People commonly refer to the act of breathing as respiration. More correctly respiration is a process that occurs within cells.

Respiration converts the energy of glucose and other molecules into cellular energy. Cells are then able to use this energy to perform functions such as build proteins, replicate DNA and breakdown wastes.

Respiration is a series of chemical reactions. The series of reactions gradually releases the energy of molecules such as glucose. The released energy is transferred to molecules such as ATP and used to power activity within cells.

Cellular respiration can be both aerobic or anaerobic. Aerobic respiration uses oxygen and is the most common and most efficient method of respiration. The overall equation of aerobic respiration can be summed up as:

sugar + oxygen → carbon dioxide + water + energy

You may recognize that this is the opposite reaction to photosynthesis which uses the sun’s energy and water to convert carbon dioxide into sugar and oxygen.

Anaerobic respiration replaces the oxygen in aerobic respiration with other molecules. The products of anaerobic respiration are compounds such as methane or lactic acid rather than carbon dioxide and water.

The process of respiration occurs in the mitochondria and cytoplasm of eukaryotic cells. In prokaryotic cells respiration occurs in the cytoplasm and across the plasma membrane.

The whole process of respiration can be split into three stages: glycolysis, the citric acid cycle and oxidative phosphorylation.


Glycolysis is the splitting of glucose into two molecules of pyruvate. The word ‘glycolysis’ translates into ‘splitting sugar’. The process includes a total of nine reactions that all occur in the cytoplasm in both eukaryotic and prokaryotic cells.

Energy is both invested and paid back in separate phases of glycolysis. All up a total of two molecules of ATP are produced through glycolysis for every glucose molecule that is converted into pyruvate.

Glucose is a sugar that contains six carbon atoms. During glycolysis, glucose is split into two molecules of pyruvate which is a three carbon molecule.

A glucose molecule has a ring of carbon atoms. The first four reactions of the glycolysis pathway breakdown the ring of the glucose molecule in two molecules of G3P (glyceralderhyde-3-phosphate). Splitting the ring of atoms requires the investment of energy and uses two molecules of ATP.

The last five reactions of glycolysis converts G3P into pyruvate. These reactions produce a total of four molecules of ATP and is known as the ‘energy payoff phase’.

The glycolysis pathway therefore uses two molecules of ATP but produces four, giving a net increase of two molecules of ATP. Carbon dioxide and cellular energy molecules called ‘NADH’ are also produced as byproducts of glycolysis.

glucose → pyruvate + ATP + NADH + carbon dioxide

In eukaryotic cells, pyruvate enters mitochondria and is converted to a compound called ‘acetyl CoA’ before entering the citric acid cycle. In prokaryotes the remaining steps of respiration are performed in the cytoplasm and the plasma membrane.

Citric acid cycle

The citric acid cycle, also known as the Krebs cycle, is the second phase of cellular respiration. Through the citric acid cycle acetyl CoA is broken down to carbon dioxide.

During the citric acid cycle ATP and molecules called ‘NADH’ and ‘FADH₂’ are produced. NADH and FADH₂ are electron carrying molecules and are important for transporting electrons from the citric acid cycle to the electron transport chain in the final stage of respiration.

The citric acid cycle is a series of eight reactions. The first reaction begins with acetyl CoA being bonded to a four carbon molecule called ‘oxaloacetate’. The following seven reactions lead to the release of two molecules of carbon dioxide.

The citric acid cycle results in the production of ATP and the electron carrying molecules NADH anf FADH₂. Three molecules of NADH and one molecule of FADH₂ are formed from NAD⁺ and FAD. These two different molecules are used to carry electrons from the citric acid cycle to the electron transport chain.

Oxidative phosphorylation

Oxidative phosphorylation is by far the most productive stage of respiration. Far more usable cellular energy is produced during oxidative phosphorylation than during glycolysis and the citric acid cycle combined.

Oxidative phosphorylation includes two phases: the electron transport chain and chemiosmosis.

Electron transport chain

The electron transport chain is a chain of molecules that electrons are passed along. It utilizes electrons made available during glycolysis and the citric acid cycle.

Electrons are carried from the citric acid cycle and glycolysis pathway by the molecules NADH and FADH₂ to the electron transport chain. The electrons are donated to the electron transport chain and passed along the chain of molecules. When an electron has been passed down the entire length of the electron transport chain it is reacted with oxygen and a hydrogen ion (H⁺) to form water.

