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Biological compounds

Carbon and oxygen cycles


Almost all molecules in living things contain carbon. Carbon moves in a cyclical way, passing from one organism to another in the biosphere, and between the other spheres of the environment (lithosphere, hydrosphere and atmosphere). The carbon cycle is the key factor in maintaining the balance of carbon dioxide in the air.

It works like this:

  • Plants photosynthesise, taking carbon from the atmosphere in the form of carbon dioxide and making the carbohydrate glucose.

  • Plants also respire, giving carbon dioxide back to the atmosphere; but they take in much more carbon dioxide than they give out.

  • Animals get their carbon from eating either plants (carbohydrates) or other animals (proteins and fats), which they then digest. They respire, giving off carbon dioxide into the air.

  • Waste carbon-based material is excreted by animals, and then digested by decomposers - mainly microbes and fungi. The decomposers also respire, releasing carbon dioxide into the air.

  • When animals die, their remains may be either eaten by scavengers (for example, crows) or digested by decomposers. Both scavengers and decomposers respire, giving off more carbon dioxide into the air.

  • In certain conditions, both animal and plant remains may become fossilised, eventually forming carbon-based fossil fuels (coal, oil and gas). Both fossil fuels and plant material (wood) may later be burned, releasing still more carbon dioxide into the air.

Therefore, all of the carbon taken out of the air by plants is later returned to the air. This is the carbon cycle. This slideshow should help you to understand how the cycle works:


Oxygen is cycled in a similar way:

  • Plants produce oxygen during the process of photosynthesis. This is released into the air. Plants also use some oxygen for respiration, but they produce more than they use.

  • Animals use oxygen from the air for the process of respiration.

  • There is a balance between the excess oxygen given out by plants and the oxygen used by animals, so that the percentage of oxygen in dry air is always the same.

Back to Chemicals of the natural environment index

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