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  1. Carbon cycle
  2. The oxygen cycle

An ecosystem uses energy and inorganic nutrients to function. Energy is supplied from the sun (an external source) but inorganic nutrients are supplied and re-cycled within the ecosystem. The materials/nutrients that make up living things are used over and over again cycling between the living and non-living environments of the ecosystem.

The major nutrients required to sustain life include carbon, nitrogen, phosphorus, oxygen and water. These are re-cycled through the biological, chemical and geological processes that take place in the environment thus, the nutrient cycles can also be referred to as biogeochemical cycles.


This is defined as the processes which bring about the circulation of carbon found in the organic molecules of living things.


Carbon is derived from carbon dioxide in air, dead remains of organisms and in fossil fuels like crude oil and coal.

Through the process of photosynthesis, green plants extract carbon from the atmosphere and incorporate it into their body tissues. The plants in turn are fed on by animals and the carbon compounds get incorporated into the tissues of the animals. When the animal excretes part of the carbon compounds are released into the environment as faeces and urine.

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As plants and animals respire, carbon dioxide is released into the atmosphere. The organic materials in dead plants and animals, faeces, urine, fallen leaves, etc. are decomposed by saprophytic bacteria and fungi. The process of decomposition breaks down complex carbon compounds and releases carbon dioxide which is returned to the atmosphere.

The combustion of fuel like wood, coal and crude oil results in the oxidation of carbon-to-carbon dioxide which is then released into the atmosphere.

The weathering of rocks results in the release of carbon dioxide which dissolves in water to form carbonic acid (H2CO3). This reacts with limestone to form carbonates and bicarbonates and stored in the sea. The sea helps to control the amount of carbon dioxide in the atmosphere by traping more carbon dioxide when the concentration in air increases. The carbon dioxide is released through the process of diffusion to the atmosphere when the concentration drops. The actions of volcanoes also release carbon dioxide into the atmosphere.

The Carbon Cycle.


  1. Plants use carbon dioxide to manufacture food during the process of photosynthesis.
  2. It provides carbon which is major building block of organic matter.
  3. The carbon compounds in food are the vehicles through which trapped solar energy passes from one organism to another in food chains.
  4. The carbon cycle helps to purify the atmosphere and maintain the atmospheric level of carbon dioxide.
  5. Carbon in carbonates such as calcium carbonate is used by marine animals to build their shells. It is also found in limestone, chalk, marble and coral.
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