O₂ + H⁺ + electron → H₂O

The electron transport chain is made from a series of four large protein complexes and electron carrying molecules. The electron carrying molecules transfer electrons between the large protein complexes down the electron transport chain.

In eukaryotic cells, the protein complexes are located in the inner membrane of mitochondria. In prokaryotic cells they are found in the plasma membrane of the whole cell.

To start the electron transport chain, electrons are donated from NADH and FADH₂ to the first and second electron carriers of the electron transport chain. As electrons move down the electron transport chain through a series of protein complexes, the proteins use the electron’s energy to pump hydrogen ions to the other side of the inner mitochondrial membrane (eukaryotic cells) or the plasma membrane (prokaryotic cells).

This movement of hydrogen ions is known as a proton pump and creates a large difference in the concentration of hydrogen ions on either side of the membranes. This difference in concentrations is known as a ‘proton gradient’ and is important in the second phase of oxidative phosphorylation called ‘chemiosmosis’.

In aerobic respiration, the final electron carrier passes the electrons to an oxygen molecule. The oxygen then binds to two hydrogen ions and forms water. This is where and how the oxygen that we breathe in is used by our bodies.

In anaerobic respiration, the final electron carrier passes the electron to a substance other than oxygen such as nitrate (NO₃⁻) or an iron ion. Less hydrogen ions are pumped across the membrane if these substances are used instead of oxygen so anaerobic respiration is less efficient than aerobic respiration.

The electron transport chain doesn’t produce any ATP but simply reduces the energy of the electron and creates a proton gradient. When the electron’s energy has been reduced enough it can be accepted by oxygen or the ‘acceptor’ in anaerobic respiration. The energy of the electron is used to pump hydrogen ions across the membrane which will be used to produce ATP through chemiosmosis.


Chemiosmosis is where the vast majority of ATP is produced. The production of ATP is driven by an enzyme called ‘ATP synthase’ which adds a phosphate group to a molecule of ADP.

ATP synthase enzymes are found in the same membranes as the electron transport chain. The proton gradient (i.e. the difference in concentration of hydrogen ions) across the membrane that was produced by the electron transport chain is used to power ATP synthase enzymes.

Hydrogen ions naturally want to have a balanced concentration on either side of the gradient. ATP synthase provides a channel for hydrogen ions to flow back across the membrane to balance out the concentrations. As the hydrogen ions flow through ATP synthase, they power the enzyme to produce ATP from ADP.

The movement of hydrogen ions through ATP synthase leads to the generation of around 26-28 molecules of ATP for every glucose molecule that is broken down. Compared to the two molecules of ATP produced during glycolysis and the citric acid cycle, this final step in the oxidative phosphorylation stage is by far the most productive.

It would not be possible however without the gradual breakdown of a glucose molecule and the gradual release of its energy and this is only possible because of the glycolysis pathway and the citric acid cycle.

The overall balanced equation of cellular respiration looks like this:

glucose + oxygen → carbon dioxide + water + 30-32 ATP


  • Cellular respiration breaks down glucose to harvest its energy and convert it into usable cellular energy in the form of ATP
  • There are three distinct stages of respiration: glycolysis, the citric acid cycle and oxidative phosphorylation
  • Glycolysis splits glucose into two 3 carbon molecules called ‘pyruvate’
  • Pyruvate is transported into the mitochondria in eukaryotic cells and stays in the cytoplasm in prokaryotic cells
  • Pyruvate is converted to acetyl CoA and enters into the citric acid cycle
  • The citric acid cycle is a series of reactions that produces NADH and FADH₂, plus two ATP molecules and CO₂ as a byproduct
  • NADH and FADH₂ deliver electrons to the electron transport chain
  • Electrons are passed along the electron transport chain and the energy released pumps protons from inside the mitochondria into the intermembrane space of the mitochondria in eukaryotic cells. The same is completed across the plasma membrane of prokaryotic cells.
  • The electron transport chain finishes with the electrons being passed onto oxygen which reacts with hydrogen ions to create water as a byproduct
  • In anaerobic respiration, a molecule other than oxygen is the final electron acceptor
  • The build up of hydrogen ions is used by the ATP synthase enzyme to power the production of ATP from ADP.
  • The overall reaction is: glucose + oxygen → carbon dioxide + water + 30-32 ATP

Last edited: 25 April 2016

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