Nutrient cycles

Question 1

The simple sequence of a nutrient cycle

Answer 1

Nutrients taken up by producer as an inorganic ion (simple inorganic molecule)

Producer incorporates nutrients into complex organic molecules.

Producer eaten and nutrients passed to consumer and along food chain.

When producers/consumers die, complex molecules are broken down by saprobiontic microorganisms (decomposition).

Inorganic ion released

Question 2

Role of saprobionts in recycling chemical elements

Answer 2

Feed on remains of dead plants/animals and their waste products e.g. faeces, urea, and break down the organic molecules

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

Decomposer

Question 3

Role of mycorrhizae in recycling chemical elements

Answer 3

Symbiotic relationship between fungi and roots of plants = mycorrhizae

Fungi act as an extension of the plant roots (made of thin strands called hyphae)

Increase surface area of root system increase rate of absorption of water/nutrients

Mutualistic relationship – plant also provides fungi with carbohydrates

Question 4

Main stages of the nitrogen cycle

Answer 4

All involve microorganisms (fungi/bacteria)

Ammonification

Nitrification

Nitrogen fixation

Denitrification

Question 5

Ammonification

Answer 5

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 which excrete enzymes for extracellular digestion.

Question 6

Nitrification

Answer 6

Ammonium ions in the soil -> nitrites -> nitrates (nitrogen-containing compounds)

A two-stage oxidation reaction by nitrifying bacteria

Bacteria need oxygen to carry out conversions

Nitrates (nutrients) can be absorbed by plant root hair cells by active transport

Application – farmers aerate their soil to increase O2, allowing number of nitrifying bacteria to increase and denitrifying bacteria to decrease which maximises nitrogen availability

Question 7

Denitrification

Answer 7

Nitrates in the soil convert to nitrogen gas via denitrifying bacteria (anaerobically respire) when low oxygen concentration in soil i.e. waterlogged because more anaerobic denitrifying bacteria (and less aerobic nitrifying and nitrogen fixing bacteria)

(Reduces availability of nitrogen compounds for plants)

Question 8

Nitrogen fixation

Answer 8

Nitrogen gas (N2) converted (reduction) to nitrogen containing compounds e.g. ammonia by nitrogen-fixing bacteria

Can be ‘free living’ in the soil

Or ‘mutualistic’ (live in nodules on roots of plants e.g. legumes; acquire carbohydrates from plant while the plant acquires amino acids from bacteria)

Question 9

Nitrogen cycle importance

Answer 9

Nitrogen gas (N2) is unreactive and not easily converted into other compounds

Most plants can only take up nitrogen (by active transport in roots) in the form of nitrate

Used by plants/animals to make proteins/nucleic acids (assimilated) growth

Question 10

Stages of the phosphorous cycle

Answer 10

1. Phosphate ions in rocks released (to soil) by erosion/weathering
2. Phosphate ions taken into plants by roots and incorporated into their biomass (assimilated)

Make biological molecules such as DNA, RNA, phospholipids (and NADP and RuBP in plants).

Rate of absorption increased by mycorrhizae

3. Phosphate ions transferred through food chain e.g. as herbivores eat plants
4. Some phosphate ions lost from animals in waste products (excretion), and plants and animals die

Decomposed by Saprobionts – release enzymes for extracellular digestion

Release phosphate ions to the soil

Note: Weathering of rocks also releases phosphate ions into seas, lakes and rivers

Taken up by aquatic producers e.g. algae

Passed along food chain to birds

Guano, a waste product of birds, returns a many phosphate ions to soils in coastal areas

Question 11

The need for fertilisers

Answer 11

Replaces nutrients (nitrates and phosphates) lost from an ecosystem’s nutrient cycle when crops are harvested or livestock (animals) are removed

Nutrients removed from soil and incorporated into their biomass can’t be released back into the soil through decomposition by saprobionts

Hence, fertilisers improve the efficiency of energy transfer (more energy can be used for growth)

Nutrients can no longer be a limiting factor

Increase overall productivity of agricultural land

Question 12

Artificial fertilisers

Answer 12

Inorganic

Contain pure chemicals e.g. ammonium nitrate as powders / pellets

Inorganic substances are more water soluble so larger quantities washed away, impacting the environment

Question 13

Natural fertilisers

Answer 13

Organic

e.g. manure, compost, sewage

Cheaper/free but exact nutrients cannot be controlled

Question 14

Environmental issues caused by fertilisers (leaching)

Answer 14

Leaching of nutrients, where rain/irrigation systems wash water-soluble compounds out of soil into waterways e.g. rivers

Worse when more fertiliser added to field than used (excess)

Leaching leads to eutrophication

Leaching less likely with natural fertilisers; nitrogen/phosphorous contained in organic molecules and organic molecules are less soluble in water so need to be decomposed by saprobionts before nitrogen and phosphorous are released (as soluble inorganic molecules)

Question 15

Environmental issues caused by fertilisers (species diversity)

Answer 15

Leaching can also reduce species diversity

Favour fast growing plants e.g. grass/nettles so slower-growing plants lose out, a decrease in organisms who feed off them

Question 16

Eutrophication

Answer 16

1. Rapid growth of algae in ponds and rivers (algal bloom)
2. Algae blocks light, preventing it from reaching plants below
3. Death of the plants below as they cannot photosynthesise
4. Aerobically respiring saprobionts decompose the dead plant matter, reducing oxygen concentration of water
5. Leading to death of aquatic organisms due to lack of dissolved oxygen for aerobic respiration