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Krebs Cycle

Krebs Cycle

Author: Sophia Tutorial
Description:

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 Krebs Cycle

1. Overview

The Krebs cycle is the second stage in cellular respiration, following glycolysis, that produces two ATP molecules for the cell. ATP stands for adenosine triphosphate, and is an energy storage molecule used by cells.

Term to Know

Krebs Cycle

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

You may sometimes hear the Krebs cycle 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. Pyruvate from glycolysis will move into the mitochondria and begin the preparatory steps of the Krebs cycle.


2. Preparatory Steps

Remember that pyruvate is a three-carbon molecule that came from glycolysis, the stage of cellular respiration before the Krebs cycle. 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

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 Krebs Cycle

Once you have acetyl CoA, you’re ready to begin the actual Krebs cycle.

Step by Step

Step 1: We started with pyruvate that was turned into acetyl COA, which is a two-carbon molecule. Then we have a 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. A hydrogen will be removed from NAD+ and transferred to NADH. NADH is an electron carrier that 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: We had a six-carbon molecule, which had a carbon removed to leave us with a five-carbon molecule. The steps then repeat and CO2 is released again. NAD+ will donate a hydrogen, leaving us with NADH, 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 4: When the five-carbon molecule releases a carbon, we're left with a four-carbon molecule. ADP is now going to join with a phosphate group, transforming it into ATP, our energy storage molecule. We haven't changed any of the carbons, so we're still at a four-carbon molecule.

Step 5: NAD+ will become NADH, and FAD will become FADH2. A hydrogen will be removed and transferred, giving us FADH2 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 start with a four-carbon molecule, which then combines with a 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. Remember, you have two pyruvate molecules that you get out of glycolysis. It takes two turns of the cycle for each glucose to be broken down because one glucose yields two pyruvate molecules.


Summary

This lesson has been an overview of the preparatory steps that lead to the Kreb’s cycle and the processes that occur within 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
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.

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.

​Krebs Cycle

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