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

Question 1

what molecules have nitrogen?

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 does a saprobiont do?

Answer 3

• releases extracellular enzymes to digest dead material, faeces or urine, breaking down large organic molecules into smaller inorganic material
• assimlates digested molecules it wants and makes them into molecules

Question 4

why are saprobionts called nitrogen recyclers?

Answer 4

allows molecues it doesn’t absorb to go into the soil and be assimilated by other organisms

Question 5

definition of assimilation

Answer 5

the conversion of nutrients into living tissue

Question 6

what are the 4 stages of the nitrogen cycle?

Answer 6

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

Question 7

stage 1 of the nitrogen cycle: ammonification

Answer 7

• all living organisms contain nitrogen compounds eg. in their bodies, urine and faeces
• saprobionts break nitrogen compounds into ammonia, then to ammonium ions in the soil, which can be assimilated

Question 8

describe the word equation for ammonification

Answer 8

large organic molecule ➞ ammonia ➞ ammonium ions

Question 9

stage 2 of the nitrogen cycle: nitrification

Answer 9

• nitrifying bacteria convert ammonium ions to nitrites
• another nitrifying bacteria converts nitrites to nitrates

Question 10

describe the word equations for nitrification

Answer 10

• ammonium ions ➞ nitrites
• nitrites ➞ nitrates

Question 11

stage 3 of the nitrogen cycle: nitrogen fixation

Answer 11

• when nitrogen gas is converted into nitrogen containing compounds
• can be carried out industrially but also occurs naturally

Question 12

what are the 3 ways nitrogen fixation can occur?

Answer 12

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

Question 13

nitrogen fixation - lightning

Answer 13

• N₂ is a very unreactive molecule due to a triple covalent bond
• lightning has very high energy, so can break triple covalent bond
• can make nitrogen react with oxygen to form nitrogen oxides (NO₃⁻) which plants can assimilate

Question 14

nitrogen fixation - mutualistic nitrogen fixing bacteria

Answer 14

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

Question 15

what do the rhizobium get in exchange?

Answer 15

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

Question 16

nitrogen fixation - free living nitrogen-fixing bacteria

Answer 16

• 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 17

what happens during nitrification?

Answer 17

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

Question 18

stage 4 of the nitrogen cycle: denitrification

Answer 18

nitrate ions in soil can be converted into gaseous nitrates by anaerobic denitrifying bacteria

Question 19

what conditions are needed for denitrifcation and why?

Answer 19

• soils need to be waterlogged and have a low oxygen concentration
• therefore, there are fewer aerobic nitrifying and nitrogen-fixing bacteria