5.4 Nutrient cycles

Question 1

What is the role of saprobionts?

Answer 1

• Decompose / break down organic compounds (eg. Proteins) in organic waste
• By secreting enzymes for extracellular digestion
• Absorb nutrients and release mineral ions

Question 2

What is the role of mycorrhizae?

Answer 2

• Fungi act as an extension of plant roots, increasing surface area
• To increase rate of absorption of water and inorganic ions
• In return, fungi receive organic compounds (eg. Carbohydrates)

Question 3

Nitrogen fixation

Answer 3

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

Question 4

Ammonification

Answer 4

• Nitrogen-containing compounds (eg. Proteins) from dead organisms are broken down and converted into ammonia which forms ammonium ions in soil
• By saprobionts

Question 5

Nitrification

Answer 5

• Ammonium ions in soil are oxidised into nitrites then nitrates
• By nitrifying bacteria in aerobic conditions

Question 6

Denitrification

Answer 6

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

Question 7

What happens during the phosphorus cycle?

Answer 7

• Phosphate ions in rocks released by weathering
• Phosphate ions taken up by producers and incorporated into their biomass
• Phosphate ions transferred through food chain
• Some phosphate ions lost from animals in waste products (excretion)
• Saprobionts decompose organic compounds (eg. DNA), releasing phosphate ions

Question 8

Why are fertilisers used?

Answer 8

• To replace nutrients lost when plants are harvested and livestock removed
• Improving efficiency of energy transfer, increasing productivity / yield

Question 9

Natural fertilisers

Answer 9

Organic compounds (eg. Manure)
Require breaking down by saprobionts, releasing ions slowly
Less water soluble so less leaching

Question 10

Artificial fertilisers

Answer 10

Inorganic compounds (eg. Phosphorus)
Don’t require breaking down, releasing ions rapidly
More water soluble so more leaching

Question 11

What happens during eutrophication?

Answer 11

• Phosphates/nitrates dissolve in water, leading to leaching of nutrients into lakes
• Causes rapid growth of algae / algal bloom so light blocked
• So submerged plants die as can’t photosynthesise
• Saprobionts decompose dead plant matter, using oxygen in aerobic respiration
• So less oxygen for fish to aerobically respire so fish die