A level Biology exam revision resources written by A level Examiners


Respiration and ATP

Respiration is the process which occurs in all cells to make the chemical potential energy stored in molecules such as glucose available to the cell. This energy is required for processes such as:

  • Muscle contraction
  • Active transport
  • Biosynthesis

It is impossible to use the energy stored in molecules like glucose directly because too much energy would be released at once and this would cause damage to the very processes that require the energy. In order to make use of this stored energy the energy must be transferred to energy carrier molecules, which can individually release smaller amounts of energy. The molecule that is used to 'carry' the energy is the same in all living organisms. The molecule is called adenosine triphosphate (ATP).

ATP is made from an organic adenine unit and a ribose sugar unit with three phosphate molecules attached. A molecule of ATP is shown in the diagram below.


ATP can be broken down into adenosine diphosphate ADP and an inorganic phosphate molecule in a reaction that releases a small amount of energy which can be used to do useful work within a cell. This reaction (like most biological reactions) requires the presence of an enzyme to occur. The diagram below shows how ATP can be broken down to release energy and how this reaction can be see as part of a cycle


Note Pi stands for inorganic phosphate

Cellular Respiration   Exam Advice

Respiration is the term used to cover a series of reactions that occur within cells which lead to the formation of ATP from ADP and Pi. A level biology requires knowledge of the reaction which occur when glucose is used as the substrate for respiration. It is important to note however that other molecules may be used by living systems.

To generate the maximum number of molecules of ATP from glucose oxygen is required. Respiration using oxygen is termed aerobic respiration. Respiration without oxygen is termed anaerobic respiration.

The overall equation for respiration can be written as:


This however is slightly misleading because it doesn't clearly show the energy stored in glucose is being transferred to another molecule. The diagram below is a better representation of the process.


This reaction is a simplification of what in reality is a complex metabolic pathway, comprising more than 20 separate enzyme catalysed reactions. All these reaction take place within cells. Many take place with mitochondria.   Exam Advice

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