In nature, elements are continuously recycled in various forms between different compartments of the environment (e.g. water, air, soil, organisms).

For IGCSE Biology, you need to know the cycle for two elements: the carbon cycle and the nitrogen cycle.

Carbon Cycle

The 4 main activities responsible for the movement of carbon between different compartments of the environment are: 

• Combustion: When fossil fuels are burnt, carbon dioxide is released into the air.
• Respiration: During aerobic respiration by living organisms, carbon dioxide is released into the air.
• Photosynthesis: Plants use carbon dioxide in the air during photosynthesis to produce glucose (C₆H₁₂O₆). This is the only stage which decreases the concentration of carbon dioxide in air. 
• Decomposition: Complex carbon compounds from dead plants and animals are broken down into simpler compounds by decomposers. 

Nitrogen Cycle

• Since nitrogen gas in the atmosphere (N₂) is too unreactive to be used by plants, nitrogen must be converted into a soluble form that can be utilized by plants. This process is called nitrogen fixation. 
• Nitrogen-fixing bacteria live in either the soil or the root nodules of legumes (e.g. peas). They convert nitrogen gas into nitrates (NO₃^-), which can be absorbed by roots of plants through active transport.
• Animals can obtain nitrogen by eating plants.
• Decomposers convert nitrogen from dead organisms and urea into ammonium (NH₄⁺).
• Nitrifying bacteria converts ammonium to nitrites (NO₂^-) and nitrites into nitrates.
• Denitrifying bacteria converts nitrates back into nitrogen gas.

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