This is the chemical process that occurs in every cell of living organisms. Chemical energy in an organic chemical, usually glucose, is converted into another form of energy, usually the kinetic energy of movement. Some animals can produce light energy, e.g. glow worms.

Read more about Respiration.

Aerobic Respiration:

Oxygen is needed for aerobic respiration: the products are Carbon Dioxide and water when either carbohydrates or fats are used as the energy source. When proteins are respired, there is also some nitrogenous waste. This can be ammonia, urea or uric acid.

All the waste products must be excreted. Read more about Aerobic Respiration.

Anaerobic Respiration:

Some organisms, e.g. yeast, are able to live in places where there is very little oxygen or none at all. They get their energy by anaerobic respiration, i.e. respiring without oxygen.

Yeast converts glucose into carbon dioxide and ethanol. This is less efficient: there is still plenty of energy left in the ethanol. Acetobacter is a bacterium that can get the energy from ethanol by converting it to carbon dioxide and water (aerobic respiration). Acetobacter converts wine into vinegar.


This appears to be the opposite of respiration. Carbon Dioxide and water are converted into Glucose using light energy in the presence of Chlorophyll. Photosynthesis produces Oxygen as a waste product. Read more about Photosynthesis.