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Krebs Cycle

Krebs Cycle

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

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Tutorial

What's Covered

Welcome to this lesson on the Krebs cycle. Today you will be learning about the processes that occur in the Krebs cycle in order to help produce ATP for the cell. Specifically, you will look at:

  1. Overview
  2. Preparatory Steps
  3. The Kreb’s Cycle

1. Overview

The Krebs cycle is the second stage in cellular respiration, following glycolysis, that produces two ATP molecules for the cell.

Term to Know

Krebs Cycle

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

Remember, ATP stands for adenosine triphosphate and 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 within the mitochondria of the cell and is an aerobic process, meaning it requires oxygen to occur. 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.

2. Preparatory Steps

In order for the Krebs cycle to begin, you have pyruvate, which is a three-carbon molecule that you got from glycolysis, the stage of cellular respiration that comes before the Krebs cycle. A carbon dioxide will be released, leaving you with a two-carbon fragment of pyruvate, which will then combine with something called coenzyme A. When that two-carbon fragment of pyruvate combines with coenzyme A, it produces something called acetyl CoA. Acetyl CoA. So once you have this acetyl CoA, you’re now ready to begin the actual Krebs cycle.

Terms to Know

Coenzyme A


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

Acetyl CoA

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


3. The Kreb’s Cycle

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

Step by Step

Step 1: You started with pyruvate that was turned into acetyl COA, which is a two-carbon molecule. Then you have this four-carbon molecule that's going to combine with that two-carbon molecule to produce a six-carbon molecule.

Step 2: Then 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.

Term to Know

NAD+ and NADH

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.

Step 3: You had this six-carbon molecule and had a carbon removed and what you’re left with is a five-carbon molecule. Then the steps repeat again and CO2 is going to be released again. NAD plus will donate a hydrogen, leaving us with NADH and again, that's an electron carrier molecule.

Did You Know

This carbon dioxide that's being released is actually being breathed out. Each time you exhale, you're exhaling carbon dioxide, which is being released from the Krebs cycle within the mitochondria of your cells.

Step 5: Then you’re going to a four-carbon molecule from which you had a five-carbon molecule releasing a carbon, and you'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. Our ADP is gaining a phosphate and turning into ATP, which is our energy storage molecule. You haven't changed any of our carbons, so we're still at a four-carbon molecule.

Step 6: The next thing that's going to happen here is NAD plus will become NADH, also FAD will become FADH2. A hydrogen will be removed and transferred, so we have FADH2, which is also another type of an electron carrier molecule. We still have then our four-carbon molecule that we started with.

Term to Know

FADH2 and FAD

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.

Big Idea

You'll notice you start with this four-carbon molecule, then it combines with this two-carbon molecule to produce a six-carbon molecule. Then as you go through the cycle, you end back up where you started, with the four-carbon molecule. Each turn of the cycle will work on one pyruvate molecule and if you remember from glycolysis, you have two pyruvate molecules that you get out of glycolysis. SIt takes two turns of the cycle for each glucose to be broken down, because one glucose yields two pyruvates.


Summary

So this lesson has been an overview of the preparatory steps that lead to the Kreb’s cycle and the processes that occur in the Krebs cycle.


Keep up the learning and have a great day!

Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND

TERMS TO KNOW
  • ​Krebs Cycle

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

  • Coenzyme A


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

  • Acetyl CoA

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

  • FADH2 and FAD

    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+ and NADH

    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.