Krebs cycle
Krebs cycle.
Before starting the Krebs cycle, a preparatory step,
called the prep step is involved.
An enzyme called coenzyme A, binds the pyruvate to oxaloacetate.
NAD+ is reduced to NADH.
Carbon dioxide, CO2 is released.
A series of reactions takes place, and the end product is acetyl CoA.
Acetyl CoA is a two carbon molecule.
It is acetyl CoA, which enters the Krebs cycle.
The molecule oxaloacetate, is a substrate molecule,
which facilitates the Krebs cycle.
Oxaloacetate is a four carbon molecule.
At the end of the cycle the oxaloacetate used, is regenerated.
2 acetyl CoA combines, with 2 oxaloacetate molecules,
to produce 2 citrate molecules.
In this step coenzyme A, is released.
Citrate is also called as citric acid.
This is reason that the Krebs cycle, is also called as the citric acid cycle.
Citric acid is an 6 carbon molecule.
The citric acid cycle, strips the citrate of carbon molecules,
and harvests the energy, in the carbon bonds.
This is the basic principle of deriving energy,
from carbon back bone molecules.
First, 2 carbon molecules is stripped, from the citrate molecule.
This carbon combines with oxygen, to produce 2 CO2 molecules.
CO2 is released, as a by-product.
2 NAD+ molecules are reduced, to produce 2 NADH molecule.
NADH molecule are energy carrier molecules.
What is left is an 4 carbon intermediate A molecule.
In this discussion, for simplicity,
we will not name all the intermediate molecules.
We will simply refer to them, as intermediate A, B, C, and D.
Next, 2 more carbon molecules is stripped, from the intermediate A molecule.
CO2 is released as a by-product.
2 NAD+ molecules are reduced to 2 NADH molecules.
This produces a 2 carbon intermediate B molecules.
The two, 2 carbon intermediate B molecules, phosphorylates 2 ADP to 2 ATP molecules.
This is called substrate level phosphorylation.
What is left is another 2 carbon intermediate C molecule.
The two intermediate C molecules, reduce 2 FAD molecules,
to 2 FADH molecules.
FADH molecules are energy carrier molecules.
What is left is a 2 carbon intermediate D molecule.
The two intermediate D molecules, reduce 2 NAD+ molecules,
to 2 NADH molecules.
Oxaloacetate molecules, is regenerated with the intermediate D molecules.
The Krebs cycle started with oxaloacetate molecule.
It ends with regenerating the same molecule.
The Krebs cycle systematically breaks down,
the carbon bonds, in citrate.
This energy is stored in energy storehouse molecules, ATP,
and energy carrier molecules NADH, and FADH.
CO2 is generated, as a bi-product, and is released.
NADH and FADH move on, to the electron transport chain,
and help to generate ATP.
For this discussion we have taken glucose, as an example.
Energy molecules are derived from glucose.
Fats and proteins also enter the Krebs cycle after some processing.
Fats are converted to glucose.
Proteins are broken down to amino acids,
and then converted to acetyl CoA.
So, all forms of food, ultimately end up in the Krebs cycle,
to produce energy molecules.
We can summarise, what has happened in cellular respiration, so far.
In glycolysis,
2 ATP molecules,
2 NADH molecules and,
2 pyruvate molecules are produced.
In the prep stage,
2 CO2 molecules,
2 NADH molecules and,
2 acetyl CoA molecules are produced.
In the Krebs cycle,
4 CO2 molecules,
2 ATP molecules,
6 NADH molecules and,
2 FADH molecules are produced.
The 6 carbon atoms in glucose, combines with oxygen,
and is released as 6 CO2 molecules.
We have so far produced 4 energy storehouse ATP molecules.
We have also produced a total of,
10 NADH and,
2 FADH energy carrier molecules.
These 12 energy carrier molecules,
will help produce more ATP in the electron transport chain.