Krebs Cycle

Captain's Log:

When I got ready for my adventure this morning, I had no idea how different this experience would be from the ones I had already encountered. Unfortunately, I had to leave early in the Krebs Cycle so that I could make it on time for my photosynthesis appointment. First, our acetyl CoA molecule traveled to find oxaloacetic acid which had four carbons of its own. We formed citric acid, causing us all to be in a rather bitter mood since there were six carbons in such a small place. I guess one of the carbons grew sick of the situation quickly because he left and joined oxygen to form carbon dioxide. I felt pretty uncomfortable in ketoglutaric acid because I was the only carbon left from pyruvic acid. All of the other carbons seemed to know each other pretty well since they travel in oxaloacetic acid in the Krebs Cycle most of the time. Even though I was interested in seeing what would happen next, I got kicked out of the group to join some oxygen that was nearby. I really wasn't very upset about having to leave the Krebs Cycle because I heard that those carbons do not know how to have any fun. They just go through the same routine everyday for countless times. They are very good at producing energy, but they didn't need me to help them with the ATP production. Instead, I got to travel through the bloodstream to the lungs, where I was exhaled. It was exciting to leave Tom's body for the first time because I was all on my own. I've decided to just hang out in the air for a while until I reach my next destination. I'm planning on going to a plant because I heard that carbon dioxide is always in high demand for something called photosynthesis. It's kind of like a factory, they are going to take my carbon dioxide molecule and separate us. The oxygen will go back into the air while I will become part of a sugar just like I was a couple of days ago!

An Overview:

The importance of the Krebs Cycle can not be overstated; almost all of our ATP is produced in this cycle. We can see two main problems as a result of glycolysis:

not enough ATP produced

a lack of NAD⁺ molecules

Remember that the body needs NAD⁺ molecules to act as electron carriers receiving the extra hydrogen atom and electrons from G3P to form NADH. Since the cell contains only a small supply of NAD⁺, there must be a way to recycle NAD⁺ from NADH to continue the shuttling of electrons. Sometimes the body reacts by immediately passing the hydrogen atom from NADH to an organic compound without the aid of an electron transport system. This process is called fermentation. However, fermentation occurs in an anaerobic environment and will only support the muscles for a short period of time before invoking lactic acid poisoning. Therefore, our bodies have evolved to "fix" the problems of our energy history. Unlike fermentation, the Krebs Cycle is able to gather the energy trapped in the NADH molecules and reproduce more NAD⁺. Also the conversion of NADH to NAD⁺ is coupled with the production of 24 ATPs.

A Closer Look:

Step 1:

This step consists of the combination of pyruvic acid and coenzyme A to form acetyl CoA. (This is described fully on the previous page)

Step 2:

Acetyl - CoA, consisting of two carbons, joins oxaloacetic acid, made up of four carbons, to form citric acid, a six-carbon compound.

Step 3:

One of citric acid's two recently acquired carbons is oxidized to form CO₂. At the same time, a NADH is produced leaving ketoglutaric acid.

Step 4:

The last carbon remaining from pyruvic acid is removed in the form of CO₂. Another NADH is formed, while the only ATP directly generated in the Krebs Cycle is produced. Left behind is succinic acid, a four-carbon molecule lacking any of pyruvic's original carbons

Step 5:

Two hydrogen atoms and their corresponding electrons are released from succinic acid and attach to FAD to form FADH₂. The remaining molecule is malic acid.

Step 6:

The continuing oxidation of succinic acid releases another hydrogen atom and two electrons forming NADH. This completes one turn of the circle as oxaloacetic acid is reproduced and ready to pick up another Acetyl CoA and form citric acid.

Summary:

The Krebs Cycle begins with oxaloacetate and combines with Acetyl CoA to cycle through one complete turn. After Acetyl CoA is oxidized to CO₂ and H₂O, the electrons drive proton pumps that generate ATP that is greatly needed by the cell. Remember that the NADH molecules are important because they contain extracted electrons that ultimately reduce NAD⁺. However, when the electrons do not have enough energy to reduce NAD⁺, they are stored temporarily in the FADH₂ molecule, as in Step 5. Each NADH molecule is responsible for the production of three ATP molecules while FADH₂ is responsible for the production of two ATP molecules. In the table below, the products of glycolysis, oxidation of pyruvic acid, and the Krebs Cycle are summarized:

Summary of ATP production through respiration

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