Respiration

Now let's take a look at the steps by which energy is extracted from carbohydrates and some of the reactions that are involved. To do that, we'll focus on this part of the energy diagram, respiration. (This diagram is also shown in Example 7 in your workbook.)

Respiration portion of the energy cycle diagram. [67013.jpg]

The oxidation of glucose is in some ways very much like the oxidation of fats. Fats are converted, through a number of steps, into pyruvic acid and acetyl CoA, which then enter the citric acid cycle to generate carbon dioxide and hydrogen, which is then oxidized to form water. Similarly, glucose is converted, through a sequence of steps, into pyruvic acid and acetyl CoA, which then enter into the citric acid cycle to generate carbon dioxide and hydrogen, which is then oxidized to make water.

Respiration portion of the energy cycle diagram with fats added in. [67014.jpg]

Glucose to Pyruvic Acid

The conversion of glucose, which has six carbons, to pyruvic acid, which has three carbons, may seem like a pretty simple process. We just split the molecule and we've essentially accomplished what we want to do. Of course, it's not like that at all.

Equation for the reaction converting glucose to pyruvic acid showing just the carbon atoms. [67015.jpg]

It is true that the molecule does split. However, if you take into account the location of the hydrogen and oxygen atoms, which is a very important thing to do, something else emerges.

If you start counting the hydrogen and oxygen atoms, you will see that along with splitting the molecule, but we must remove four hydrogen atoms. So, this is also an oxidation reaction. (This reaction equation is also shown in Example 8 in your workbook. Use it for taking notes.)

Equation for the reaction converting glucose to pyruvic acid. [67016.jpg]

But it's not even that simple, if you look at what happens to each individual carbon atom.

	The first carbon atom gets reduced by losing an oxygen atom and gaining two hydrogen atoms along with the electrons.
	The second carbon atom gets oxidized by losing two hydrogen atoms.
	The third carbon atom gets oxidized by losing one hydrogen atom (odd - they are usually lost in pairs) and gaining an oxygen atom. It also loses a carbon bond because of the split.
	Similarly, the fourth carbon atom is oxidized by losing one hydrogen atom (ahh - that makes a pair) and gaining an oxygen atom. It also loses a carbon bond of carbon because of the split.
	The fifth carbon atom is oxidized by losing two hydrogen atoms.
	The sixth carbon atom is reduced by losing an oxygen atom.

Annotated equation for the reaction converting glucose to pyruvic acid. [67017.jpg]

If you start thinking about how many steps have to occur in order to change a six-carbon glucose molecule into two three-carbon pyruvic acid molecules, you can see that this is not a simple reaction. There are about a dozen steps, just to change the glucose to pyruvic acid. If you're interested in seeing what those steps are, you might check your textbook or one of the other textbooks that's available in the lab.

Pyruvic Acid to Acetyl CoA

The next step in the oxidation of glucose is to change the pyruvic acid to acetyl CoA. This is, by the way, the same reaction that occurs at this step in the oxidation of fats. Once fat or glucose has been converted to pyruvic acid, it doesn't matter what it used to be. (This equation is also shown in Example 9 in your workbook. Use it for taking notes.)

Equation for the reaction converting pyruvic acid to acetyl CoA. [67018.jpg]

The reaction that converts pyruvic acid into acetyl CoA accomplishes three things.

	Two hydrogens (underlined in the diagram) with electrons are removed by an oxidizing agent. The oxidizing agent that accomplishes this is NAD⁺, the oxidized form of NAD. As it picks up the two hydrogens and electrons, it is converted into NADH, the reduced form of NAD. One of the hydrogens actually remains free as H⁺. (Note: the removed hydrogens should be shown with the NAD or on the right side of the equation, but not both as shown here.)
	One carbon and two oxygen atoms are removed (circled in the diagram). So this is a decarboxylation reaction as well as an oxidation reaction.
	Also, coenzyme A attaches to the second carbon to form acetyl CoA.

Annotated equation for the reaction converting pyruvic acid to acetyl CoA. [67019.jpg]

Citric Acid Cycle and Electron Transport System

Once pyruvic acid has been converted to acetyl CoA, then that enters into the citric acid cycle, which you have studied before. In the process of citric acid reacting to form one chemical, then another, then another, and so forth, carbon dioxide is given off along with hydrogen and electrons. These then enter the electron transport system and eventually combine with the oxygen to make water. This is the process that occurs when glucose is oxidized with oxygen available to drive this last part of the process.

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