AQA A2 Biology Unit 4 The Nutrient Cycles RC


  • Saprobiotic Microorganisms (Saprophyte): Organism that obtains food from dead or decaying remains of other organisms
  • Niches: All conditions and resources required for an organism to survive, reproduce and maintain population
  • Ecosystem: Unit in ecology made up of all interacting biotic and abiotic factors in a specific area
  • Aerobic: In the presence of oxygen
  • Anaerobic: Without oxygen
  • Biomass: Total mass of living material in a specific area at a given time, usually measured as dry mass as water value is variable
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6.1 The Carbon Cycle

  • The main source of carbon for terrestrial organisms is carbon dioxide in the atmosphere
  • Photosynthetic organisms remove it from the air to form macromolecules e.g. carbohydrates, fats and proteins
  • Respiration returns carbon dioxide back to the air
  • The concentration of CO₂ is higher at night than day due to no photosynthesis occuring while respiration still occur 
  • Main Reasons due to human activities:
    • Combustion of Fossil Fuels: Coal, oil and peat releases CO₂ previously trapped
    • Deforestation: Removes photosynthesising biomass so less CO₂  is removed
  •  CO₂ is a greenhouse gas and contributes to global warming
  • The ocean is a CO₂  sink so keeps it constant
  • When organisms die Saprophytes break them down into small soluble molecules using enzymes
  • The carbon is then released as CO₂during respiration of the decomposer
  • When decay is prevented fossils are formed
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6.2 The Greenhouse Effect and Global Warming

The Greenhouse Effect

  • Natural process that occurs all the time
  • Due to solar radiation from the sun reaching the earth
  • Greenhouse gases trap the heat in the Earth’s atmosphere causing it to heat up Greenhouse Gases
  • The major greenhouse gas is CO₂ which is increasing due to human activities
    • Methane is also produced when microorganisms break down organic molecules , it occurs in two situations:
    • Decomposers break down dead remains of organisms
  • Microorganisms in intestines of primary consumers e.g. cattle digest food Global Warming
  • Due to the layer of greenhouse gases building up it traps the heat from the sun causing the Earth to heat up
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Consequences of Global Warming

  • Changes in temperature and precipitation, the timing of seasons and frequency of extreme events e.g. storms
  • Climate change will effect niches available due to organisms being adapted to particular niches
  • Animals could migrate to new areas causing competition and lose of native species
  • Melting ice gap could cause extinction of wild plants and animals e.g. polar bears and sea levels will rise
  • Low land would be flooded and sea water would extend further up rivers making cultivation difficult
  • Droughts could occur due to higher temperatures meaning xerophytes could only survive
  • Greater rainfall would occur in some areas
  • Insect lifecycles will be altered and due to them carrying human and crop pathogens tropical diseases could spread toward poles
  • Benefit could be more rainfall filling reservoirs, higher temperatures causing higher rate of photosynthesis so more productivity and a larger harvest
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6.3 The Nitrogen Cycle

  • All living organisms require a source of nitrogen to form nucleic acids and proteins
  • Plants take most of their nitrogen up via nitrate ions (NO₃-) from the soil
  • The ions are absorbed by active transport from the root hairs
  • Animals obtain their nitrogen compounds by eating the plants
  • Nitrate ions are soluble
  • When plants and animals die decomposition occurs and the nitrates are restored to the soil
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The Stages of the Nitrogen Cycle


  • The production of ammonia from organic ammonium compounds e.g. urea, proteins and nucleic acids
  • Saprobiotic microorganisms e.g. fungi feed on these materials releasing ammonia which forms ammonium ions in the soil
  • This is where nitrogen returns to non living components of the ecosystem


  • Ammonium ions to nitrate ions is an oxidation reaction so releases energy
  • It is carried out by nitrifying bacteria in two stages:
    • 1. Oxidation of ammonium ions to nitrite ions (NO₂¯)
    • 2. Oxidation of nitrite ions to nitrate ions (NO₃¯)
  • Nitrifying bacteria require oxygen to carry out the conversions so the soil needs to have air spaces
  • Farmers keep soil light and aerated by ploughing and having good drainage  
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The Stages of the Nitrogen Cycle Cont

Nitrogen Fixation

  • Nitrogen gas is converted to nitrogen containing compounds
    • 1. Free living nitrogen fixing bacteria reduce gaseous nitrogen to ammonia to manufacture amino acids, when the bacteria dies and decay they release the nitrogen compounds
    • 2. Mutualistic nitrogen fixing bacteria live in nodules on roots of plants  and they obtain carbohydrates from the plant while in return the plant acquires amino acids from the bacteria


  • When soil is waterlogged there is a shortage of oxygen and the type of microorganism present changes
  • Fewer aerobic nitrifying and nitrogen fixing bacteria are found meaning more anaerobic denitrifying bacteria are present
  • This bacteria converts soil nitrates into gaseous nitrogen reducing availability of nitrogen containing compounds for plants
  • To prevent the build up of denitrifying bacteria the soil has to be well aerated
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6.4 Use of Natural and Artificial Fertilisers

  • Intensive food production makes large demands on the soil
  • Due to this the minerals are removed from the soil, in agriculture the remains of the consumer are rarely returned to the same area so mineral ions fall
  • The mineral ions need to be replenished so fertilisers are added:
    • Natural (organic): consist of dead/decaying remains as well as animal waste
    • Artificial (inorganic): Mined from rocks and deposits then converted into different forms to give the appropriate balance of minerals , compounds contain nitrogen, phosphorus and potassium
  • Plants require minerals for growth, nitrogen is needed for proteins and DNA
  • With nitrogen plants grow taller and have a greater leaf area
  • This increases the rate of photosynthesis and improves crop productivity
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6.5 Consequences of using Nitrogen Fertilisers

Effects of Nitrogen Fertilisers

  • Nitrogen is essential for proteins and growth and causes the increase in leaf area
  • This increases the rate of photosynthesis and improves crop productivity
  • The nitrogen containing fertilisers have bad effects to:
    • Reduced species diversity as nitrogen rich soils favour growth of grasses so they out compete other species that die
    • Leaching leads to pollution of watercourses
    • Eutrophication caused by leaching of fertiliser into watercourses
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  • The process by which nutrients are removed from the soil
  • Rain water will dissolve soluble nutrients e.g. nitrates and carry them into the soil beyond plant roots
  • The leached nitrates reach the watercourses e.g. rivers that drain into freshwater lakes
  • They can harm drinking water, prevent efficient oxygen transport in babies and cause stomach cancer
  • The leached nitrates are harmful to environment as they cause eutrophication
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  • The process by which nutrients build up in bodies of water
  • Most rivers contain low nitrate levels so it is a limiting factor for plant/algae growth
  • Nitrate concentration increases due to ,leaching so the plants grow exponentially
  • Algae grow at the surface so the upper layers of water become densely populated with algae, ‘algae bloom’
  • The layer absorbs light and prevents it from reaching the lower depths
  • Light becomes the limiting factor for growth so plants and deeper depths die
  • The lack off dead plants and algae is no longer limiting for the growth of saprobiotic algae so they grow exponentially
  • Saprobiotic bacteria require oxygen for respiration creating a demand for oxygen
  • The concentration of oxygen in the water is reduced and nitrates are reduced from decaying organisms
  • Oxygen then becomes the limiting factor for aerobic organisms e.g. fish so they die
  • Without aerobic organisms there is less competition for anaerobic organisms
  • These organisms further decompose dead material realising more nitrates and toxic waste link hydrogen sulphide
  • Animal slurry, human sewage, ploughing and artificial fertilisers cause leaching
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