[13.3] nutrient cycles (1/2) the nitrogen cycle

Question 1

why is nitrogen important?

Answer 1

• DNA nucleotides
• amino acid R groups
• ATP

Question 2

what form do organisms normally use nitrogen in?

Answer 2

• nitrate ion form (from nitrate compounds)
• ammonium ion form (ammonium compounds)

Question 3

what is the role of a saprobiont?

Answer 3

• releases extracellular enzymes to digest dead material, faeces or urine, breaking down large organic molecules into smaller inorganic material
• assimlates the digested molecules it wants (absorbs them and makes them into molecules)
• allows the molecues it doesn’t absorb to go into the soil and be assimilated by other organisms - they are called nutrient recyclers

Question 4

definition of assimilation

Answer 4

the conversion of nutrients into living tissue

Question 5

what are the 4 stages of the nitrogen cycle?

Answer 5

1. ammonification
2. nitrification
3. nitrogen fixation
4. denitrification

Question 6

stage 1 of the nitrogen cycle: ammonification

Answer 6

• all living organisms contain large proteins, DNA, RNA etc. in their bodies, urine and faeces
• saprobionts are able able to break these nitrogen compounds down into ammonia and then into ammonium ions in the soil, which can be assimilated

Question 7

describe the word equation for ammonification

Answer 7

large organic molecule ➞ ammonia ➞ ammonium ions

Question 8

stage 2 of the nitrogen cycle: nitrification

Answer 8

• nitrifying bacteria now convert ammonium ions to nitrtes
• then another nitrifying bacteria converts nitrites to nitrates

Question 9

describe the word equations for nitrification

Answer 9

• ammonium ions ➞ nitrites
• nitrites ➞ nitrates

Question 10

stage 3 of the nitrogen cycle: nitrogen fixation

Answer 10

• a process by which nitrogen gas is converted into nitrogen containing compounds
• it can be carried out industrially and also occurs naturally

Question 11

what are the 3 ways nitrogen fixation can occur?

Answer 11

• lightning
• haber process
• nitrogen fixing bacteria in soil or root nodules

Question 12

3. nitrogen fixation - lightning

Answer 12

• N₂ is a very unreactive molecule
• this is because it has a triple covalent bond
• lightning has very high energy
• it can break that triple covalent bond and make the nitrogen react with oxygen to form nitrogen oxides
• these can become nitrogen oxides (NO₃⁻) which plants can assimilate

Question 13

3. nitrogen fixation - mutualistic nitrogen fixing bacteria

Answer 13

• rhizobium bacteria live in root rodules on certain plants called legumes
• they take in nitrogen and convert it to ammonia, which changes to ammonium ions inside the legumes
• the plants assimilate this into AA / DNA / ATP

Question 14

what do the rhizobium get in exchange?

Answer 14

• carbohydrates / amino acids from the plant
• this is a mutualistic relationship

Question 15

3. nitrogen fixation - free living nitrogen-fixing bacteria

Answer 15

• nitrogen-fixation bacteria live in soil
• they take in nitrogen from the air and convert it to ammonia then ammonium ions
• plants will assimilate the ammonium ions, but they can be converted further

Question 16

what happens during nitrification?

Answer 16

• nitrifying bacteria convert ammonium ions ➞ nitrates
• then another nitrifying bacteria converts nitrites ➞ nitrates
• plants need nitrates to make AA for protein etc.

Question 17

stage 4 of the nitrogen cycle: denitrification

Answer 17

• nitrate ions in the soil can be converted back into gaseous nitrates by anaerobic denitrifying bacteria
• this can only happen when soils become waterlogged and have low oxygen concentration so there are fewer aerobic nitrifying bacteria and nitrogen-fixing bacteria

Question 18

why do farmers often plough their fields to introduce oxygen into the soil? (3)

Answer 18

• prevents anaerobic conditions
• this prevents denitrifying bacteria breaking down nitrates into nitrogen gas
• more nitrogen remains in the soil allowing plants to make more AA / DNA / ATP / RNA for growth