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

• 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

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

• 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

• 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

Ammonification

• 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

Nitrification

• 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  

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

Denitrification

• 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

• 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

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

Leaching

• 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

• 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