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

Lactate fermentation occurs in animal muscle cells during strenuous activity, where pyruvate is reduced to lactate by lactate dehydrogenase, oxidizing NADH to NAD⁺. This reversible process involves the conversion of a carbonyl group into an alcohol group. In contrast, alcohol fermentation, performed by certain bacteria and yeast, converts pyruvate to ethanol and carbon dioxide through a two-step process involving decarboxylation and reduction, utilizing pyruvate decarboxylase and alcohol dehydrogenase, respectively. Both processes highlight the importance of fermentation in energy production under anaerobic conditions.

Anaerobic Respiration Concept 1

Now remember, if oxygen is not present, then pyruvate will go through anaerobic respiration. Here, without oxygen, pyruvate is redirected through fermentation. So, we take a look here, we have glycolysis, where fermentation is the generation of energy in the absence of oxygen. In the absence of oxygen, fermentation regenerates NAD⁺ allowing glycolysis to continue. If we come back up here, we have glucose undergoing glycolysis, releasing ATP, and forming our pyruvate.

Here we're going to have our NADH. NADH will undergo fermentation, helping to regenerate NAD⁺. Remember, NAD⁺ can then oxidize glucose through glycolysis to make more pyruvate, right?

Again, this only occurs if oxygen is not present. If oxygen were present, then pyruvate would go through Acetyl CoA formation to make Acetyl CoA, proceed through stages 3 and 4 of catabolism to make NADH, FADH₂, and more ATP. But again, this pathway is blocked to us. We can't go this way because there is no oxygen present for pyruvate to utilize in order to make Acetyl CoA.

Anaerobic Respiration Concept 2

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. 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⁺. When we examine this reaction, we have pyruvate being reduced, 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, meaning the enzyme is the same either way we go. Although we're accustomed to seeing dehydrogenases as a class of enzymes used for oxidation reactions, this reversible reaction can be used for either oxidation or reduction.

So here again, we're discussing lactate fermentation, using lactate dehydrogenase to reduce our pyruvate to lactate.

Anaerobic Respiration Concept 3

In this video, we're going to take a look at alcohol fermentation. Here, this process is used by certain bacteria and yeast to convert pyruvate into ethanol and carbon dioxide. Pyruvate to ethanol happens through a two-step process. In these processes, we're going to say one carbon atom is lost as carbon dioxide. If we take a look 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 two, once we've created our ethanol, we're going to say one NADH is oxidized to one NAD⁺.

Here, we'd say that our ethanol is going to be converted into ethanol. Right? So here, if we're looking at this, we're going to say that our aldehyde group is reduced into an alcohol group. So we're creating an alcohol. So we'd say that this is an alcohol dehydrogenase.

Right. So when it comes to our alcohol fermentation, this is the two-step process that we would employ to change pyruvate first into ethanol and then from ethanol to ethanol.

Anaerobic Respiration Example 1

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, and from ethanol to ethanol. To change it into Ethanol, we use Pyruvate decarboxylase because it's a decarboxylation reaction, we lose CO₂. And then ethanol is 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 Acetyl Aldehyde. "Acetyl" refers to 2 carbons with a carbonyl involved, and "aldehyde" refers to that end carbon being an aldehyde. So this one was a little bit tricky, 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 perform decarboxylation to change pyruvate into ethanol, but then we have to go one step further and reduce that ethanol into ethanol. Therefore, this does not work. So here, the only correct answer is option B.

Select the best statement about fate of pyruvate.

• A. Conversion of pyruvate to lactate is irreversible because it is an oxidation reaction.
• B. Conversion of pyruvate to lactate involves reduction of aldehyde group of pyruvate.
• C. Conversion of pyruvate to acetaldehyde is irreversible because it is a reduction reaction.
• D. Pyruvate oxidation involves transfer of acetyl group to CoA.
• 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)

• A. a) anaerobic b) anaerobic c) aerobic d) anaerobic
• B. a) aerobic b) anaerobic c) aerobic d) anaerobic
• C. a) aerobic b) aerobic c) anaerobic d) aerobic
• D. a) anaerobic b) aerobic c) anaerobic d) aerobic