Aerobic and anaerobic respiration

Aerobic respiration

Respiration using oxygen to break down food molecules is called aerobic respiration. 'Aero' means air, which contains oxygen, leading to the name aerobic respiration. Glucose is the molecule normally used for respiration - it is the main respiratory substrate. Glucose is oxidised to release its energy, which is then stored in ATP molecules.

The word equation for aerobic respiration is:

glucose + oxygen → carbon dioxide + water (+ ATP made)

You need to be able to recognise the chemical formulas:

C6H12O6 + 6O2 → 6CO2 + 6H2O

Respiration is a series of chemical reactions, but this equation summarises the overall process. Aerobic respiration breaks down glucose and combines the broken down products with oxygen, making water and carbon dioxide. The carbon dioxide is a waste product of aerobic respiration because cells do not need it.

The first stages of respiration occur in the cytoplasm of plant and animal cells, but most of the stages of respiration that release energy happen in the mitochondria. Microorganisms do not have mitochondria - they carry out respiration only in the cytoplasm.

Anaerobic respiration

Most organisms cannot respire without oxygen but some organisms and tissues can continue to respire if the oxygen runs out. In conditions of low or no oxygen the process of anaerobic respiration occurs. The 'an' in 'anaerobic' means without.

During anaerobic respiration, the oxidation of glucose is incomplete - not all of the energy can be released from the glucose molecule as it is only partially broken down. The reaction therefore releases much less energy than aerobic respiration - around only a nineteenth of the energy released during aerobic respiration. This means that fewer molecules of ATP can be made.

Glucose in human muscle cells is converted to lactic acid during anaerobic respiration:

glucose → lactic acid (+ ATP made)

The lactic acid is a waste product.

Some plants, microorganisms and fungi such as yeast can respire anaerobically - it's preferable to release less energy and make less ATP but remain alive.

Glucose in yeast cells is converted to carbon dioxide and ethanol, which we refer to simply as 'alcohol':

glucose → ethanol + carbon dioxide (+ ATP made)

This reaction is also called fermentation.

This table compares aerobic and anaerobic respiration:

Aerobic respirationAnaerobic respiration
OxygenPresentAbsent or in short supply.
Oxidation of glucoseCompleteIncomplete. All of the energy available from glucose is not released.
Reactants of respirationGlucose and oxygenGlucose.
Products of respirationCarbon dioxide and water (and ATP)Mammalian muscle - lactic acid (and ATP). Yeast - ethanol and carbon dioxide (and ATP). Some plants - ethanol and carbon dioxide (and ATP).
Amount of ATP madeLarge amountSmaller amount, but produced more quickly.