Nutrient Cycles

Question 1

Significance of phosphorus to living organisms

Answer 1

Plants convert inorganic phosphorus into biological molecules (e.g. DNA, ATP, NADP)
Phosphorus is passed to consumers via feeding

Question 2

Role of bacteria in nitrogen fixation

Answer 2

Mutualistic nitrogen-fixing bacteria in nodules of legumes & free-living bacteria in soil
Use enzyme nitrogenase to reduce gaseous nitrogen into ammonia

Question 3

Role of bacteria in ammonification

Answer 3

1. Saprobionts feed on and decompose organic waste containing nitrogen
2. NH3 released
3. NH3 dissolves in water in soil to form NH4+

Question 4

Role of bacteria in denitrification

Answer 4

Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen

Question 5

2 main environmental issues caused by the use of fertilisers

Answer 5

1. Leaching: nitrates dissolve in rainwater and ‘runoff’ into water sources
2. Eutrophication: water source becomes putrid as result of algal bloom

Question 6

What happens during eutrophication

Answer 6

1. Aquatic plants grow exponentially since nitrate level is no longer a limiting factor
2. Algal bloom o water surface prevents light from reaching the bottom and plants die
3. Oxygen levels decrease as population of aerobic saprobionts increases to decay dead matter, so fish die.
4. Anaerobic organisms reproduce exponentially and produce toxic waste which makes water putrid.

Question 7

Why is phosphorus cycle slow process?

Answer 7

• Phosphorus has no gas phase, so there is no atmospheric cycle
• Most phosphorus is stored as phosphate ion in rocks

Question 8

What happens during atmospheric fixation of nitrogen?

Answer 8

1. High energy of lightning breaks N2 into N
2. N reacts with oxygen to form NO2-
3. NO2- dissolves in water to form NO3-

Question 9

Outline role of bacteria in nitrification

Answer 9

2-step process carried out by saprobionts in aerobic conditions:
2NH4+ + 3O2 -> 2NO2- + 2H20 + 4H+
2NO2- + O2 -> 2NO3

Question 10

Explain significance of nitrogen to living organisms

Answer 10

Plant roots uptake nitrates via active transport & use them to make biological compounds e.g.:
- amino acids
- NAD/ NADP
- Nucleic acids

Question 11

Outline role of Mycorrhizae

Answer 11

Mutualistic relationship between plant & fungus increases surface area of root system = increases uptake of water & mineral ions

Question 12

Name 2 categories of fertiliser & state purpose of using fertiliser

Answer 12

• Organic: decaying organic matter & animal waste
• Inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium
• To increase gross productivity for higher yield

Question 13

2 main environmental issues caused by use of fertilisers

Answer 13

1. Leaching: nitrates dissolve in rainwater & ‘runoff’ into water sources
2. Eutrophication: water source becomes putrid as result of algal bloom

Question 14

How can risk of eutrophication be reduced?

Answer 14

• Sewage treatment marshes on farms
• Pumping nutrient-enriched sediment out of water
• Using phosphate-free detergent