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The Citric Acid (Krebs) Cycle: Products and Steps

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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

In this lesson, we return to the process of cellular respiration for the second act of creating energy from food. In this act, products from glycolysis feed into the next stage, the citric acid cycle.

The Purpose of the Citric Acid Cycle

Glycolysis breaks glucose down into 2 pyruvate molecules.
Glucose Breaks Down into Pyruvate

In previous lessons, we started to learn about cellular respiration, the process that turns food into chemical energy. We learned how this process begins to use the food we eat and the air we breathe. It's a three-phase process, beginning with glycolysis, followed by the citric acid cycle, and, finally, the electron transport chain. In this lesson, we'll learn how the products of glycolysis feed into the citric acid cycle and how the products of the citric acid cycle ultimately end up with the products of glycolysis in the electron transport chain.

In glycolysis, you'll remember that we broke down sugary glucose molecules from the food at our picnic. One glucose molecule in glycolysis became two three-carbon sugars called pyruvate. During this process, we also netted two ATP molecules, or units of chemical energy, as well as two NADH + H+ molecules, or electron carriers.

Step Between Glycolysis and Citric Acid Cycle

Now, before we can get to the next stage of cellular respiration, the citric acid cycle, there's some prep work that needs to be done. Before you barbecue your steaks at the picnic, you probably want to thaw and season them first to get them ready to go. So, let's get those products from glycolysis ready for the grill.

Each pyruvate, which is produced in the cytoplasm, enters the mitochondria to be converted into acetyl coenzyme A (acetyl-CoA). Remember that two pyruvates are created from glycolysis, meaning two acetyl coenzyme A molecules are produced. Acetyl coenzyme A is a two-carbon molecule.

Pyruvate oxidation converts pyruvate into 2 acetyl coenzyme As, 2 CO2 molecules, and 2 NADH + H+.
Glycolysis-Citric Acid Cycle Link

Are you wondering where the third carbon from pyruvate went? It's found in a second product of the reaction as a carbon dioxide molecule, which eventually diffuses out of the cell and into your bloodstream. It's part of the same carbon dioxide that you exhale!

This preparation for the citric acid cycle is called pyruvate oxidation because the pyruvate is oxidized, or loses electrons, to form NADH + H+. One NADH + H+ is produced per pyruvate. This brings our total for this reaction to two acetyl coenzyme As, two carbon dioxide molecules, and two NADH + H+. This reaction links glycolysis to the citric acid cycle.

Citric Acid Cycle Steps

With two acetyl coenzyme As inside the mitochondrial matrix, we are finally able to start the steps of the citric acid cycle, or second stage of cellular respiration. The citric acid cycle is also known as the Krebs cycle. No matter what you call it, you'll notice the name fits the bill. This stage of cellular respiration is a cyclical process of 8 different chemical reactions. While all the reactions that occur are important, in this lesson, we're just going to focus on the reactions that are essential to create products that are important to the next phase of cellular respiration.

The citric acid cycle consists of 8 chemical reactions.
Eight Steps of Citric Acid Cycle

To start, oxaloacetic acid, a four-carbon molecule, combines with acetyl coenzyme A from pyruvate oxidation . The coenzyme A molecule separates, donating the acetyl group to oxaloacetic acid so that it becomes a six-carbon molecule - this is called citric acid. Do you see where the citric acid cycle got its name?

Citric acid is oxidized by the electron carrier NAD+. In turn, NAD+ is reduced to become NADH + H+. This reaction also releases a carbon dioxide molecule and turns the six-carbon citric acid molecule into a five-carbon molecule.

Another reaction produces the same result, creating a four-carbon molecule, carbon dioxide, and NADH + H+. Remember that this carbon dioxide is the same carbon dioxide that we exhale, just like the carbon dioxide made earlier during pyruvate oxidation!

In a later step, this four-carbon molecule undergoes a change that eventually ends up converting an ADP molecule to ATP. This is the only citric acid step that releases chemical energy.

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