Nutrient Cycles (AQA A Level Biology): Revision Note

Nutrient cycles

• In a functioning ecosystem the elements that living organisms need are constantly recycled

  • These elements are incorporated into biological molecules within the tissues of living organisms, and then released back into the environment when decomposers break down dead or waste matter

• Examples of nutrient cycles are:

  • the nitrogen cycle

  • the phosphorous cycle

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 during the nitrogen cycle as follows:

  • Nitrogen fixation:

    • N₂ gas is converted into ammonium compounds by nitrogen-fixing bacteria; these bacteria can be free-living in the soil, or may live within root nodules of legume plants

    • Ammonium compounds are converted into nitrates, which are then absorbed by plants and used to build plant proteins

  • Ammonification:

    • Nitrogen from living organisms is returned to the soil in the form of ammonia by the action of saprobionts such as bacteria and fungi

    • This ammonia forms ammonium ions in the soil

  • Nitrification:

    • The ammonium ions in the soil are converted into nitrates by nitrifying bacteria

      • Nitrosomonas bacteria convert ammonium ions into nitrites

      • Nitrobacter bacteria then convert nitrites into nitrates

  • Denitrification:

    • Denitrifying bacteria use nitrates in the soil for respiration

      • This occurs in anaerobic conditions, such as in waterlogged soil

    • This process produces nitrogen gas, which returns to the atmosphere

The phosphorus cycle

• Plants and animals require phosphorus for production of, e.g.:

  • phospholipids

  • nucleic acids (DNA and RNA)

  • ATP

• The phosphorus cycle includes the following processes:

  • phosphorus in rocks is released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) due to weathering

  • Phosphate ions are taken up from the soil by plants, or absorbed from water by algae

  • Phosphate ions are transferred to consumers during feeding

  • Phosphate ions in waste products and dead organisms are released into the soil or water during decomposition by saprobionts

  • The phosphate ions can be taken up and used again by producers, or may be trapped in sediments that may turn into phosphorus-containing rock once again