The carbon cycle

Carbon is passed from the atmosphere, as carbon dioxide, to living things. It is then passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again. This is known as the carbon cycle.

The slideshow shows how the cycle works.

Stage one of the carbon cycle. Overnight, CO2 from burning fuel (combustion) and respiration by cows, birds and plants pass into the atmosphere.

Stage one of the carbon cycle

Carbon enters the atmosphere as carbon dioxide from respiration and combustion.

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Removing carbon dioxide from the atmosphere

Plants use carbon dioxide from the atmosphere for photosynthesis. The carbon becomes part of complex molecules in the plants, such as proteins, fats and carbohydrates.

Passing carbon from one organism to the next

When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal. Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these organisms.

Returning carbon dioxide to the atmosphere

Carbon dioxide is released to the atmosphere through respiration by animals, plants and microorganisms. It is also released by the combustion of wood and fossil fuels (such as coal, oil and natural gas). The use of fossil fuels is gradually increasing the carbon dioxide levels in the atmosphere.

Decomposition or decay also releases carbon dioxide. This process happens faster in warm, moist conditions with plenty of oxygen because it involves microorganisms. Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen.

  1. ammonia Ammonia is a gas at room temperature but readily dissolves in water to form an alkaline solution.
  2. combustion The process of burning by heat.
  3. compound A substance formed by the chemical union of two or more elements.
  4. condense Condensation is a change of state in which gas becomes liquid by cooling.
  5. decomposer An organism which eats dead organisms or animal droppings, and breaks them down into simple materials.
  6. decomposition The process of breaking down material to release nutrients back into the soil.
  7. denitrifying bacteria The bacteria that convert nitrates in the soil into nitrogen gas which is released into the atmosphere.
  8. egested material Undigested material that has passed out of the alimentary canal or gut.
  9. evaporate Evaporation is the process in which a liquid turns into a gas.
  10. fossil fuel Fuel which is finite, eg oil, coal and natural gas.
  11. Haber process The industrial chemical process that makes ammonia by reacting nitrogen and hydrogen together.
  12. ion Electrically charged particle, formed when an atom or molecule gains or loses electrons.
  13. microorganism Another name for a microbe. It is microscopic and is an organism, such as a virus or bacteria.
  14. molecule A collection of two or more atoms held together by chemical bonds.
  15. nitrifying bacteria The bacteria that produce nitrate which is released into soil.
  16. nitrogen-fixing bacteria The bacteria found free living in the soil or in the root nodules of some plants such as peas and clover that convert nitrogen gas into nitrate.
  17. organism Living entity, eg animals, plants or microorganisms.
  18. photosynthesis A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.
  19. respiration Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.
  20. state Solid, liquid or gas. Evaporation is a change of state from liquid to gas.
  21. transpiration The loss of water from leaves by evaporation. It is much faster when stomata are open than when they are closed.
  22. urea A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine.