Anaerobic Respiration: Study with Video Lessons, Practice Problems & Examples

Anaerobic respiration occurs when oxygen is absent, preventing the electron transport chain from producing ATP. Instead, pyruvate undergoes fermentation in the cytosol, generating energy less efficiently than aerobic respiration. In lactate fermentation, pyruvate is converted to lactate, while in alcohol fermentation, pyruvate is transformed into ethanol and carbon dioxide through a two-step process involving decarboxylation and reduction. Both processes regenerate NAD⁺, allowing glycolysis to continue, but yield only 2 ATP compared to over 20 ATP from aerobic pathways.

Anaerobic Respiration Concept 1

Hey everyone, so in this video we'll talk about the limitations of anaerobic respiration. Now, here we're going to say without oxygen as the final electron acceptor, the electron transport chain or ETC is not able to produce ATP. Now remember, when it comes to stage 4 of our food catabolism, we're dealing with ETC and oxidative phosphorylation. We're going to say here that we're dealing with our different complexes 1 to 4 within our ETC, this portion that's highlighted. Remember, NAD⁺ goes to our complex 1 to draw well, NADH drops off its electrons at complex 1 to become NAD⁺ again. FADH₂ takes its electrons to complex 2 to become FAD again. Through our chain of these all travelling electrons, O₂ is supposed to serve as the final electron acceptor and with the use of ATP synthase produce ATP. But without oxygen being there, we can't go from ETC to oxidative phosphorylation. So, what happens instead? Well, pyruvate is redirected through fermentation in the cytosol. So we're not able to go and take the pyruvate and go into the mitochondrial matrix because of the absence of oxygen. So the pyruvate stays within the cytosol in order to do anaerobic respiration. Alright. So this is just a quick overview of why exactly anaerobic respiration is happening. Now, on the next video, we'll take a more detailed look at the process of food catabolism, particularly what happens without the presence of oxygen.

Anaerobic Respiration Concept 2

So, if we take a closer look at our food catabolism, remember here we're basically having our monosaccharides entering the cytosol. At this point, we should go into glycolysis. And, remember in glycolysis we make high energy molecules in the form of NADH and ATP. So here in glycolysis, we do create our ATP and we have also the generation of pyruvate. At this point, we should have gone into stage 3 where we're going from a cytosol into the mitochondrial matrix. But again, we're dealing with anaerobic respiration so there's no oxygen available. So pyruvate is unable to go towards acetyl CoA formation. So instead, it continues cycling through fermentation. So here in fermentation, we have NADH becoming oxidized into NAD⁺ to start the whole cycle again. Now, here again we're going to say fermentation is the generation of energy in the absence of oxygen. We're still making energy, but it's less efficient than aerobic respiration because we don't have the Electron Transport Chain (ETC) and oxidative phosphorylation. Now, here, this is utilized by animals and certain microorganisms. We're going to say in the presence of oxygen, so O₂, it's serving as our oxidizing agent, fermentation regenerates NAD⁺, allowing glycolysis to continue. We see that in the cyclic nature of what's going on here. Because we're going from glycolysis to pyruvate formation in NADH. NADH undergoes fermentation to create NAD⁺ again and start the whole glycolysis process once again. Now, here we're going to say recall, glycolysis only makes 2 ATP. Now, this is much less than the 20 plus ATP we would have made through aerobic respiration through the ETC and oxidative phosphorylation. This is just a way of continuing to make energy when conditions are not ideal because of the absence of oxygen. Alright. So keep that in mind. Anaerobic respiration can still make ATP molecules, it just makes way fewer of them because, our pyruvate is unable to go into Acetyl CoA formation and then go from stages 3 to 4 for the creation of even more ATP molecules.

Anaerobic Respiration Example 1

This example question asks, "Why is anaerobic respiration, also called fermentation, in eukaryotic cells inefficient?" Alright, so for the first one it says, "No metabolic processes are able to continue without oxidation." That's not true. If there's no oxygen available, that's why fermentation happens. Our pyruvate does not go into the mitochondrial matrix; it stays within the cytosol. So, this is not true. Here, ETC or electron transport chain is not able to produce ATP without oxygen as the final electron acceptor. Alright. This one's tricky. The ETC doesn't technically make the ATP. It's the oxidative phosphorylation step that makes the ATP. And we're going to say without oxygen involved, yes, we won't be able to go through stage 4 of food catabolism to make ATP. But this thing is not true because it's not the ETC that's even making the ATP. It's oxidative phosphorylation step where it happens. I know we tack on ETC and oxidative phosphorylation together, but technically it's the oxidative phosphorylation step. Stage, complex 5 or ATP is finally being made through ATP synthase.

