Respiration is an exothermic reaction (releases energy) that supplies all the energy needed by living organisms. This reaction happens continuously in all living cells. The reaction can be aerobic (with oxygen) or anaerobic (without oxygen).

Aerobic respiration (with oxygen)

In aerobic respiration, glucose reacts with oxygen in the mitochondria of cells to give carbon dioxide, water and energy.

  • Glucose + oxygen → carbon dioxide + water (+ energy)

Anaerobic respiration in plants

In plant and yeast cells, glucose is converted into ethanol and carbon dioxide.

  • Glucose → ethanol + carbon dioxide

In yeast cells, this is called fermentation. This is an important step in the manufacture (making) of both bread and alcohol.

Anaerobic respiration in animals

Glucose is not broken down completely, making it a less efficient way of transferring energy than aerobic respiration.

In animals, glucose is converted to lactic acid:

  • Glucose → lactic acid

Anaerobic respiration happens when insufficient (not enough) oxygen reaches the muscles during periods of intense activity.

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