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