Nitrogen Cycle (Edexcel IGCSE Biology)

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The Nitrogen Cycle

  • Nitrogen is present as N2 gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms
  • Nitrogen is cycled through ecosystems by the processes of the nitrogen cycle

Uptake of nitrogen by living organisms

  • N2 in the atmosphere is made available to living organisms by the process of nitrogen fixation
  • Nitrogen fixation is carried out by nitrogen-fixing bacteria which convert N2 gas into ammonium compounds; these compounds are converted into nitrates in the soil
    • Nitrogen gas can also be fixed by lightning when it strikes the earth, or during the production of chemical fertilisers
  • After nitrogen fixation has occurred plants absorb the nitrates in the soil and use the nitrogen to build plant proteins

Transfer of nitrogen between living organisms

  • Animals feed on plants and digest the proteins in the plant tissues, providing nitrogen to build animal proteins
  • Nitrogen may then be passed from one consumer to another up the food chain in the same way

Release of nitrogen from tissues

  • Nitrogen from living organisms is returned to the soil in the form of ammonia by the action of decomposers, such as bacteria and fungi
  • The plants can’t absorb ammonia so nitrifying bacteria convert it to nitrates, which can then be taken up again by plants
    • The conversion of ammonium compounds to nitrates is known as nitrification

Returning nitrogen to the atmosphere

  • Nitrates in the soil can be converted back into nitrogen gas by the action of denitrifying bacteria
    • This process is known as denitrification
  • Denitrifying bacteria are active in anaerobic conditions, e.g. in waterlogged or compacted soil
    • Farmers can decrease the activity of denitrifying bacteria by ploughing the soil to increase aeration

nitrogen cycle

The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.