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

Fermentation occurs when aerobic organisms lack oxygen, leading to glycolysis continuing to produce a small amount of ATP. In lactic acid fermentation, pyruvate is reduced to lactic acid, regenerating NAD⁺ for glycolysis, which is crucial for energy production in muscle cells and certain bacteria. Alcohol fermentation similarly reduces pyruvate to ethanol, also regenerating NAD⁺. Anaerobic respiration, using alternative electron acceptors like nitrate or sulfate, generates more ATP than fermentation, but less than aerobic respiration, highlighting the importance of these processes in energy metabolism.

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 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 in our previous lesson videos, cannot occur. So aerobic cellular respiration can only occur if oxygen is present. Without oxygen as the final electron acceptor, the electron transport chain will get backed up like a traffic jam. Ultimately, the amount of NADH is going to increase, whereas the amount of NAD⁺ is going to decrease significantly to dangerously low levels.

If we take a look at our image below the top half of this image, notice that we have glycolysis as the very first step of cellular respiration. Again, if oxygen is present, then cellular respiration would occur as we've discussed in our previous lesson videos, where pyruvate oxidation occurs, then the Krebs Cycle, then the electron transport chain, and chemiosmosis. If there's no oxygen present, these stages will not occur. Instead, if there's no oxygen, then fermentation will take place. The process of fermentation uses the electrons from these NADH molecules that have increased to reduce pyruvate and generate alternative molecules that end up regenerating NAD⁺ levels that have dangerously decreased.

One of the big takeaways from fermentation is that it helps regenerate the NAD⁺ levels that have gotten dangerously low. Depending on the specific type of organism, the pyruvate that gets reduced can be turned into either lactic acid or it can be reduced to alcohol. Later in our course, we'll discuss lactic acid fermentation and alcohol fermentation as well. Fermentation ultimately makes very little amounts of ATP. Thus, only some unicellular organisms can survive on just fermentation alone, but multicellular organisms cannot survive on just fermentation because it produces so little ATP that it's not enough to drive the energy processes needed by multicellular organisms. However, fermentation is advantageous because it allows for the regeneration of NAD⁺ as already indicated, and that regeneration of NAD⁺ is critical to allowing glycolysis to continue even in the absence of oxygen.

Even when there is no oxygen, glycolysis can continue and produce the small amount of ATP it does, because fermentation regenerates the NAD⁺ needed. Let's take a look at this image below; recall that the electron carriers, NADH and FADH₂s, can be represented as electron taxis. Notice here we have these electron taxicabs and other electron carriers that we're showing as other vehicles. Notice that what we're showing you here in this image is, there is no oxygen acting as the final electron acceptor. 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. When there's no oxygen, what happens is the amount of NADH increases significantly, and the electron transport chain is going to get backed up like a traffic jam.

Notice here what we have is a traffic jam because there's no final electron acceptor. 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. Over here we have this fermentation plant with 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 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.” The fermentation plant can take the NADHs that are building up and reduce pyruvate to generate either lactic acid in some organisms or ethanol. or alcohol in other organisms. This regeneration of NAD⁺ or the empty taxi cab is needed to allow glycolysis to continue forward.

The empty taxicab allows for glycolysis to occur, and glycolysis can produce a little bit of ATP even when there's no oxygen gas and the NADHs are backed up in this traffic jam. This here is a loop that can continuously happen so that glycolysis can run even in the absence of oxygen. Glycolysis only produces a small amount of ATP, just 2 ATP molecules, and this amount of ATP is not enough to allow multicellular organisms like ourselves to survive in the absence of oxygen. This 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. But for now, this concludes our introduction to what happens to aerobic organisms if there's no oxygen and how fermentation takes place when there's no oxygen. We'll be able to get some practice applying these concepts as we move forward in our course. 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 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, and 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, 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 going to be converted to lactic acid. Through this lactic acid fermentation, the big part is that it regenerates the NAD⁺, the empty electron taxicab. This empty electron taxicab, the NAD⁺, is needed in order for glycolysis to continue forward, and so 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 is actually going to occur 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 down below 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 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, 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, alcohol fermentation is not only going to produce ethanol, but it's also going to regenerate the 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. 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. Also, once again, the NAD⁺ is going to get regenerated through alcohol fermentation, and 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. 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 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 that the final electron acceptor is not oxygen.

Some of the alternative electron acceptors include nitrate or NO₃^-, sulfate or SO₄^2-, 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 right 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. 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. Instead of the electrons making their way to the final destination of Orlando or 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. So, 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 aerobic and anaerobic respiration?

Aerobic respiration requires oxygen and produces a large amount of ATP by fully oxidizing glucose through glycolysis, the Krebs cycle, and the electron transport chain. The final electron acceptor is oxygen, forming water. Anaerobic respiration, on the other hand, does not require oxygen and uses alternative electron acceptors like nitrate (NO₃^-) or sulfate (SO₄^2-). It generates less ATP compared to aerobic respiration but more than fermentation. Both processes involve glycolysis, but anaerobic respiration continues with the electron transport chain using different final electron acceptors.

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 regenerate NAD⁺. This regeneration of NAD⁺ allows glycolysis to continue, producing a small amount of ATP (2 ATP molecules per glucose). Although this ATP is insufficient for prolonged activity, it provides a temporary energy source until oxygen levels are restored, allowing aerobic respiration to resume.

What are the end products of alcohol fermentation?

Alcohol fermentation produces ethanol (a type of alcohol) and carbon dioxide (CO₂) as end products. In this process, pyruvate from glycolysis is reduced by NADH to form ethanol and CO₂, while regenerating NAD⁺. This regeneration of NAD⁺ allows glycolysis to continue in the absence of oxygen, producing a small amount of ATP. Alcohol fermentation is commonly used in brewing beer and wine production.

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 processes, such as lactic acid fermentation and alcohol fermentation, convert NADH back to NAD⁺, allowing glycolysis to produce a small amount of ATP (2 ATP molecules per glucose) even without oxygen.

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 acceptors allow the electron transport chain to continue operating, producing more ATP than fermentation but less than aerobic respiration. The specific acceptor used depends on the organism and its environment.