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Respiration is a process that provides cells with energy for many processes such as protein synthesis and reproduction.

Respiration releases energy from molecules like glucose obtained in food. Respiration uses a process called oxidation to breakdown glucose.

Oxidation is an energy-releasing process. When fire is made from burning wood, this energy is being released from the wood as it is oxidised.

During respiration, the energy released by oxidation is stored in a molecules called adenosine triphosphate (ATP). This simple molecule holds enough chemical energy to drive useful chemical reactions in the cell.

ATP is used to drive reactions in active transport and DNA replication.

There are two main types of respiration:

  • Aerobic respiration uses oxygen.
  • Anaerobic respiration does not use oxygen.
Respiration releases energy using the same type of reaction that produces fire!
Respiration releases energy using the same type of reaction that produces fire!

Aerobic respiration uses oxygen to break down and release energy from glucose.

In both animal and plant cells, aerobic respiration occurs in specialised organelles called mitochondria.

The chemical equation for aerobic respiration is:

$$$\begin{align*} \ce{C6H12O6 + 6O2} &\rightarrow \ce{6CO2 + 6H2O } \\ \text{glucose + oxygen} &\rightarrow \text{carbon dioxide + water (+ energy!)} \end{align*}$$$

This equation shows why we need to keep breathing. Cells constantly require oxygen and produce carbon dioxide.

Aerobic respiration releases a large amount of energy. Most of this energy is stored in ATP molecules, but some is released as heat.

Doing exercise makes you hot because more heat is produced by respiration.

Scuba divers carry air tanks so that they have a continuous source of oxygen for aerobic respiration.
Scuba divers carry air tanks so that they have a continuous source of oxygen for aerobic respiration.

There are four stages in aerobic respiration. Their main features, locations and ATP yields per glucose molecule are summarised in the table below.

Stage Location Process ATP
Glycolysis Cytosol Two pyruvate molecules are formed 2
Link reaction Matrix of mitochondria Pyruvate is oxidised and bonds with coenzyme A 0
Krebs cycle Matrix of mitochondria A cycle of reactions which produce hydrogen and electrons 4
Oxidative phosphorylation Inner membrane of mitochondria Protons provide energy to add phosphate to ADP 32

Overall, 38 molecules of ATP can be produced. However, two of these are used in the respiration process, so the maximum net yield is 36 molecules of ATP per glucose molecule.

This is the yield in ideal conditions. In reality, the yield is slightly lower.

When there is insufficient oxygen for aerobic respiration, cells can break down glucose using anaerobic respiration.

In humans, the chemical equation for anaerobic respiration is:

$$$\begin{align*} \ce{C6H12O6} &\rightarrow \ce{2C3H6O3} \\ \text{glucose} &\rightarrow \text{lactic acid (+ energy!)} \end{align*}$$$

This reaction has two disadvantages over aerobic respiration:

  • Produces less energy: anaerobic respiration produces roughly $$^1/_{20}$$ of the ATP produced aerobically.
  • Produces lactic acid: lactic acid causes fatigue (inability to contract muscles).

Other organisms have different methods of anaerobic respiration:

Yeast produce ethanol and carbon dioxide instead of lactic acid. This process is called fermentation.

Yeast is used to make alcohol and bread.
Yeast is used to make alcohol and bread.

Not all cells have constant access to oxygen and some have no mitochondria. It is still possible for cells to produce ATP in the absence of oxygen.

ATP can be created through anaerobic respiration.

Anaerobic respiration uses glycolysis to produce ATP.

This process is not very efficient as only two ATP molecules are produced per glucose molecule as opposed to 36 in aerobic respiration.

Anaerobic respiration cannot rely only on glycolysis as a constant supply of NAD$${^+}$$ is required to oxidise glucose.

NADH must be therefore be re-oxidised. In aerobic respiration, this is done via the electron transport chain in oxidative phosphorylation. In anaerobic respiration, this is done by fermentation.

Anaerobic bacteria can live in hot springs like this one. Bacteria in the water are responsible for the remarkable colours.
Anaerobic bacteria can live in hot springs like this one. Bacteria in the water are responsible for the remarkable colours.

During vigorous exercise, muscles build-up lactic acid due to increased anaerobic respiration.

There is a limit to the amount of energy that can be produced by aerobic respiration. When aerobic respiration is insufficient to drive muscle, anaerobic respiration kicks in to provide additional energy.

The build-up of lactic acid in muscles:

  • Increases the acidity of muscle tissue
  • Can be painful
  • Reduces ability of muscle to contract (muscle fatigue)

Muscle fatigue can cause cramping, a painful sensation of tightness in muscles.

The build-up of lactic acid creates an oxygen debt. After exercise, the body requires more oxygen than at rest.

Oxygen is used to break down the lactic acid in the muscle cells. Lactic acid is also converted back to glucose by the liver and kidneys.

Athletes train to reduce the amount of lactic acid their muscles produce.
Athletes train to reduce the amount of lactic acid their muscles produce.

