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Krebs Cycle: Phase Two of Cellular Respiration

Krebs Cycle: Phase Two of Cellular Respiration


This lesson will describe in detail the processes that occcur in the krebs cycle stage of cellular respiration.

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The Krebs Cycle

Source: Video and Images Created by Amanda Soderlind

Video Transcription

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Welcome to this lesson on the Krebs cycle. Today we will be discussing the processes that occur in the Krebs cycle in order to help produce ATP for the cell. So the Krebs cycle is the second stage in cellular respiration, following glycolysis, that produces two ATP molecules for the cell. And remember, ATP stands for adenosine triphosphate. So ATP is an energy storage molecule used by cells. And the Krebs cycle, you might also sometimes hear it referred to as the citric acid cycle. But they're both the same thing.

The Krebs cycle occurs in the mitochondria of the cell. So let's take a look at this picture right here, this diagram. And it's just showing you our three phases of cellular respiration. And the Krebs cycle, colored in green here, shows you that it's our second phase and that it occurs in the mitochondria. So the structure surrounding it here is the mitochondria.

The Krebs cycle requires oxygen, meaning it's an aerobic process. It needs oxygen to occur. And basically what happens in the Krebs cycle is that pyruvate from glycolysis will move into the mitochondria and began the preparatory steps of the Krebs cycle.

So let's take a look here at those preparatory steps that we're talking about. So in order for the Krebs cycle to begin, we have pyruvate, which is a three-carbon molecule that we got from glycolysis, which is the stage of cellular respiration that comes before the Krebs cycle. And a carbon dioxide will be released, leaving us with a two-carbon fragment of pyruvate, which will then combine with something called coenzyme A.

So when that two-carbon fragment of pyruvate combines with coenzyme A, it produces something called acetyl COA. Acetyl COA. So once we have this acetyl COA, we're now ready to begin the actual Krebs cycle.

So let's take a look at this diagram here. So as we had mentioned, we started with pyruvate. That was turned into acetyl COA, which is a two-carbon molecule. And then we have this four-carbon molecule here that's going to combine with that two-carbon molecule to produce a six-carbon molecule.

Then what's going to happen is carbon dioxide will be released. NAD plus will transfer-- a hydrogen will be removed from NAD plus and transferred to NADH. Now, NADH is just an electron carrier. And it will carry electrons into the electron transport chain to make more ATP later.

So we had this six-carbon molecule. We had a carbon removed. So now what we're left with is a five-carbon molecule. And then we're going to repeat these steps again.

CO2 is going to be released again. And NAD plus will donate a hydrogen, leaving us with NADH. And again, that's an electron carrier molecule.

And this carbon dioxide that's being released is actually being breathed out. So each time you exhale, you're exhaling carbon dioxide, which is being released from the Krebs cycle within the mitochondria of your cells. That's kind of an interesting fact.

So then we're going to a four-carbon molecule. So we had our five-carbon molecule. We're releasing a carbon. And we're left with a four-carbon molecule.

Now what's going to happen is ADP, which stands for adenosine diphosphate, di- meaning two, is going to join with a phosphate group, transforming it into ATP, adenosine triphosphate, which means three. Let me move this up just a little bit. There you go. So our ADP is gaining a phosphate and turning into ATP, which is our energy storage molecule. So we haven't changed any of our carbons, so we're still at a four-carbon molecule.

The next thing that's going to happen here is NAD plus will become NADH. And also FAD will become FADH2. So a hydrogen will be removed and transferred, so we have FADH2, which is also another type of an electron carrier molecule. So we still have then our four-carbon molecule that we started with.

So you'll notice we start with this four-carbon molecule. It combines with this two-carbon molecule to produce a six-carbon molecule. And then as we go through the cycle, we end back up where we started, with our four-carbon molecule.

So each turn of the cycle will work on one pyruvate molecule. Now if you remember from glycolysis, we have two pyruvate molecules that we get out of glycolysis. So it takes two turns of the cycle for each glucose to be broken down, because one glucose yields two pyruvates.

So this lesson has been an overview on the processes that occur in the Krebs cycle.

Terms to Know
Acetyl CoA

A molecule produced from pyruvate which enters the Krebs cycle to produce ATP.

Coenzyme A

An enzyme used to change pyruvate into acetyl CoA which enters the Krebs cycle.


FAD is a molecule that picks up hydrogen atoms and becomes FADH2 which is then transferred to the electron transport chain to produce more ATP from the energy in electrons.


NAD+  is a molecule that picks up hydrogen atoms and becomes NADH which is then transferred to the electron transport chain to produce more ATP from the energy in electrons.

​Krebs Cycle

The second stage of cellular respiration in which pyruvate from glucose is used to produce ATP, NADH and FADH2.