Now, here, glycolysis only produces 2 ATP molecules per one glucose molecule. This answers better because here it's telling us why it's inefficient. We're only making 2 ATP molecules per one glucose molecule versus the 20 plus ATP molecules we would have made if we had gone through the ETC and oxidative phosphorylation, stage 4 of food catabolism. So, this is a much better answer. And then overproduction of NAD⁺ causes glycolysis to shut down. No. The production of NAD⁺ is what's allowing us to continue with glycolysis. This is saying the opposite. So, this is not true.

The only answer here that talks about the inefficiency of the absence of oxygen is part C. Under fermentation, we're only making 2 ATP molecules versus the 20 plus ATP molecules we could have made if we went through the mitochondrial matrix through Stage 3 and then Stage 4. So again, C is our final answer.

Anaerobic Respiration Concept 3

In this video, we'll take a look at lactate fermentation. Now, here, this process occurs in animal muscle cells, and it happens during strenuous activity. Now, here we're going to say that pyruvate is going to be reduced by Lactate Dehydrogenase to Lactate. And in this process, 1 NADH is oxidized to 1 NAD⁺. Now, if we take a look at this reaction, we have pyruvate, and pyruvate here is going to be reduced, and what we're doing here is we're changing this carbonyl into an alcohol group. That's where the reduction happens. In this process, NADH is going to be oxidized to NAD⁺. This is a reversible process. That means the enzyme is the same either way we go. I know we're accustomed to seeing dehydrogenases as a class of enzymes that we use for oxidation reactions. But because this is a reversible reaction, it can be used for oxidation or reduction. It's a reversible process. Alright. So here again, we're talking about Lactate Fermentation. So basically, we're using lactate dehydrogenase to reduce our pyruvate to lactate.

Anaerobic Respiration Concept 4

In this video, we're going to take a look at alcohol fermentation. Here, this process is by certain bacteria and yeast that convert pyruvate to ethanol and carbon dioxide. Pyruvate to ethanol happens by a 2-step process. In these processes, we're going to say 1 carbon atom is lost as carbon dioxide. If we take a look here at the first step, we have pyruvate here; we've highlighted the carboxyl group of pyruvate. We use pyruvate decarboxylase because decarboxylation is occurring. Remember, decarboxylation means that we're losing CO2. Losing CO2 creates an aldehyde group. And because this is 1 to 2 carbons long, this is ethanol. Remember, aldehydes end with "al".

In step 2, once we've created our ethanol, we're going to say 1 NADH is oxidized to 1 NAD⁺. Here, we'd say that our ethanol is going to be converted into ethanol. In this, our aldehyde group is reduced into an alcohol group, so we're creating an alcohol. We'd say that this is an alcohol dehydrogenase. So when it comes to our alcohol fermentation, this is the 2-step process that we would employ in order to change pyruvate first into ethanol and then from ethanol to ethanol.

Anaerobic Respiration Example 2

So here it says, Pyruvate is converted into Ethanol and Carbon Dioxide by which of the following enzymatic reactions? Alright, so pyruvate is directly converted to ethanol by alcohol dehydrogenase. Now, this is not true, we know that it is a 2 step process, so this cannot work. Next, pyruvate is converted to Acetaldehyde by pyruvate decarboxylase, then reduced to ethanol by alcohol dehydrogenase. Alright. So remember we have pyruvate. It's going to be changed into ethanol. In order to change it into ethanol, we do use pyruvate decarboxylase because it's a decarboxylation reaction. We lose CO₂. And then ethanol is going to be reduced to ethanol through the use of alcohol dehydrogenase. But here they're using the term Acetaldehyde. Well, remember, we're going to say ethanol here, is a 2 carbon chain, the end carbon is an aldehyde. Another name for ethanol, its more common name, is Acetaldehyde. Acetyl here is talking about 2 carbons with a carbonyl involved, and then aldehyde is talking about that end carbon being an aldehyde. So this one was a little bit tricky, little covered up in terms of which is the correct answer. Ethanol is the same thing as Acetaldehyde. So B is our answer. Now if we look at the other options here, pyruvate is converted to lactate. No. It's pyruvate being converted into ethanol. Pyruvate is converted to ethanol by oxidative decarboxylation. So remember, we do do decarboxylation to change pyruvate into ethanol, but then we have to go one step further and reduce that acetaldehyde into ethanol. Okay, so this does not work. So here, the only correct answer is option B.

How is aerobic respiration different from anaerobic respiration?

• A. Anaerobic respiration produces ethanol or lactate, while aerobic respiration produces water and more CO₂.
• B. Fermentation takes place in the mitochondrial matrix, whilst pyruvate oxidation takes place in the cytoplasm of the cell.
• C. Aerobic respiration produces less ATP than anaerobic.
• D. Aerobic respiration can be described as reduction reactions, while anaerobic is oxidation reactions.
• E. Both fermentation and pyruvate oxidation produce NAD⁺.

Select statement that explains importance of conversion of NADH to NAD⁺ during anaerobic respiration.