Glycolysis is the first step in cellular respiration. It takes place in the cytosol.

Glycolysis does not require oxygen. It occurs in both anaerobic and aerobic respiration.

During glycolysis, one molecule of glucose is broken down into two molecules of 3-carbon pyruvate in several steps. This produces 4 molecules of ATP directly but also uses 2 molecules of ATP.

  • ATP is produced by substrate-level phosphorylation: a phosphate group is added directly onto an ADP molecule to create ATP.

  • Two molecules of NADH are created and transported into the mitochondria along with pyruvate to be used in aerobic respiration.

The pyruvate ion
The pyruvate ion

The link reaction is also called pyruvate decarboxylation. The process is exclusive to aerobic respiration. It takes place in the matrix of mitochondria.

This reaction links glycolysis to the next stage of respiration. The link reaction uses pyruvate to form acetyl-CoA. This releases NADH and carbon dioxide. The link reaction does no directly produce ATP.

  • Pyruvate (from glycolysis) is transported into the mitochondria, where it is oxidised to form acetate.

  • The acetate produced by the decarboxylation of pyruvate associates with coenzyme A to form acetyl coenzyme A (acetyl-CoA). This will be used in the Krebs cycle.

Two molecules of pyruvate produce two molecules of NADH. Three molecules of ATP can be produced from a single NADH.

NADH will produce more ATP if it is reduced in the mitochondria than if it is reduced in the cytosol.

NADH cannot pass across the mitochondrial membrane without using up one ATP molecule. NADH produced in the cytosol therefore gives a net gain of two ATP rather than three ATP.

The overall link reaction is as follows: $$$\begin{gather*}\ce{2 Pyruvate + 2NAD^{+} +2H_2O \\ \downarrow \\ 2 Acetate +2NADH +2H^{+} +2CO_2}\end{gather*}$$$

The Krebs cycle, also known as the citric acid cycle, is the third step of aerobic respiration.

The Krebs cycle is a series of reactions that 'turns' twice for each glucose molecule being processed. It occurs in the matrix of mitochondria.

The Krebs cycle uses one acetyl molecule each turn. One molecule of glucose produces two acetyl molecules, so this is why it turns twice.

Each turn, the Krebs cycle produces three molecules of NADH and one molecule of FADH$$_2$$. These are used in the next stage of respiration. One turn also produces one molecule of ATP.

The acetyl molecule (two carbons) binds with oxaloacetate (four carbons), forming citrate (six carbons). The citrate then undergoes a series of redox reactions, releasing energy, and finally loses two molecules of CO$$_2$$, regenerating oxaloacetate.

ATP in the Krebs cycle is produced using substrate-level phosphorylation.

Oxidative phosphorylation is the fourth and final stage in aerobic respiration. It uses an electron transport chain to provide a proton gradient.

This stage occurs in the inner membrane of mitochondria.

This stage produces the most ATP; 32 out of the total 36 produced from one molecule of glucose. It is the only stage that requires oxygen.

Firstly, NADH and FADH$$_2$$ are oxidised to NAD$$^+$$ and FAD$$^+$$. This releases electrons onto carrier molecules.

The carriers undergo a series of redox reactions. At each stage the electrons lose energy, which is used to pump protons across the membrane from the matrix into the inter-membrane space.

As the hydrogen ions flow back, they must pass through a protein in the membrane called ATP synthase. The protons release energy which is used to bond free phosphate groups to ADP, forming ATP.

This model of ATP production is known as the chemiosmotic theory. Chemiosmosis refers to the movement of ions across a membrane.

The free hydrogen ions and electrons recombine and react with the oxygen to form water.

The use of oxygen gives aerobic respiration its name (aerobic means 'requiring air' - usually oxygen), yet it is not until the last step in aerobic respiration that it is required. Oxygen acts as the final electron acceptor in the electron transport chain.

$$$\ce{O_2 + 4H^+ + 4e^- \rightarrow 2H_2O}$$$

Oxygen allows aerobic respiration to continue. Without oxygen, the electron transport chain would grind to a halt and the organism would die.

Cyanide is a lethal toxin that affects the electron transport chain. Cyanide binds more strongly to the electron carrier than oxygen, preventing the release of the final electron. This halts aerobic respiration, killing the organism.

Cyanide is a deadly toxin that is used for killing pests.
Cyanide is a deadly toxin that is used for killing pests.

In animals, NADH re-oxidation is achieved through lactic acid fermentation. Pyruvate is broken down into lactate in order to regenerate NAD$${^+}$$.

$$$\mathrm{Pyruvate + NADH\rightarrow Lactate + NAD^+ }$$$

Muscle cells switch to lactic acid fermentation when the need for ATP is great. This is the reason for muscle pain and fatigue after exercise. Lactate is then transported to the liver where it is broken down.

Some eukaryotes, such as yeast, use ethanol fermentation. Pyruvate is broken down into ethanol in order to regenerate NAD$${^+}$$.

$$$\ce{Pyruvate + NADH\rightarrow Ethanol + CO_2 + NAD^+}$$$