A2-LEVEL AQA BIOLOGY NOTES 
TOPIC 4: Nutrient Cycles

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In an ecosystem, materials must be recycled. Nitrogen and carbon are in high demand in all aspects of an ecosystem and their recycling occurs in the Nitrogen and Carbon cycles, aided by decomposers.

Role of Decomposers

• Dead and waste organic material is broken down by decomposers
  
• Bacteria and fungi feed in a different way to other organisms, they are saprotrophs:
  
  • Secrete enzymes onto dead waste material
    
  • Enzymes digest material into small molecules
    
  • Molecules then absorbed into the body
    
  • Molecules are stored or respired to releases energy
    

Nitrogen Cycle

• Living things require nitrogen to make proteins and nucleic acids
  
• Nitrogen is cycled between biotic and Abiotic components
  
• Bacteria (decomposers called ‘saprobionts’) are involved in ammonification, nitrogen fixation, nitrification and denitrification
  
• Mycorrhizae = the symbiotic relationship between fungi and plants that enable plants to efficiently absorb water and inorganic ions from the soil
  
• Saprotrophic nutrition = where digestion occurs extracellularly, e.g. by another organism with which the saprotroph has a symbiotic relationship
  
• 1. Nitrogen Fixation
  
  • Nitrogen is incredibly uncreative when in the atmosphere
    
  • Plants need a supply to be ‘fixed’
    
  • Nitrogen fixation can occur when:
    
    • Lightning strikes
      
    • Haber process
      
    • Nitrogen fixing bacteria (Rhizobium) fixes it
      
  • Bacteria has a mutualistic relationship with the plant providing in with nitrogen and receiving carbon compounds in return
    
• 2. Nitrification
  
  • Chemotropic bacteria absorbs ammonium ions
    
  • Ammonium ions are released in putrefaction of proteins
    
  • Nitrosomonas bacteria oxidise ammonium to nitrates
    
  • Nitrobacteria oxidise nitrites to nitrates
    
  • As reaction requires oxygen, it only occurs in well aerated soil
    
• 3. Denitrification
  
  • Bacteria can convert nitrates back to nitrogen gas
    
  • When bacteria involved are growing under anaerobic conditions, they can produce nitrogen gas and nitrous oxide
    

Phosphorus Cycle

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Use of Artifical Fertilisers

• Farming and agriculture disrupts nutrient cycles as crops are harvested before they can decompose
  
• The land becomes deficient in nutrients such as nitrogen and phosphorus
  
• Artificial fertilisers aim to replace the lost supply of nitrates and phosphorus in the soil
  
• However, fertilisers can cause problems - particularly if used in excess
  
  • Leaching
    
    • Rainwater leaks into soil and erodes land
      
    • Fertilisers in the soil get drained by the movement of the rainwater towards bodies of water, e.g. lakes and streams
      
  • Eutrophication
    
    • Follows leaching
      
    • Excess fertilisers in lakes cause overgrowth of aquatic plants such as algae
      
    • Algae overgrowth blocks sunlight from penetrating deeper regions of water
      
    • Plants rooted to the bottom of the lake cannot photosynthesise
      
    • Therefore carbon dioxide builds up and oxygen isn’t produced
      
    • Oxygen gets used up by animals living in the water
      
    • Animals eventually run out of oxygen and suffocate
      
    • Destruction of the ecosystem