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

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 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. 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.

If we take a look at our image below the top half of this image, notice that we have glycolysis here as the very first step of cellular respiration. 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 these stages are only going to occur if oxygen is present. If there's no oxygen present, then these stages are not going to occur. Instead, if there's no oxygen, then fermentation is going to take place. The process of fermentation is 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 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.

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. Later in our course, we'll discuss lactic acid fermentation and alcohol fermentation as well. Fermentation ultimately is going to make very little amounts of ATP. Only some unicellular organisms can survive on just fermentation alone, but multicellular organisms cannot survive on just fermentation because it makes so little ATP that it's not enough to drive the energy processes needed by multicellular organisms. But fermentation is advantageous because it will allow for the regeneration of NAD⁺ as we've already indicated. That regeneration of NAD⁺ is critical to allow glycolysis to continue even in the absence of oxygen. Even when there's 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.

In order to get a better understanding of this, let's take a look at this image that we have below. Recall that the electron carriers, NADH and FADH₂s, can be represented as electron taxi cabs. Notice here we have these electron taxi cabs and all of these other electron carriers here that we're showing as these other vehicles. Notice what we're showing you here in this image is that there is no oxygen acting as the final electron acceptor. When there's no oxygen, the amount of NADHs are going to increase significantly, and so the electron transport chain is going to get backed up like a traffic jam. Notice here we have 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. However, even when there's no oxygen, fermentation can take place. 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." This electron carrier here, this electron taxi cab, 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 is able to take the NADHs that are being built up, and it's basically able to take those electrons and it's able to reduce pyruvate to generate either lactic acid in some organisms or ethanol or alcohol in some other organisms.

This here really just 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 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. 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 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, we're showing you lactic acid fermentation. Notice that when there's absolutely no oxygen, the normal processes 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. 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 allows for glycolysis to produce 2 ATP molecules. Even a little bit, 2 ATP molecules, 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 can also occur in bacteria as well. When it occurs 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 are doing strenuous 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, so it can only occur for a small amount of time. We then have to stop performing the exercise to get more oxygen and allow our normal aerobic cellular respiration to take place. Lactic acid fermentation can also occur in bacteria found in yogurt, and that gives yogurt, once again, its sour taste. This concludes our brief introduction to lactic acid fermentation; 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 really 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. Moreover, 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. 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. 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.

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. Glycolysis only produces a little bit of ATP, and 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 uses some 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 oxygen. In anaerobic respiration, the final electron acceptor is some other molecule instead of oxygen. Some of the alternative electron acceptors include nitrate or NO3-, sulfate or SO422-, 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 makes a lot more ATP than fermentation. Even though both occur without oxygen, anaerobic respiration will make more ATP than fermentation, which only makes a little bit of ATP. Yet, anaerobic respiration still makes a lot less ATP than aerobic cellular respiration. Aerobic cellular respiration, which uses oxygen, will make the most amount of ATP, followed by anaerobic respiration, and the least amount of ATP will be made by fermentation. If we look at the diagram below, notice that it's the same image of the electron transport chain from our previous lesson videos, but the only difference here is that there is an alternative final electron acceptor. This final electron acceptor is not oxygen. During anaerobic respiration, it will be some other destination such as maybe nitrate or sulfate, symbolizing that electrons are not heading towards oxygen but towards another element or compound. These alternative final electron acceptors are present in anaerobic respiration. 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.

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.