A level Biology exam revision resources written by A level Examiners


There are four main stages that the complex series of reactions which make up respiration can be divided into:

  1. Glycolysis
  2. Link reaction
  3. Krebs cycle
  4. Electron transport chain

Before going into these stages in detail there are several points which are worth considering:

  • The different stages of respiration take place in different parts of the cell. This allows the cell to keep the various metabolites separate, and to control the stages more easily.
  • Since the amount of detail required at A level is still at the summary level, it is meaningless to try to balance all the equations in each step precisely.
  • The release of carbon dioxide takes place before oxygen is involved. So it isn't true to say that respiration turns oxygen into carbon dioxide. It is better to say that respiration turns glucose into carbon dioxide. Respiration also turns oxygen into water using hydrogen atoms from glucose.

1. Glycolysis   Exam Advice

This stage occurs in both aerobic and anaerobic respiration. It takes place in the cytoplasm the reactions involved can be summarised as shown below.


The above summary shows that glucose which is a six carbon molecule is split into 2 molecules of pyruvate which are 3 carbon molecules. The diagram also introduces NAD. This is a complex organic molecule called a coenzyme. It is not necessary to know the formula of NAD. The significance of this molecule is that it can accept hydrogen atoms which are released in the reactions. As NAD accepts a hydrogen atom it is said to be reduced. This hydrogen atom is very important as it stores energy, which is later used by the respiratory chain to make more ATP. As NAD is reduced the glucose molecule is oxidised. This means energy is transferred from glucose to Reduced NAD. The diagram below shows the energy changes that occur during glycolysis.


These reactions are said to be coupled redox reactions. This means that the energy released from one oxidation reaction allows the reduction reaction to occur. Reduction and oxidation are important processes (redox reactions occur throughout respiration). They are summarised in the table below.

Addition of hydrogen
Removal of hydrogen

Gain of electrons
Loss of electrons

An increase in energy level (energy required)
A decrease in energy level (energy given out)

If there is no oxygen present the other 3 stages do not occur. As can be seen from the glycolysis equation some ATP is generated. The final products of anaerobic respiration are not the same in plants, animals and fungi. A summary of anaerobic respiration is shown in the diagram below.


2. The link reaction

This reaction links glycolysis and the Krebs cycle. It can be summarised as shown below


During this stage the organic molecule coenzyme A is joined to the pyruvate molecule. A molecule of carbon dioxide is given off therefore acetylcoenzyme A can be thought of as a two carbon molecule. A hydrogen atom is also given off which is used to reduce a molecule of NAD.   Exam Advice

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