nutrient cycles

Question 1

why do organisms need nitrogen?

Answer 1

• To make amino acids and proteins
• For use in nitrogenous bases to make DNA and RNA nucleotides
• To make ATP and ADP
• (Also used to make chlorophyll and NADP for photosynthesis and NAD in respiration

Question 2

describe the nitrogen cycle

Answer 2

1. Organisms cannot use atmospheric nitrogen- plants absorb nitrogen as Nitrate ions from the soil by ACTIVE TRANSPORT through the root hair cells, using ATP and a specific carrier protein.
2. The plants use the nitrates to make biological molecules such as amino acids, proteins and DNA. (Always give an example)
3. Consumers then eat the plants and hydrolyse proteins to amino acids. These are assimilated into new tissue in the consumer. Plants and animals die + animals also excrete nitrogenous waste such as urine.
4. AMMONIFICATION- saprobiotic bacteria secrete extracellular enzymes which hydrolyse proteins and other nitrogen containing compounds in the dead organisms and release ammonium ions (NH4+) into the soil.
5. NITRIFICATION- NH4+ ions are oxidised to nitrites (NO2-) then nitrates (NO3-) by nitrifying bacteria, so plants can take up nitrites by active transport. Needs aerobic respiration hence why farmers plough fields to get O2 in.
6. DENITRIFICATION- Nitrates can be reduced to nitrogen gas (N2) by denitrifying bacteria. This occurs anaerobically e.g. water-logged soils so plough reduces amount of denitrifying bacteria.
7. NITROGEN FIXATION- Nitrogen fixing bacteria found in the soil and in the root nodules of some leguminous plants can reduce nitrogen gas to ammonium ions

Question 3

why do organisms need phosphorus?

Answer 3

• To make in ATP and ADP
• To make phospholipids to make new cell membranes
• For use in the sugar phosphate backbone to make DNA and RNA nucleotides
• (Also used to make NADP for photosynthesis and NAD in respiration)

Question 4

describe phosphorus cycle

Answer 4

1. Weathering and erosion of sedimentary rock deposits cause phosphate ions (PO4
   3-) to dissolve into soil and oceans/lakes.
2. The phosphate ions are then be absorbed by plants and incorporated into biological molecules
   such as DNA, phospholipids and ATP. Consumers then eat the plants and hydrolyse biological molecules e.g. phospholipids via digestion.
   The phosphate ions are assimilated into new tissue in the consumer e.g. DNA nucleotides.
3. Saprobiotic bacteria and fungi secrete extracellular enzymes which hydrolyse the
   phosphorus-containing molecules e.g. DNA!! in the dead organisms and faeces, releasing phosphate ions back into the soil or water.
4. Excess phosphate ions are excreted in waste products such as urine and guano in birds, returning them directly to the soil. The phosphate ions can form sedimentary rocks through deposition, completing the cycle.

Question 5

what is the role of mycorrhizae in nutrient cycles?

Answer 5

Mycorrhizae are relationships between fungi and plant roots.
The hyphae of the fungi increase the surface area for the absorption of water and nutrient (e.g. nitrates
and phosphates).
This is a mutualistic relationship – both the plants and the fungi benefit.
The plant benefits from improved water and inorganic ion uptake and the fungi receives organic
compounds made in photosynthesis such as glucose from the plant.

Question 6

why are fertilisers needed?

Answer 6

In farming, when crops and livestock are grown, they are not left in the environment to die or egest/excrete, but are removed. Therefore the nitrates and phosphates which are taken up by
the plants or eaten by the animals are used to form the organic molecules in their tissues and are not returned to the environment .
So fertilisers (natural or artificial) replace the nitrates and phosphates lost by harvesting plants
and removing livestock.

Question 7

what are the 2 types of fertiliser?

Answer 7

1) natural e.g. manure
2) artificial e.g. manufactured powders + pellets containing nitrates, phosphates + potassium

Question 8

give advantages + disadvantages of natural fertilisers

Answer 8

adv:
1) more macronutrients + greater nutrient range
2) improves soil quality
3) cheap + readily available
dis:
1) releases nitrates slowly bc of all the processes
2) exact nitrate + phosphate conc unknown

Question 9

give advantages + disadvantages of artifical fertilisers

Answer 9

adv:
1) nitrate + phosphate conc known
2) nutrients provided immediately
3) nutrients spread evenly
4) more compact to transport
dis:
1) conc needed in smaller amounts

Question 10

list 3 environmental issues with fertilisers

Answer 10

1) reduces species diversity- plants adapted to high-nitrate soils outcompete other species
2) leaching- excess ions from inorganic fertilisers are washed from the soil into streams, rivers and lakes by the rain
3) eutrophication- the leaching of nitrates and phosphates can cause a rapid growth of algae

Question 11

describe eutrophication

Answer 11

1. leaching is nitrates + phosphates from fertilisers entering streams and rivers, increasing growth of algae on river surface- an algal bloom.
2. the algal bloom blocks the light available to aquatic plants, so they stop photosynthesising + die.
3. Saprobiotic bacteria decompose the dead plant matter, using oxygen for their aerobic respiration
   and reproduce rapidly, increasing in number, reducing oxygen available for other organisms e.g. fish in the river. They can’t aerobically respire + die.