OCR A Level Biology

Revision Notes

6.5.4 The Nitrogen Cycle

The Nitrogen Cycle

  • In natural ecosystems, nutrients (such as nitrogen and phosphorus) are recycled through food webs
  • For example, these nutrients get passed on from producers to primary consumers when they feed on the producers and from primary consumers to secondary consumes when they feed on primary consumers
  • Microorganisms are very important in ensuring that the nutrients that are stored within dead organisms and in the waste products of organisms (i.e. faeces and urine) are recycled and made available to producers once again
    • This process is known as decomposition


  • True decomposers are known as saprobionts
  • Saprobionts mainly come from the fungi and bacteria kingdoms
  • Saprobionts secrete enzymes onto their food (dead organisms and waste material), which then digest the material externally
    • This process is also known as extracellular digestion
    • The products of this external digestion are then absorbed by the saprobionts
  • This method of obtaining nutrients from dead or waste organic matter via extracellular digestion is known as saprobiotic nutrition
  • Saprobionts secrete a wide range of digestive enzymes that allow them to hydrolyse (break down) a large variety of biological molecules, releasing a large range of products (nutrients) as a result
    • For example, mineral ions such as ammonium and phosphate ions that are important for the growth and function of producers (e.g. plants) may be released as products during saprobiotic nutrition
  • Importantly, not all of the products of extracellular digestion get absorbed by saprobionts and many remain in the surrounding environment (e.g. the soil) and are available to be absorbed by other organisms (e.g. plants)
    • This is why saprobionts are such an essential component of ecosystems and food webs
    • Without them, the nutrients locked up in dead and waste matter would never be made available again and producers such as plants would not have access to sufficient nutrients
    • In addition, some saprobionts even excrete important nutrient mineral ions as waste products from their own metabolism


  • Many plants have evolved symbiotic (mutually beneficial) relationships with fungi
    • The fungi are composed of long, thin filaments known as hyphae, which interact with the roots of the plants
    • These hyphae greatly increase the surface area of the root systems of the plants, increasing the amount of water and mineral ions (e.g. nitrates and phosphates) that can be absorbed by the plants’ roots
    • In return, the fungi receive organic compounds (e.g. glucose) from the plant
  • These relationships between plant roots and fungi are known as mycorrhizae

Mycorrhizae, downloadable AS & A Level Biology revision notes

An example of a mycorrhiza (plural = mycorrhizae) – the mutual, symbiotic relationship between a fungus and the roots of a plant

The Nitrogen Cycle

  • Nitrogen is the most abundant gas in the atmosphere, though is relatively inert. It does have a role in biology thanks to its ability to be converted (fixed) into biomass
  • Bacteria and other microorganisms play a vital role in the nitrogen cycle
  • Nitrogen-fixing bacteria (Azotobacter and Rhizobium species) are able to convert gaseous nitrogen into ammonium ions (NH4+)
  • Ammonium ions are nitrified by nitrifying bacteria (Nitrosomonas and Nitrobacter species) into nitrite (NO2) and nitrate (NO3), which are highly soluble and can be absorbed by roots
  • Denitrifying bacteria (e.g. Thiobacillus denitrificans) use nitrates for respiration and return gaseous nitrogen to the atmosphere
  • Other bacteria and fungi are involved in saprobiotic decomposition e.g. decay of dead biomass or excreta

Nitrogen cycle advanced 1, downloadable AS Level & A Level Biology revision notes Nitrogen cycle advanced 2, downloadable AS Level & A Level Biology revision notes
The roles of various bacteria in the recycling of nitrogen

Summary of the Role of Microorganisms in Recycling Minerals Table

Role in recycling minerals, downloadable AS & A Level Biology revision notes


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