Nutrient Cycle

Question 1

Describe the role of bacteria in nitrogen fixation

Answer 1

● Nitrogen gas (N2) converted into ammonia (NH3), which forms ammonium ions (NH4+) in soil
● By nitrogen-fixing bacteria

Question 2

Describe the role of bacteria in ammonification

Answer 2

● Nitrogen-containing compounds eg. proteins / urea from dead organisms / waste are broken down and decomposed
● Converted to ammonia, which forms ammonium ions in soil
● By saprobiont

Question 3

Describe the role of bacteria in denitrification

Answer 3

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

Question 4

Explain the role of saprobionts in recycling chemical elements

Answer 4

● Decompose (break down) organic compounds
● By secreting enzymes for extracellular digestion (saprobiotic nutrition)
● Absorb soluble needed nutrients and release minerals ions

Question 5

Mycorrhizae

Answer 5

symbiotic association between fungi and plant roots

Question 6

Explain the role of mycorrhizae

Answer 6

● Fungi (hyphae) act as an extension of plant roots to increase surface area of root system
● To increase rate of uptake / absorption of water and inorganic ions
● In return, fungi receive organic compounds

Question 7

Give examples of biological molecules that contain nitrogen

Answer 7

ATP
Proteins
Chlorophyll
NAD

Question 8

Suggest why ploughing (aerating) soil increases its fertility

Answer 8

● More ammonium converted into nitrite and nitrate / more nitrification / more (active) nitrifying bacteria
● Less nitrate converted to nitrogen gas / less denitrification / fewer (active) denitrifying bacteria

Question 9

Give examples of biological molecules that contain phosphorus

Answer 9

Phospholipids
DNA
ATP

Question 10

Describe the phosphorus cycle

Answer 10

1. Phosphate ions in rocks released (into soils / oceans) by erosion / weathering
2. Phosphate ions taken up by producers / plants / algae and incorporated into their biomass
   ○ Rate of absorption increased by mycorrhizae
3. 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 eg. DNA in dead matter / organic waste,
   releasing phosphate ions

Question 11

Explain why fertilisers are used

Answer 11

● To replace nitrates / phosphates lost when plants are harvested and livestock are removed

● So improve efficiency of energy transfer → increase productivity / yield

Question 12

Describe the difference between artificial and natural fertilisers

Answer 12

Natural
● Organic, eg. manure, compost, sewage
● Ions released during decomposition by saprobionts

Artificial
Contain inorganic compounds of nitrogen, phosphorus and potassium

Question 13

Explain the key environmental issue arising from use of fertilisers

Answer 13

● 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 (algal bloom) 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

Question 14

Explain the key advantage of using natural fertiliser over artificial fertili

Answer 14

● Less water soluble so less leaching → eutrophication less likely
● Organic molecules require breaking down by saprobionts → slow release of nitrate / phosphate etc.

Question 15

Describe the role of bacteria in nitrification

Answer 15

● 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 (oxygen)