Anaerobic Reactions
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Anaerobic Reactions

In the absence of oxygen to drive the last part of this process, the citric acid cycle suffers a sort of gridlock and is unable to function. Acetyl CoA and pyruvic acid pile up because they cannot be processed. Under these conditions, a couple different things can happen to the pyruvic acid, let's take a look at what those are.

Diagram of overall respiration process. [67020.jpg]

Pyruvic Acid to Lactic Acid

One of the alternate fates of pyruvic acid is that it can be converted into lactic acid. This commonly occurs in muscle tissue that is trying to use oxygen faster than it's coming in. Note that a ketone group has been converted into an alcohol group. Pyruvic acid picks up two additional hydrogen atoms to become lactic acid, so this is a reduction reaction. (This equation is also shown in Example 10 in your workbook.)

Equation for reaction converting pyruvic acid to lactic acid. [67021.jpg]

Pyruvic Acid to Ethanol

Another anaerobic alternative for pyruvic acid is that it can be converted into acetaldehyde and then into ethanol. This kind of reaction generally occurs in yeast. (This equation is also shown in Example 11 in your workbook.)

In the first reaction, pyruvic acid releases carbon dioxide as it becomes acetaldehyde. That process, in yeast, generates the carbon dioxide that is necessary for causing breads and baked goods to rise before they're baked.

In the second reaction, an aldehyde is converted into an alcohol. This is another reduction reaction.

Equation for reaction converting pyruvic acid to acetaldehyde and ethanol. [67022.jpg]

 

Reduction vs. Oxidation

Why should these alternate reactions for pyruvic acid be reduction reactions? In the previous page, dealing with respiration and the oxidation of glucose, we talked about how the reactions were oxidation reactions. Well, remember that we're now talking about reactions occurring in the absence of oxygen. In the absence of oxygen, we're talking about reduction changes instead of oxidation changes.

Let's briefly review an important aspect of what happens in the respiration process. The hydrogen atoms and electrons, which enter the electron transport system in order to react with oxygen, are actually attached to other molecules. Commonly, they start in the form of NADH and there is a sequence of reactions in which oxygen finally combines with hydrogen to form water.

67020.jpg (5759 bytes)

 

This abbreviated representation of the electron transport system (also shown in Example 12 in your workbook) begins with NAD and ends with oxygen. Everything in between is simply represented by X and Y. NAD passes hydrogen and electrons to X (turning it into XH2) which, in turn, passes them to Y (turning it into YH2) which, in turn, passes them to oxygen (turning it into water). If oxygen takes the electrons, then Y is free to take some more from X (in the XH2 form).

Biochemical redox series. [67027.jpg]

If oxygen is not available to take electrons from Y (in the YH2 form) it will remain "stuck" in the reduced (YH2) form and not be able to take them from X which will remain "stuck" in the reduced (XH2) form and it will not be able to take them from NAD. Consequently, NAD will remain in the reduced (NADH) form.

Notice that one of the functions of having the oxygen available to take hydrogens and electrons to make water is that, at the top of this list the reduced form of NAD (NADH) was oxidized. It was able to give it's electrons and hydrogens to something else and regenerate the oxidized form which can then be used to carry on additional oxidation reaction that it needs to do. If oxygen is not available to take electrons, NAD+ will not be available, only the reduced form will be available.

Let's take a look at the consequences of that.

Look at the first step in the oxidation of glucose. Previously, we didn't go into the details of it, but in the conversion of glucose to pyruvic acid, four hydrogens with their elctrons had to be removed. Guess what it was that removed those four hydrogens. It was the NAD+, the oxidized form of NAD. In order for the process of oxidizing glucose to get energy to continue, NAD+ must be available. NADH2 must be reoxidized to replenish NAD+.

67016.jpg (15721 bytes)

If oxygen is not available to cause the reoxidation of the NAD, then something else has to do it, and that's where this reaction comes in. Pyruvic acid takes on the task of oxidizing the reduced form of NAD. It removes hydrogen and electrons from NADH and attaches them to its central carbonyl group, forming the lactic acid. That process also converts the reduced NADH to the oxidized NAD+, which will allow the glycolysis or splitting of glucose to continue.

Annotated equation (with NAD) for the reaction converting pyruvic acid to lactic acid. [67028.jpg]

 

Yeast accomplishes essentially the same thing, but using different enzymes and coming up with a different chemical product. But again, a reduction reaction occurs. Yeast converts the pyruvic acid to acetaldehyde and then uses the acetaldehyde as the oxidizing agent to take hydrogen from the reduced NADH, applies those hydrogens to the aldehyde carbonyl group to make an alcohol, and frees up the oxidized NAD+ to continue with the reactions that are needed (such as the glycolysis or splitting of glucose).

Annotated equation for the reaction converting pyruvic acid to acetaldehyde to ethanol. [67029.jpg]

 

Other kinds of cells and microorganisms have other ways of dealing with anaerobic conditions, but these two particular processes are fairly important and widespread.

Annotated equations for both muscle conversion of pyruvic acid to lactic acid and yeast converion of pyruvic acid to ethanol. [67030.jpg]

 

At this point, I recommend that you review this energy cycle diagram and make sure that you understand how these processes are interrelated, particularly with regard to the role of carbohydrates in respiration and photosynthesis.

Diagram of overall energy cycle. [67031.jpg]

As you do that review, you might also try to create a balance sheet by counting carbon atoms and things such as that. Then refine that balance sheet by also counting hydrogen and oxygen atoms. Here are some appropriate questions to consider.

When glucose is formed in photosynthesis, how many carbon atoms are contained in glucose? In order for that to happen, how many carbon dioxide molecules are needed? And how many water molecules? And how many oxygen molecules are formed?
When glucose is changed to pyruvic acid, and then pyruvic acid to acetyl CoA, and in the citric acid cycle, how many carbon dioxide molecules come out? And how many hydrogen atoms come out?
Do we get the same number of hydrogen atoms coming out of these reactions as the number that are necessary to use up the oxygen that was created in photosynthesis?

You might not come up with answers to all of those questions, but think about them. Get some sense of the balance that's involved in this process. Also, I recommend that you review how the oxidation of fats enters into this process.

In the other sections of this lesson, we'll continue by looking at the structure of glucose and how it can be converted into other kinds of chemicals.

 

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E-mail instructor: Sue Eggling

Clackamas Community College
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