Nitrogen Cycle

Question 1

List the stages of the Nitrogen Cycle

Answer 1

1. Nitrogen Fixation
2. Nitrification
3. Assimilation
4. Ammonification
5. Denitrification

Question 2

Why can’t atmospheric nitrogen be used by plants and animals as it is?

Answer 2

It has a triple bond between the two nitrogen atoms, so it is too stable

Question 3

How does nitrogen fixation work and where does it mostly occur?

Answer 3

Nitrogen Fixing Bacteria in root nodules of leguminous plants or free in the soil ‘fix’ atmospheric nitrogen into ammonium ions. (Catalysed by nitrogenase, requires 15 ATP molecules to fix each molecule of N2)

Question 4

Apart from by bacteria, how can nitrogen fixation occur?

Answer 4

• Haber Process

- Oxidising to Nitrate from lightning

Question 5

How does Nitrification work?

Answer 5

Nitrifying bacteria oxidise Ammonia to nitrite ions, then nitrate ions. (Exothermic oxidation, released energy used to make ATP)

Question 6

What happens after nitrification?

Answer 6

Assimilation; plants use the nitrate ions to make proteins

Question 7

What carries out ammonification and what is ammonification?

Answer 7

Microbial Saprobionts breaking down proteins in detritus to form ammonia

Question 8

What are the two stages of ammonification?

Answer 8

• Digestion of proteins using extracellular protease enzymes

- Removal of amino groups using deaminase enzymes

Question 9

When does de-nitrification occur, what carries it out, and what happens in it?

Answer 9

When the soil runs out of oxygen, nitrates are converted back to atmospheric nitrogen by anaerobic bacteria

Question 10

Describe the role of microorganisms in producing nitrates from the remains of dead organisms.

Answer 10

Saprobiotic bacteria break down dead material into ammonia/ammonium ions which are then converted into nitrites and then nitrates by nitrifying bacteria / nitrification