5.4 nutrient cycles

Question 1

Role of saprobionts in recycling chemical elements

Answer 1

Feed on remains of dead plants/animals and their waste products and break down organic molecules

By secreting enzymes for extracellular digestion
- saprobionts absorb soluble needed nutrients

Question 2

Role of mycorrhizae in recycling chemical elements

Answer 2

Symbiotic relationship between fungi and roots of plants = mycorrhizae

Fungi act as extension of the plant roots (made of hyphae)
- increase surface area of root system > increase rate of water/nutrient absorption

Question 3

Main stages of nitrogen cycle

Answer 3

Ammonification

Nitrification

Nitrogen fixation

Denitrification

Question 4

Ammonification

Answer 4

Nitrogen-containing compounds e..g proteins from dead organisms/animal waste broken down

Converted to ammonia which goes on to form ammonium ions (NH4+) in the soil

By saprobionts
- by secreting enzymes for extracellular digestion

Question 5

Nitrification

Answer 5

Ammonium ions in the soil > nitrites > nitrates

Two stage oxidation reaction

By nitrifying bacteria
- bacteria need oxygen to carry out conversion

Nitrates can be absorbed by plant root hair cells by active transport

Question 6

Application of nitrification

Answer 6

Farmers aerate their soil

Increase o2

Number of nitrifying bacteria increases and denitrifying bacteria decreases

Maximises nitrogen availability

Question 7

Denitrification

Answer 7

Nitrates in the soil > nitrogen gas

By denitrifying bacteria (anaerobically respire)
- when low oxygen conc. in soil e.g. waterlogged
- more anaerobic denitrifying bacteria

Reduces availability of nitrogen compounds for plants

Question 8

Nitrogen fixation

Answer 8

Nitrogen gas (N2) converted to nitrogen containing compounds e.g. ammonia

By nitrogen-fixing bacteria
- can be free living in soil
- or mutalistic (live in nodules on roots of plants e..g legumes - acquire carbohydrates from plant while plant acquires amino acids from bacteria)

Question 9

Nitrogen cycle importance

Answer 9

Nitrogen gas is unreactive and not easily converted into other compounds
- most plants take up nitrogen (by active transport in roots) in form of nitrate
- used by plants/animals to make proteins/ nucleic acids > growth

Question 10

Stages of the phosphorus cycle

Answer 10

Phosphate ions in rocks released by erosion or weathering

Phosphate ions taken into plants by roots and incorporated into their biomass
- DNA, RNA, phospholipids
- rate of absorption increased by mycorrhizae

Phosphate ions transferred through food chain

Some phosphate ions lost from animals in waste products and plants and animals die
- decomposed by saprobionts - release enzyme for extracellular digestion
- release phosphate ions into soil

Question 11

Need for fertilisers

Answer 11

Replaces nutrients lost when crops are harvested & livestock removed

Improve efficiency of energy transfer
- nutrient no longer a limiting factor
- increase productivity of agricultural land

Question 12

artificial fertilisers

Answer 12

Inorganic

Contain chemicals

More water soluble so larger quantities washed away, impacting the environment

Question 13

Natural fertilisers

Answer 13

Organic

E.g. manure, compost, sewage

Cheaper but exact nutrients cannot be controlled

Question 14

Leaching of nutrients

Answer 14

Result of fertilisers
- rain/irrigation systems wash water-soluble compounds out of soil into waterways

Question 15

Eutrophication

Answer 15

Result of fertilisers
- rapid growth of algae in ponds and rivers (algal bloom)

• algae blocks light, preventing it from reaching plants below
• death of plants below as they cannot photosynthesise
• aerobically respiring saprobionts decompose dead plant matter, reducing oxygen concentration of water
• leading to death of aquatic organisms due to lack of dissolved oxygen for aerobic respiration

Reduces species diversity as favours fast growing plants > slower-growing plants lose out > less organisms who feed off them

Question 16

Leaching less likely with natural fertilisers

Answer 16

Nitrogen/phosphorus contained in organic molecules

Organic molecules less soluble in water so need to be decomposed by saprobionts before nitrogen and phosphorus are released