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3. The Krebs cycle

The acetyl CoA then enters the Krebs Cycle. This is a cyclic metabolic pathway, and is also known as the citric acid cycle or the tricarboxylic acid cycle. The 2-carbon acetylcoenzyme A enters the Krebs cycle by reacting with a 4-carbon intermediate compound in the matrix of the mitochondria where the Krebs cycle takes place. The diagram below shows the Krebs cycle in summary   Exam Advice


Note FAD is another coenzyme which (like NAD) when reduced contains a hydrogen atom which came from the initial glucose molecule.

The intermediate compound in the Krebs cycle are all organic acids. This is why when an amino acid is deaminated the organic acid created can often be fed directly into the Krebs cycle. See section on Protein metabolism

Review of the products of stages 1-3

From a single molecule of glucose there has been a generation of the following:

  • 6 CO2
  • 4 ATP
  • 10 reduced NAD
  • 2 reduced FAD

This shows that amount of ATP generation so far is quite limited. The bulk of the ATP generation occurs in the final stage and involves the reduced coenzymes.

4. The electron transport chain

The reactions of this stage occur on the membranes of the cristae of the mitochondria. A summary of the process is shown below.


This is clearly very diagrammatic and only shows an outline. At a level it is not necessary to balance the equations which occur during the electron transport chain. The following section covers the level of detail required.

The electron transport chain takes place within the inner mitochondrial membrane, using integral membrane proteins, which act as electron carrier molecules. In the electron transport chain the hydrogen atoms from the reduced coenzymes gradually release all their energy to form ATP, and are finally combined with oxygen to form water.

The process starts when a reduced coenzyme gives up its hydrogen atom (becomes oxidised). The hydrogen atom acts as a source of protons (H+) and electrons (e-).


The electrons are passed down the electron carrier molecules. This occurs because each molecule binds electrons more tightly than the previous one. In these molecules the electrons release some of their energy, which is used to pump protons across the inner mitochondrial membrane by active transport (the protons are actually transported through the electron carrier molecules).

The proton gradient which has been created acts as a store of energy just as the potential energy in the water can be used to generate electricity in a hydroelectric power station, the energy in the proton gradient can be used to generate ATP.

When the protons return through the membrane they release the energy which is used to form ATP from ADP and Pi

When the electrons leave the final electron carrier molecule they are accepted by oxygen atoms which then accepts protons forming water   Exam Advice

How many molecules of ATP are generated by respiration?   Exam Advice

Few (2)

Lots (>30)
Electron transport chain

It is worth noting that although glucose is the molecule which is most commonly used in respiration other substances can also be used to make ATP. Triglycerides can be broken down to fatty acids and glycerol, both of which can enter the Krebs Cycle. Proteins are not normally used to make ATP, but in times of starvation they can be broken down and used in respiration. They are first broken down to amino acids, which are deaminated and fed into the Krebs cycle.

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