nitrogen cycle

Question 1

oxidation of N

Answer 1

• N bonded to H = negative oxidation state
• N bonded to O = positive oxidation state
• N2 - cannot be oxidised due to its strong triple bond

Question 2

steps in the nitrogen cycle

Answer 2

1. N fixation
2. Ammonia assimilation
3. Nitrification
4. assimilatory nitrate reduction
5. ammonification
6. dentrification

Question 3

Nitrogen fixation

Answer 3

• nitrogen gas in the atmosphere is converted by biologically useable forms - ammonia
• diazotrophic process
• requires lots of energy due to the strong triple bond in N2
• Rhizobium bacteria found inside root noodles on leguminous plants - form a mutualistic relationship (they give the plant N in return for carbs)
• Azotobacter found in soil (not mutualistic relationship)
• reductive process - anaerobic conditions needed

Question 4

ammonia assimilation

Answer 4

process when ammonia (NH3) or ammonium (NH4+) is absorbed by organisms and incorporated into organic N compounds, forming part of their biomass
- energy efficient

Question 5

nitrification

Answer 5

1. ammonia oxidation
   - NH3 + O2 + 2H+ goes to NH2OH + H2O (catalysed by AMO) (uses energy)
   - This then reacts to form No2- (nitrite) + 5H+ (catalysed by FAO)
   - done by the bacteria Nitrosomonas
2. nitrite oxidation
   - NO2- + 52O goes to NO3- (nitrate) + 2H+ +2e + energy
   - done by the bacteria Nitrobacter
   - this can then by used by plants
   - chemolithotroph process - energy used to fix co2

Question 6

assimilatory nitrate reduction

Answer 6

• process where nitrate is reduced and incorporated into an organisms biomass
• essential for plants and microorganisms when ammonia is in low supply (nitrate more abundant in soils)
• however the reduction of nitrate to ammonia requires energy which is costly

Question 7

ammonification

Answer 7

• breakdown of organic N compounds from dead organisms and waste products into ammonia or ammonium
• heterotrophic bacteria responsible
• small proportion of ammonia produced may be released into the atmosphere contributing to atmospheric N

Question 8

denitrification

Answer 8

• reduction of nitrate back to N2 or N2O - moved from the biosphere into the atmosphere
• anaerobic process where N2 is used as the final electron acceptor in the electron chain
• rain wets the soils making them anaerobic
• pseudomonas dentrificans bacteria involved

Question 9

photochemical smog

Answer 9

• pollutants (N2O) react with sunlight
• look at diagram