Nutrient cycles

• What is the first step in the nitrogen cycle?

  Molecules containing nitrogen are passed through a food chain
• What happens to organisms in the nitrogen cycle after they die?

  They are decomposed by bacteria (Saprobiants)
• What is ammonification in the nitrogen cycle?

  It is the process where saprobiants break down nitrogen-containing organic matter to produce ammonia
• What do saprobiants do during ammonification?

  Saprobiants break down nitrogen-containing organic matter
• What is produced when ammonia is dissolved in water?

  Ammonium is produced
• What occurs during nitrification in the nitrogen cycle?

  Ammonium is oxidised into nitrites and then into nitrates
• Which bacteria oxidise nitrites into nitrates?

  Pseudomonas bacteria
• Which bacteria are involved in the oxidation of ammonium to nitrites?

  Nitrifying bacteria Nitrobacter
• How do root hair cells utilize nitrates in the nitrogen cycle?

  They actively transport nitrates into the plant
• Under what conditions does denitrification occur?

  In anoxic conditions
• What is the role of denitrification in the nitrogen cycle?

  It converts nitrates in the soil into nitrogen gas, releasing it back into the atmosphere
• What are the main processes of the nitrogen cycle?

  1. Nitrogen molecules passed through food chain
  2. Ammonification by saprobiants
  3. Nitrification (ammonium to nitrites to nitrates)
  4. Active transport of nitrates into plants
  5. Denitrification (nitrates to nitrogen gas)
• Explain the role of Saprobiants in the Nitrogen Cycle:

  • Decompose (break down) organic compounds e.g. proteins, urea, DNA
  • By secreting enzymes for extracellular digestion
  • Absorb soluble needed nutrients and release mineral ions e.g. phosphates
• 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 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
  • RNA
  • Chlorophyll
  • ATP or ADP
  • NAD
  • NADP
• Describe the role of bacteria in Nitrogen Fixing:

  • Nitrogen gas (N2) converted into ammonia (NH3), which forms ammonium ions in soil (NH4+)
  • By nitrogen fixing bacteria such as Rhizobium (May be found in root nodules)
• Describe the role of bacteria in Ammonification:

  • Nitrogen containing compounds e.g. proteins or urea from dead organisms / waste is decomposed
  • Converted into ammonia which forms ammonium ions
  • By Saprobiants - secrete enzymes for extracellular digestion
• Describe the role of bacteria in Nitrification:

  • Ammonium ions in soil are converted into nitrites then nitrates through oxidation reactions - for uptake by root hair cell by active transport
  • By nitrifying bacteria in aerobic conditions
• Describe the role of bacteria in denitrification:

  • Nitrates in soil converted into nitrogen gas (reduction)
  • By denitrifying bacteria in anoxic conditions
• Suggest why ploughing soil increases fertility:

  • More ammonium converted into nitrite and nitrate / more nitrification / more active nitrifying bacteria
  • Less nitrate converted into nitrogen gas / less denitrification / fewer active nitrifying bacteria
• What are Non-essential amino acids?

  Amino acids we can make
• What are essential amino acids?


  Amino acids we can't make
• Give examples of biological molecules that contain phosphorus:

  • Phospholipids
  • DNA
  • RNA
  • ATP
  • ADP
  • NADP
  • TP
  • GP
  • RuBP
• Describe the Phosphorus Cycle:

  1)Phosphate ions in rock released into soils / oceans by erosion

  2)Phosphate ions taken up by producers / plants / algae and incorporated into their biomass

  -Rate of absorption is increased by mycorrhizae

  3)Phosphate ions transferred through food chain

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

  5)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 saprobiants

  • So improve efficiency of energy transfer -> increases productivity / yield
• What are natural fertilisers?

  They are organic e.g. manure, compost, sewage -> ions released during decomposition by saprobiants
• What are artificial fertilisers?

  They 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 can't photosynthesise
  3. So Saprobiants decompose dead plant matter, using oxygen in aerobic respiration
  4. So less oxygen for fish to aerobically respire, leading to death
• Explain the key advantage of using natural fertiliser over artificial:

  • Less water soluble so less leaching -> eutrophication less likely
  • Organic molecules require breaking down by saprobiants -> slow release of nitrate / phosphate