Nitrogen Cycle

• Why is the nitrogen cycle important?

  It converts nitrogen gas into usable compounds
• What percentage of the atmosphere is nitrogen gas?

  30 percent
• Why can't animals and plants obtain nitrogen through gas exchange?

  Nitrogen gas has a triple bond, making it inert
• What do microorganisms do in the nitrogen cycle?

  They break nitrogen bonds and convert gas
• How do plants absorb nitrogen-containing substances?

  Through active transport in the roots
• What are the four key processes of the nitrogen cycle?

  • Nitrogen fixation
  • Ammonification
  • Nitrification
  • Denitrification
• What biological molecules contain nitrogen?

  Proteins, ATP, and nucleic acids
• What is nitrogen fixation?

  Converting nitrogen gas into nitrogen compounds
• Where does nitrogen fixation occur in plants?

  In root nodules
• Which plants have root nodules for nitrogen fixation?

  Leguminous plants
• What do nitrogen-fixing bacteria convert nitrogen gas into?

  Ammonium or nitrates
• What is ammonification?

  Conversion of organic nitrogen into ammonium
• What is nitrification?

  Conversion of ammonium to nitrites and nitrates
• How many stages are there in nitrification?

  Two stages
• What do nitrifying bacteria do in the soil?

  Convert ammonium into nitrites and nitrates
• How do plants absorb nitrates?

  By active transport from the soil
• What happens to nitrates after plants absorb them?

  They are used to create amino acids and ATP
• How do animals obtain nitrogen?

  By eating and digesting plant material
• What do decomposers do in the nitrogen cycle?

  Break down proteins into nitrogen compounds
• What is denitrification?

  Conversion of nitrates back into nitrogen gas
• Why is denitrification undesirable in agriculture?

  It leads to loss of nitrogen from soil
• When does denitrification occur?

  When soil is waterlogged and anaerobic
• What is the oxidation process in nitrification?

  Ammonium to nitrite to nitrate
• What must be specified in exam questions about decomposers?

  The nitrogen-containing compound involved
• What nitrogen-containing compound is found in animal waste?

  Urea in urine
• What are the key points of the nitrogen cycle?

  • Nitrogen fixation: Converts nitrogen gas to compounds
  • Ammonification: Converts organic nitrogen to ammonium
  • Nitrification: Converts ammonium to nitrites and nitrates
  • Denitrification: Converts nitrates back to nitrogen gas
• How do farmers prevent denitrification in soil?

  By plowing to aerate the soil
• What is the role of aerobic bacteria in nitrification?

  They require oxygen to convert nitrogen compounds
• What is the nitrogen cycle?

  A series of processes converting nitrogen compounds
• How do Saprobionts decompose dead organic matter?

  • Saprobionts secrete enzymes onto the dead organic matter. This is called extracellular digestion.
  • The enzymes break down large, insoluble molecules into smaller, soluble molecules.
  • These smaller molecules can then be absorbed by the saprobiont.
  • The saprobiont will then either store the molecules or use them in respiration.
• Nitrogen fixation

  This is where nitrogen fixing bacteria convert nitrogen gas into ammonia, which forms ammonium ions (NH4+).

  An example of this kind of bacteria is Rhizobium, which is found in the root nodules of leguminous plants. Rhizobium are mutualistic:

  • The bacteria provide the plant with nitrogen compounds
  • The plant provides the bacteria with carbohydrates
• Ammonification

  Organic material is broken down by saprobiotic bacteria to release ammonium ions back to the soil.

  It is now in a form that can be taken up by plants.

  • Plants use nitrates to form proteins and nucleic acids etc.
• Nitrification

  • Ammonium ions that are released from ammonification are converted into nitrates by bacteria in the soil.
  • Nitrifying bacteria (e.g. Nitrosomonas) firstly convert NH4+ ions to nitrites.
  • Another bacteria (e.g. Nitrobacter) then convert nitrites to nitrates.
• Dentrification

  • Denitrifying bacteria convert nitrates in the soil back into atmospheric nitrogen (N2).
  • This takes place in anaerobic conditions (e.g waterlogged soils).