis can still function.

• A. NAD⁺ is reduced to NADH during glycolysis, pyruvate oxidation, and the Krebs cycle.
• B. NAD⁺ has more chemical energy than NADH.
• C. NAD⁺ donates high energy electrons to the electron transport chain.
• D. In the absence of NAD+, glycolysis can still function.

Here’s what students ask on this topic:

What is the difference between fermentation and anaerobic respiration?

Fermentation and anaerobic respiration are both processes that occur in the absence of oxygen, but they differ significantly. Fermentation involves the conversion of pyruvate into lactic acid or ethanol, regenerating NAD+ to sustain glycolysis, which produces only 2 ATP molecules. It does not involve the electron transport chain. In contrast, anaerobic respiration uses alternative electron acceptors like nitrate (NO₃^-) or sulfate (SO₄^2-) instead of oxygen in the electron transport chain, producing more ATP than fermentation but less than aerobic respiration. Thus, while both processes allow ATP production without oxygen, anaerobic respiration is more efficient than fermentation.

How does lactic acid fermentation occur in human muscle cells?

Lactic acid fermentation occurs in human muscle cells during intense exercise when oxygen levels are low. In this process, pyruvate, produced from glycolysis, is reduced by NADH to form lactic acid (lactate) and NAD+. This regeneration of NAD+ allows glycolysis to continue, producing 2 ATP molecules per glucose molecule. Although this provides a small amount of energy, it is not sufficient for prolonged activity, and the accumulation of lactic acid can lead to muscle fatigue. Once oxygen becomes available again, normal aerobic respiration resumes, and lactic acid is converted back to pyruvate or metabolized further.

What are the end products of alcohol fermentation?

Alcohol fermentation results in the production of ethanol (a type of alcohol) and carbon dioxide (CO₂). During this process, pyruvate from glycolysis is first converted to acetaldehyde, which is then reduced by NADH to form ethanol. This reaction also regenerates NAD+, allowing glycolysis to continue producing ATP. Alcohol fermentation is commonly used in brewing and winemaking, where yeast cells convert sugars into ethanol and CO₂, contributing to the alcohol content and carbonation of beverages like beer and wine.

Why is fermentation important for glycolysis in the absence of oxygen?

Fermentation is crucial for glycolysis in the absence of oxygen because it regenerates NAD+, which is essential for glycolysis to continue. During glycolysis, NAD+ is reduced to NADH. Without oxygen, the electron transport chain cannot operate, leading to a buildup of NADH and a shortage of NAD+. Fermentation addresses this by converting pyruvate into lactic acid or ethanol, using NADH and regenerating NAD+. This allows glycolysis to proceed, producing a small amount of ATP (2 ATP molecules per glucose), which is vital for cellular energy needs when oxygen is unavailable.

What are the alternative electron acceptors used in anaerobic respiration?

In anaerobic respiration, alternative electron acceptors are used instead of oxygen to complete the electron transport chain. Common alternative electron acceptors include nitrate (NO₃^-), sulfate (SO₄^2-), and carbon dioxide (CO₂). These molecules accept electrons at the end of the electron transport chain, allowing the process to continue and generate ATP. Although anaerobic respiration produces more ATP than fermentation, it generates less ATP compared to aerobic respiration, which uses oxygen as the final electron acceptor.