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Electron transport chain

A series of electron carriers involved in the production of ATP in the mitochondria of the cell.

Adenosine diphosphate

The molecule produced when ATP is hydrolysed to release energy from the terminal phosphate. ADP and inorganic phosphate (Pi) are the products of the reaction.

Catabolic reactions

Reactions where a larger molecule is broken down into smaller molecules ("breaking down" reactions).

Chemiosmotic theory

The theory developed by Peter Mitchell to explain how the production of ATP is linked to the electron transport chain in the mitochondria.

Active transport

The process which uses energy to move substances against a concentration gradient or across a partially permeable membrane using a special transport protein.


Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse


A chemical reaction where more energy is released as bonds form than is taken in to break the bonds, so the surroundings are heated.

Adenine (A)

The nitrogenous base, adenine, which pairs with T, thymine.

ATP (adenosine triphosphate)

Chemical reactions need energy to take place. The energy is needed to break the bonds in molecules to enable new bonds to form. One molecule acts as the common energy currency in all cells – adenosine triphosphate, or ATP. ATP is synthesised in the mitochondria of the cells. Anything which interferes with the synthesis or breakdown of ATP in the cell rapidly leads to the death of the cell and the whole organism.

The structure of ATP

ATP is made up of

  • a nitrogenous base, adenine (also involved in the structure of DNA and RNA)
  • a five-carbon (pentose) sugar, ribose (also involved in the structure of RNA)
  • three inorganic phosphate groups
Structure of ATP

The structure of ATP is closely linked to its function in the cell.

ATP in action

ATP acts as an energy store. When the final phosphate bond of an ATP molecule is broken by the enzyme ATPase in an exothermic hydrolysis reaction, around 30.5 kJ of energy is released per mole of ATP. Some of this energy is lost as heat but most is available to drive other reactions in the cell such as muscle contraction, protein synthesis or active transport. The products of the reaction are adenosine diphosphate, or ADP, and inorganic phosphate ions (Pi).

ATP + H2O → ADP + Pi ΔH -30.54 kJmol-1

The breakdown of ATP is a reversible reaction. The condensation reaction, catalysed by ATP synthase, produces ATP and water and requires an input of 30.54 kJ per mole of energy. It is usually linked to catabolic reactions such as cellular respiration.

ADP + Pi → ATP + H2O ΔH +30.54 kJmol-1

Breakdown and synthesis of ATP

The breakdown and synthesis of ATP

ATP production

The production of ATP is a very complex process. Scientists have worked out that the main way in which ATP is synthesised in the mitochondria involves the removal of hydrogen atoms from several of the intermediates in a metabolic pathway – for example during cellular respiration. Hydrogen atoms are picked up by hydrogen carriers which become reduced. Electrons from the hydrogen atoms are then passed along a series of electron carriers known as the electron transport chain. The process involves a number of reduction-oxidation reactions through a series of electron carriers known as the electron transport chain. Each reaction releases a small amount of energy which is used to drive the synthesis of a molecule of ATP.

The process is complex but very elegant – it was worked out by a scientist called Peter Mitchell and is called the chemiosmotic theory.