5.4 Nutrient cycles

• Explain the role of saprobionts in recycling chemical elements

  ● Decompose (break down) organic compounds eg. proteins / urea / DNA in dead matter / organic waste

  ● By secreting enzymes for extracellular digestion (saprobiotic nutrition)

  ● Absorb soluble needed nutrients and release minerals ions eg. phosphate ions
• Explain the role of mycorrhizae

  Mycorrhizae = symbiotic association between fungi and plant roots

  ● Fungi (hyphae) act as an extension of plant roots to increase surface area of root system

  ● To increase rate of uptake / absorption of water and inorganic ions

  ● In return, fungi receive organic compounds eg. carbohydrates
• Give examples of biological molecules that contain nitrogen

  Amino acids / proteins or enzymes / urea / DNA or RNA / chlorophyll / ATP or ADP / NAD or NADP
• Draw a diagram to show the key stages of the nitrogen cycle

  Picture

  
• Describe the role of bacteria in nitrogen fixation

  ● Nitrogen gas (N2) converted into ammonia (NH3), which forms ammonium ions (NH4+) in soil

  ● By nitrogen-fixing bacteria (may be found in root nodules)
• Describe the role of bacteria in ammonification

  ● Nitrogen-containing compounds eg. proteins / urea from dead organisms / waste are broken down / decomposed

  ● Converted to ammonia, which forms ammonium ions in soil

  ● By saprobionts - secrete enzymes for extracellular digestion
• Describe the role of bacteria in nitrification

  ● Ammonium ions in soil converted into nitrites then nitrates, via a two-step oxidation reaction ○ For uptake by plant root hair cells by active transport

  ● By nitrifying bacteria in aerobic conditions (oxygen)
• Describe the role of bacteria in denitrification

  ● Nitrates in soil converted into nitrogen gas (reduction)

  ● By denitrifying bacteria in anaerobic conditions (no oxygen, eg. waterlogged soil
• Suggest why ploughing (aerating) soil increases its fertility

  ● More ammonium converted into nitrite and nitrate / more nitrification / more (active) nitrifying bacteria

  ● Less nitrate converted to nitrogen gas / less denitrification / fewer (active) nitrifying bacteria
• Give examples of biological molecules that contain phosphorus

  Phospholipids / DNA or RNA / ATP or ADP / NADP / TP or GP / RuBP
• Describe the phosphorus cycle

  1. Phosphate ions in rocks released (into soils / oceans) by erosion / weathering
  2. Phosphate ions taken up by producers / plants / algae and incorporated into their biomass

  ○ Rate of absorption increased by mycorrhizae

  3. Phosphate ions transferred through food chain eg. as herbivores eat producers

  4. Some phosphate ions lost from animals in waste products (excretion)

  5. Saprobionts decompose organic compounds eg. DNA in dead matter / organic waste, releasing phosphate ions
• Explain why fertilisers are used

  ● To replace nitrates / phosphates lost when plants are harvested and livestock are removed ○ Those removed from soil and incorporated into biomass can’t be released back into the soil through decomposition by saprobionts

  ● So improve efficiency of energy transfer → increase productivity / yield
• Describe the difference between artificial and natural fertilisers

  Natural

  • Organic, eg. manure, compost, sewage → ions released during decomposition by saprobionts

  Artificial

  • Contain inorganic compounds of nitrogen, phosphorus and potassium
• Explain the key environmental issue arising from use of fertilisers

  ● Phosphates / nitrates dissolve in water, leading to leaching of nutrients into lakes / rivers / oceans

  ● This leads to eutrophication

  1. Rapid growth of algae in pond / river (algal bloom) so light blocked
  2. So submerged plants die as they cannot photosynthesise
  3. So saprobionts decompose dead plant matter, using oxygen in aerobic respiration
  4. So less oxygen for fish to aerobically respire, leading to their death
• Explain the key advantage of using natural fertiliser over artificial fertiliser

  ● Less water soluble so less leaching → eutrophication less likely

  ● Organic molecules require breaking down by saprobionts → slow release of nitrate / phosphate etc