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

Fermentation occurs in the absence of oxygen, allowing glycolysis to continue by regenerating NAD⁺ from NADH. Lactic acid fermentation converts pyruvate into lactic acid, producing 2 ATP, and occurs in human muscle cells during intense exercise. Alcohol fermentation, on the other hand, converts pyruvate into ethanol, also regenerating NAD⁺ and producing small amounts of ATP. Anaerobic respiration utilizes alternative electron acceptors like nitrate or sulfate, generating more ATP than fermentation but less than aerobic respiration, highlighting the importance of these processes in energy production.

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. 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 what happens if aerobic organisms don't have any oxygen around? Without oxygen, aerobic cellular respiration, as we've discussed it 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 while the amount of NAD⁺ is going to decrease significantly down to dangerously low levels. 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 it in our previous lesson videos where pyruvate oxidation would occur, then the Krebs Cycle, then the Electron Transport Chain and Chemiosmosis. These stages here are only going to occur if oxygen is present.

However, if there's no oxygen present, then these stages are not going to occur. Instead, fermentation is going to take place. The process of fermentation is going to use the electrons from these NADH molecules, that have increased, to reduce pyruvate and generate alternative molecules that end up regenerating NAD⁺, which have gotten dangerously low. One of the big takeaways of fermentation is that it's going to help regenerate those NAD⁺ molecules. Depending on the specific type of organism, the pyruvate that gets reduced can be converted to either lactic acid or it could be reduced to alcohol.

Latter in our course, we'll discuss lactic acid fermentation and alcohol fermentation as well. Fermentation is going to make very little amounts of ATP, and so really only some unicellular organisms can survive on just fermentation alone. Multicellular organisms 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. Fermentation is advantageous because it will allow for the regeneration of NAD⁺ as we've already indicated. This regeneration is critical to allow glycolysis to continue even in the absence of oxygen.

In order to get a better understanding of this, let's take a look at this image we have down below. Recall that once again, the electron carriers NADH and FADH₂s can be represented as these electron taxicabs. Notice here we have these electron taxicabs and all of these other electron carriers here that we're showing as these other vehicles. There is no oxygen acting as the final electron acceptor, and so the electron transport chain is backed up like a traffic jam, 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. We have a 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". This electron carrier, this electron taxicab, is basically going to take this exit so that we can help out glycolysis and help glycolysis make a little bit of ATP. The fermentation plant is able to take the NADHs that are being built up and use those electrons to reduce pyruvate to generate either lactic acid in some organisms or ethanol or alcohol in some other organisms.

This 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. The empty taxi cab 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. 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 again, glycolysis only produces a small amount of ATP, just 2 ATP molecules, and so the amount of ATP produced is not enough to allow multicellular organisms like ourselves to survive in the absence of oxygen.

This really shows how fermentation is critical to allowing glycolysis to continue in the absence of oxygen. We'll get to talk even more about fermentation moving forward in our course when we talk about lactic acid fermentation and alcohol fermentation. This 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. 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.

Lactic Acid Fermentation

In this video, we're going to briefly introduce lactic acid fermentation. Lactic acid fermentation is really just 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, we're showing you lactic acid fermentation. 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 will be able to occur and the reason for that is because lactic acid fermentation, represented by this red arrow here, is going to ultimately generate lactic acid or lactate, and the pyruvate is going to get reduced instead of being oxidized as 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 going to be converted to lactic acid. Through this lactic acid fermentation here, the big part is that it regenerates the NAD⁺ here, the empty electron taxicab. This empty electron taxicab, the NAD⁺ is needed in order for glycolysis to continue forward. Ultimately, lactic acid fermentation is allowing for glycolysis to proceed forward even in the absence of oxygen and glycolysis is able to produce a small amount of ATP, just 2 ATP molecules, and a little bit is certainly better than no ATP molecules. 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 also it can occur in bacteria as well, and when it does occur in bacteria this is what gives yogurt its sour taste. Notice on the right-hand side of the image we're showing you a human muscle here to remind you that lactic acid fermentation will actually occur in our muscle cells when we're doing extraneous exercise and our muscles are low on oxygen. Lactic acid fermentation will occur and our muscles are able to still produce a little bit of energy via ATP, but that small amount of ATP produced via glycolysis is not going to be enough to sustain our muscle cells and so it can only occur for a small amount of time and 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. But also, lactic acid fermentation can also occur in bacteria that are found in yogurt, and that gives yogurt once again its sour taste. This here concludes our brief introduction to lactic acid fermentation and in our next video, we'll get to 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, with the main difference being that the pyruvate in alcohol fermentation is reduced by NADH to form ethanol instead of forming lactic acid, and ethanol is a type of alcohol. Alcohol fermentation not only produces ethanol but also regenerates the NAD⁺, allowing glycolysis to continue even in the absence of oxygen. Taking a look at our image below, notice that we're showing you alcohol fermentation, which is very similar to lactic acid fermentation.

The only difference is that ethanol is produced instead of lactic acid. Once again, the NAD⁺ is regenerated through alcohol fermentation. When NAD⁺ is regenerated, it 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 produces beer from barley, which humans can drink, and also wine from grapes. Over here on the right-hand side, notice that we're just showing you an image of a brewery with 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.

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. And 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. It is practically the same as aerobic respiration, except that the final electron acceptor is not going to be oxygen. In anaerobic respiration, the final electron acceptor is going to be some other molecule instead of oxygen. Some of the alternative electron acceptors include nitrate or NO3-, sulfate or SO42-, and even carbon dioxide can act as the final electron acceptor in anaerobic respiration.

Really, 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.

But really, the only difference here is that there is some alternative final electron acceptor. Notice that 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. So, instead of the electrons making their way to the final destination of oxygen, the final destination is going to be some other destination such as maybe New Orleans to represent the 'n' in the nitrate or San Antonio to represent the 's' in the sulfate. These alternative final electron acceptors are going to be present in anaerobic respiration.

This here 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.

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.

Here’s what students ask on this topic:

What is the difference between aerobic and anaerobic respiration?

Aerobic respiration requires oxygen and involves glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport chain, producing up to 36-38 ATP per glucose molecule. Anaerobic respiration, on the other hand, does not require oxygen and uses alternative electron acceptors like nitrate or sulfate. While it also involves glycolysis and the electron transport chain, it produces less ATP than aerobic respiration but more than fermentation. The key difference lies in the final electron acceptor: oxygen in aerobic respiration and other molecules in anaerobic respiration.

How does lactic acid fermentation work in human muscle cells?

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

What are the end products of alcohol fermentation?

Alcohol fermentation converts pyruvate into ethanol and carbon dioxide. This process also regenerates NAD⁺ from NADH, allowing glycolysis to continue in the absence of oxygen. The overall reaction can be summarized as: $C_6{H}_{12}{O}_6\to C_2{H}_5\mathrm{OH}+{\mathrm{CO}}_2$. This process is used in brewing beer and making wine.

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

Fermentation is crucial for glycolysis to continue in the absence of oxygen because it regenerates NAD⁺ from NADH. During glycolysis, NAD⁺ is reduced to NADH, and without oxygen, the electron transport chain cannot operate to oxidize NADH back to NAD⁺. Fermentation steps in to convert NADH back to NAD⁺, allowing glycolysis to proceed and produce a small amount of ATP (2 ATP per glucose molecule). This is essential for cells to generate energy when oxygen is not available.

What are the alternative electron acceptors used in anaerobic respiration?

In anaerobic respiration, alternative electron acceptors are used instead of oxygen. 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 than aerobic respiration.