nutrient cycles

Question 1

Explain the role of saprobionts in recycling chemical elements

Answer 1

Decompose organic compounds
By secreting enzymes for extracellular digestion
Absorb soluble needed nutrients and release minerals ions

Question 2

Explain the role of mycorrhizae

Answer 2

Fungi act as an extension of plant roots to increase surface area of root system
To increase rate of uptake of water and inorganic ions
In return, fungi receive organic compounds eg. carbohydrates

Question 3

Describe the role of bacteria in nitrogen fixation

Answer 3

Nitrogen gas converted into ammonia
which forms ammonium ions in soil
By nitrogen-fixing bacteria

Question 4

Describe the role of bacteria in ammonification

Answer 4

Nitrogen-containing compounds
decomposed
Converted to ammonia, which forms ammonium ions in soil
By saprobionts - secrete enzymes for extracellular digestion

Question 5

Describe the role of bacteria in nitrification

Answer 5

Ammonium ions in soil converted into nitrites then nitrates
via a two-step oxidation reaction
For uptake by plant root hair cells by active transport
By nitrifying bacteria in aerobic conditions

Question 6

role of bacteria in dentrification

Answer 6

● Nitrates in soil converted into nitrogen gas
● By denitrifying bacteria in anaerobic conditions

Question 7

Suggest why ploughing (aerating) soil increases its fertility

Answer 7

more nitrification
less dentrification

Question 8

phosphorous cycle

Answer 8

Phosphate ions in rocks released by erosion / weathering
Phosphate ions taken up by producers / plants / algae and incorporated into their biomass
Rate of absorption increased by mycorrhizae
Phosphate ions transferred through food chain eg. as herbivores eat producers
4. Some phosphate ions lost from animals in waste products (excretion)
5. Saprobionts decompose organic compounds
releasing phosphate ions

Question 9

why are fertilisers used

Answer 9

To replace nitrates phosphates lost when plants are harvested and livestock are removed
So improve efficiency of energy transfer → increase productivity / yield

Question 10

Explain the key environmental issue arising from use of fertilisers

Answer 10

Phosphates nitrates dissolve in water, leading to leaching of nutrients into lakes / rivers / oceans
● This leads to eutrophication
1. Rapid growth of algae in pond / river ) so light blocked
2. So submerged plants die as they cannot photosynthesise
3. So saprobionts decompose dead plant matter, using oxygen in aerobic respiration
4. So less oxygen for fish to aerobically respire, leading to their death