The final stage of energy transformation in cellular respiration includes:

  • the electron transport chain
  • oxidative phosphorylation of adenosine diphosphate (ADP) by chemiosmosis.

Electron transport chain

The reduced coenzymes nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) shuttle electrons and hydrogen ions (H+) from the Krebs cycle (citric acid cycle) in the matrix to the electron transport chain embedded on the cristae of mitochondria.

The oxidation and reduction of the electron carriers in the electron transport chain complexes releases small amounts of energy.

Electron transport chain

This energy is then used to power proton pumps that pull hydrogen ions across the inner membrane into the intermembrane space.

These H+ ions are now trapped between two mitochondrial membranes, building a concentration and electrical gradients (an electrochemical gradient) between the intermembrane space and the matrix.


Chemiosmosis - the movement of hydrogen ions down their electrochemical gradient through special channels - drives the production of adenosine triphosphate (ATP).

Oxygen must be present in the matrix to oxidize the last component of the electron transfer system.

When oxygen is combined with available H+ ions in the matrix, water is formed. This allows additional electrons to enter the electron transfer chain and release the energy needed to pump more hydrogen ions into the intermembrane space.

ATP is produced when the high concentration of H+ ions diffuses through the channel of the ATP synthase complex that is embedded in the inner membran of mitochondria.

Electron transport chain and oxidative phosphorylation

Mitochondrial oxidative phosphorylation