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The Citric Acid (Krebs) Cycle: Products and Steps

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  1. 0:05 The Purpose of the Citric Acid Cycle
  2. 1:01 Step Between Glycolysis and…
  3. 2:17 Citric Acid Cycle Steps
  4. 4:42 Lesson Summary
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Taught by

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 barbeque 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 (acetyll-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 A 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.

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?

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

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.

Through the last few steps, this four-carbon molecule is further oxidized by one more NAD+ and one FAD, another type of electron carrier, to become NADH+ H+ and FADH2. This four-carbon molecule that results is familiar. We eventually end up back where we started! What goes around comes around, right?

This cycle recreates the four-carbon oxaloacetic acid, which is ready to collect the second acetyl coenzyme A from pyruvate oxidation and re-enter the citric acid cycle to complete the reactions again. The wheel will keep turning, twice for each glucose molecule that entered the cell from whatever you ate for dinner today.

Lesson Summary

Okay! So now for every glucose from the food we enjoy, we've taken two rides around the the citric acid cycle, the second stage of cellular respiration, using products from glycolysis. Let's see where we stand for the final stage of cellular respiration, the electron transport chain.

Two molecules of pyruvate were produced from glycolysis and converted into two molecules of acetyl coenzyme A, two carbon dioxide, and two NADH + H+ molecules through pyruvate oxidation, an intermediate step between glycolysis and the citric acid cycle. Each acetyl coenzyme A proceeded once through the citric acid cycle. Therefore, in total, it created 6 NADH + H+ molecules, two FADH2 molecules, four carbon dioxide molecules, and two ATP molecules. That's a lot of products!

Perhaps it didn't create a lot of chemical energy, but our reduced electron carriers from this stage of cellular respiration have picked up lots of energetic electrons for the last step - the electron transport chain - to help our cells create plenty more energy from our food! So just stay tuned!

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