3.5.4 Nutrient Cycles

Question 1

What happens to nutrients in an ecosystem

Answer 1

They are recycled

Question 2

Examples of two nutrient cycles

Answer 2

Nitrogen cycle
Phosphorus cycle

Question 3

What’s the role of saprobionts in decomposition

Answer 3

Extracellular digestion, occurs in nitrogen and phosphorus cycle
Enzymes are released onto dead material, urine, faeces
Digestion, large insoluble molecules are hydrolysed into smaller soluble ones outside the body
Absorption of these products eg amino acids, glucose
Saprobionts release ammonium ions (N cycle), phosphate ions (P cycle)

Question 4

What’s mycorrhizae

Answer 4

Fungus, facilitates uptake of water and inorganic ions by plants

Question 5

Role of mycorrhizae in nutrient cycles

Answer 5

microscopic threads (hyphae) acting as extensions of the plants root system, increase surface area for absorption of water and minerals
Acts as a sponge holding water and minerals around the roots, plants can resist drought and take up scarce ions readily
Mutualistic relationship, plant has better water uptake, fungus recurve organic compounds like sugar from plant

Question 6

What’s the importance of the nitrogen cycle

Answer 6

Plants and animals can obtain nitrogen gas from gas exchange
Microorganisms needed to convert N2 into nitrogen containing substances they can absorb

Question 7

Which biological molecules contain nitrogen

Answer 7

ATP, adenine is a nitrogenous base
Proteins, amino acids are made of 3 nitrogenous based
DNA, nitrogenous bases

Question 8

How to plants take up nitrogen

Answer 8

Nitrate ions from soil are absorbed via active transport

Question 9

How do animals take up nitrogen

Answer 9

Through food

Question 10

Steps in the nitrogen cycle

Answer 10

1. Ammonification
2. Nitrification
3. Nitrogen fixation
4. Denitrification

Question 11

What happens in nitrogen fixation

Answer 11

N2 in atmosphere converted to nitrogen containing compounds
Root nodules in some plants contain nitrogen fixing bacteria
Nitrogen fixing bacteria in soil reduce nitrogen to ammonia
Artificially lightening produces nitrogen oxides which are taken up as nitrates by plants

Question 12

What happens in ammonification

Answer 12

Urea, proteins, nucleus acids are eaten by saprobionts, forming ammonia then ammonium ions in soil

Question 13

What happens in nitrification

Answer 13

Ammonium ions oxidised to nitrite ions then oxidised again to nitrate ions
Both reactions require nitrifying bacteria

Question 14

What happens in denitrification

Answer 14

Waterlogged soil contains less aerobic nitrifying and nitrogen fixing bacteria and more anaerobic denitrifying bacteria, nitrates in soil are converted to N2 gas, this reduced nitrogen containing compounds for plants,

Question 15

What are sources natural fertilisers

Answer 15

Dead and decaying remains of plants and animals
Manure
Slurry

Question 16

What are sources of artificial fertilisers

Answer 16

Mined from rocks, converted into different forms, blended to create an appropriate balance of minerals
Chemicals contain nitrogen phosphorus and potassium (NPK)

Question 17

Why are fertilisers used

Answer 17

In agriculture crops are harvested, soil is depleted of nitrates and phosphates, these ions are replaced by the use of fertilisers

Question 18

How are fertilisers causing environmental issues

Answer 18

Excessive use of them

Question 19

How do fertilisers cause leaching

Answer 19

Leaching, nutrients are removed from soil
Rainwater dissolves soluble nutrients like nitrate and phosphate ions, carried deep into soil so plant roots can’t reach them, nitrates move into water sources like lakes
This can further cause eutrophication

Question 20

How do both fertilisers cause eutrophication

Answer 20

Low conc of nitrate ions in lakes, limiting factor for plant growth
Due to leaching nitrate ion conc increases, algae and plant populations grow
Upper surface of water becomes densely populated with algae
This layer of algae absorbs light, cannot penetrate lower, it becomes a limiting factor for algae at the bottom of the lake causing it to die
There is enough dead matter for saprobionts to use as food causing their population to grow
Saprobionts require oxygen for respiration increasing the demand of oxygen
There is a reduced oxygen conc causing nitrates to be released from dead organisms
Oxygen becomes a limiting factor for aerobic organisms, these organisms then die
Less competition for anaerobic organisms, they decompose dead material releasing nitrates and toxic waste making water putrid

Artificial and natural fertilisers cause leaching and eutrophication but artificial ones are the main cause

Question 21

Why is phosphorus important

Answer 21

Used in ATP, phospholipids, nucleic acids to make DNA

Question 22

Explain the phosphorus cycle

Answer 22

Phosphate ions exist in rock deposits, weathering and erosion allows them to dissolve into water
Plants absorb these, become part of their biomass, pass into animals through feeding, excess ions are excreted
Bacteria and fungi break down dead plants and animals releasing ions into water and soil. Phosphate ions in excreta are released by decomposition and transported by lakes to form rocks