Nutrient cycles

• Biological molecules containing nitrogen:

  • amino acid / protein / enzyme
  • nucleic acids- DNA, RNA
  • ATP
  • chlorophyll
• Benefit of root nodule bacteria being mutualistic:

  The bacteria obtain glucose to be used in respiration and the plant obtains ammonium ions, so both components benefit.
• How do farmers increase nitrification?

  They plough soil and ensure the soil has good drainage. This prevents anaerobic conditions.
• Describe ammonification:

  Saprobionts decompose organic nitrogen compounds such as urea by releasing extracellular enzymes. This adds ammonium ions to the soil.
• Describe nitrification:

  Nitrifying bacteria, which require oxygen, oxidise ammonium ions first into nitrites and then nitrates.
• Describe nitrogen fixation:

  Nitrogen fixing bacteria (which live free in the soil or in plant root nodules as mutualistic) convert nitrogen gas into ammonium ions which can be used to make amino acids.
• Describe denitrification:

  Denitrifying bacteria, which thrive in anaerobic conditions convert nitrate ions to nitrogen gas, so less is available for plants.
• How are nitrate ions absorbed by plants?

  Via active transport, so ATP is required. They are then used to synthesise nitrogenous compounds.
• Denitrification requires anaerobic conditions. Ploughing aerates the soil. Explain how ploughing would affect the fertility of the soil (2)

  Fertility would increase as ploughing introduces more oxygen into the soil. So there is more nitrification and less denitrification.
• One farming practice used to maintain high crop yields is crop rotation. This involves growing a different crop each year in the same field.

  Suggest 2 ways in which crop rotation may lead to high crop yields (2)

  1. As different crops remove different ions from the soil e.g. no3-, not all ions are removed from the soil.
  2. By growing nitrogen-fixing crops, less no3- will be removed as the crops can convert n2 gas directly to nh4+ / no3- ions.
• After harvesting, the remains of crop plants are often ploughed into the soil. Explain how microorganisms in the soil produce a source of nitrates from these remains. (5)

  • saprobionts decompose organic nitrogen compounds in the remains using extracellular enzymes
  • this produces nh4+ ions in the soil
  • then the nh4+ ions are oxidised to nitrites then nitrates
  • by nitrifying bacteria which live free in the soil and require oxygen
  • nitrates are absorbed by active transport and used to synthesise proteins for growth - this requires ATP
• Biological molecules containing phosphate ions:

  • ATP
  • nucleic acids- DNA and RNA
  • phospholipids
  • NADP
• Describe mycorrhizal association between the plant root and the fungus:

  Fungus significantly increases surface area for plant root to absorb water and inorganic ions.

  Plant root provides glucose for respiration and amino acids for growth.

  They have a mutualistic relationship.
• Refutes for evaluation questions:

  • sample size?
  • stats test?
  • applicable to all organisms?
  • challenge experiment
• 5 steps of the cycling of phosphate:

  1. Phosphate ions in sedimentary rocks dissolve in rainwater following weathering - main source of phosphates
  2. Phosphate ions in soil and water provide a source for plants
  3. Plant roots absorb phosphate ions via active transport
  4. Death of plants provides phosphate source to soils, phosphate ions transferred to animals when they eat plants
  5. Phosphate ions present in animal remains decompose slowly and dissolve into water sources