5.4 nutrient cycles

• explain the role of saprobionts in recycling chemical elements

  decompose (break down) organic compounds e.g proteins/urea/DNA in dead matter/organic waste

  by secreting enzymes for extracellular digestion (saprobiotic nutrition)

  absorb soluble needed nutrients and release mineral ions e.g mineral ions
• explain the role of mycorrhizae

  mycorrhizae - symbiotic association between fungi and plant roots

  fungi (hyphae) acts 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 e.g 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
• describe the role of bacteria in nitrogen fixation

  nitrogen gas converted into ammonia which forms ammonium ions in soil

  by nitrogen-fixing bacteria (may be found in root nodules)
• describe the role of bacteria in ammonification

  nitrogen-contains compounds e.g proteins/urea from dead organisms/waste are broken down/decomposed

  converted in 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 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, e.g 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

  phosphate ions in rocks released (into soils/oceans) by erosion/weathering

  phosphate ions taken up by producers/plants/algae and incorporated in to their biomass

  rate of absorption increased by mycorrhizae

  phosphate ions transferred through food chain e.g herbivores eat producers

  some phosphate ions lost from animals in waste products (excretion)

  saprobionts decompose organic compounds e.g 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 so increase productivity/yield
• artificial fertilisers

  contain inorganic compounds of nitrogen, phosphorus and potassium
• natural fertilisers 


  organic (e.g manure, compost, sewage) so ions released during decomposition by saprobionts
• 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

  so rapid growth of algae in pond/river (algal bloom) so light blocked

  so submerged plants die as they cannot photosynthesise

  so saprobionts decompose dead plant matter, using oxygen in aerobic respiration

  so less oxygen for fish to aerobically respire, leading to their death
• explain the key advantage of using natural fertilisers over artifical fertilisers

  less water soluble so less leaching so eutrophication less likely organic molecules require breaking down by saprobionts so slow release of nitrate/phosphate ect