• A. Cells rely on glycolysis to produce ATP and NAD⁺ in the absence of oxygen.
• B. Conversion of glucose to pyruvate in glycolysis requires NAD⁺ as an electron acceptor.
• C. Allows for conversion of glucose to Acetyl CoA in the absence of oxygen.
• D. Regeneration of NAD⁺ through fermentation ensures that glycolysis will come to a halt.

Circle the correct type of respiration under which:

a) Pyruvate converted to lactate (aerobic or anaerobic)

b) Glucose converted to pyruvate (aerobic or anaerobic)

c) Pyruvate converted to Acetyl CoA (aerobic or anaerobic)

d) Pyruvate converted to ethanol (aerobic or anaerobic)

In this practice question, it says, "Circle the correct type of respiration under which pyruvate is converted to lactate." Alright, so if pyruvate is going into lactate, the production of lactate happens under fermentation. Fermentation only occurs in the absence of oxygen, which only happens under anaerobic respiration. Glucose is converted to pyruvate. Here, glucose becoming pyruvate can also fall under anaerobic respiration because, at this point, all we've done is create the pyruvate. We still can decide where we're going to go with it. We don't know if there's oxygen present. So, if we're going by default thinking that oxygen is not present, we're going to say it happens over anaerobic respiration. Next, pyruvate is converted to Acetyl CoA. So if pyruvate is going to change into Acetyl CoA, that means pyruvate is going to go from the cytosol to the mitochondrial matrix. This would only occur if oxygen was available. And if oxygen is available, this is aerobic respiration. Then here we're going to say, pyruvic converted to ethanol. Ethanol is an alcohol. If pyruvate is becoming an alcohol, this happens under alcohol fermentation. Again, if fermentation is occurring, that means there is an absence of oxygen, and therefore, this is anaerobic respiration. So, this is how we classify each of the following statements on the type of respiration we're dealing with in terms of each of these statements.

Here’s what students ask on this topic:

What is anaerobic respiration and how does it differ from aerobic respiration?

Anaerobic respiration occurs in the absence of oxygen, preventing the electron transport chain (ETC) from producing ATP. Instead, pyruvate undergoes fermentation in the cytosol, generating energy less efficiently. In contrast, aerobic respiration uses oxygen as the final electron acceptor in the ETC, allowing for oxidative phosphorylation and the production of over 20 ATP molecules per glucose molecule. Anaerobic respiration yields only 2 ATP per glucose molecule through glycolysis and fermentation. The key difference lies in the presence of oxygen and the efficiency of ATP production.

What are the main types of fermentation in anaerobic respiration?

The main types of fermentation in anaerobic respiration are lactate fermentation and alcohol fermentation. In lactate fermentation, pyruvate is converted to lactate by lactate dehydrogenase, with NADH being oxidized to NAD⁺. This process occurs in animal muscle cells during strenuous activity. In alcohol fermentation, pyruvate is converted to ethanol and carbon dioxide through a two-step process involving decarboxylation and reduction. This type of fermentation is carried out by certain bacteria and yeast. Both processes regenerate NAD⁺, allowing glycolysis to continue.

Why is anaerobic respiration less efficient than aerobic respiration?

Anaerobic respiration is less efficient than aerobic respiration because it does not utilize the electron transport chain (ETC) and oxidative phosphorylation, which are responsible for producing the majority of ATP in aerobic respiration. In anaerobic conditions, pyruvate undergoes fermentation, yielding only 2 ATP per glucose molecule through glycolysis. In contrast, aerobic respiration can produce over 20 ATP per glucose molecule by fully oxidizing glucose in the presence of oxygen. The absence of oxygen in anaerobic respiration limits the energy yield significantly.

How does lactate fermentation work in anaerobic respiration?

In lactate fermentation, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase. During this process, NADH is oxidized to NAD⁺, which is essential for glycolysis to continue. This reaction is reversible, meaning lactate can be converted back to pyruvate when oxygen becomes available. Lactate fermentation occurs in animal muscle cells during strenuous activity when oxygen supply is insufficient. Although it allows for continued ATP production, it is much less efficient than aerobic respiration, yielding only 2 ATP per glucose molecule.

What is the role of NAD⁺ in anaerobic respiration?

NAD⁺ plays a crucial role in anaerobic respiration by acting as an electron carrier. During glycolysis, NAD⁺ is reduced to NADH, which must be oxidized back to NAD⁺ to allow glycolysis to continue. In the absence of oxygen, this oxidation occurs through fermentation processes. In lactate fermentation, NADH is oxidized to NAD⁺ as pyruvate is reduced to lactate. In alcohol fermentation, NADH is oxidized to NAD⁺ during the conversion of acetaldehyde to ethanol. This regeneration of NAD⁺ is essential for the continuous production of ATP in anaerobic conditions.