15: Nutrient Cycles

Question 1

Which molecules contain Nitrogen?
Why does the Nitrogen Cycle need to occur?

Answer 1

• DNA/RNA/ATP/ADP/amino acids
• although nitrogen is abundant, Nitrogen in its natural, gaseous and inorganic state is inert/nonreactive so cannot be used by organisms, therefore nitrogen is cycled between abiotic/biotic phases

Question 2

Why could a lack of Nitrogen lead to stunted growth?

Answer 2

• organisms are unable to synthesise proteins needed for growth (nitrogenous bases)

Question 3

What are the 4 stages of the Nitrogen Cycle?

Answer 3

1. Nitrogen Fixation
2. Nitrification
3. Denitrification
4. Decomposition/Ammonification

Question 4

Describe Nitrogen Fixation:

Answer 4

1. nitrogen-fixing bacteria covert Nitrogen gas into ammonia/ammonium ions (NH3, NH4+)
2. these bacteria live in swellings on roots called legumes
3. these have a mutualistic relationship, in which the plants receive ammonia (for making proteins + other nitrogenous bases) and the nitrogen-fixing bacteria receive sugars + water
4. some ammonia produced by bacteria ends up in soil
5. nitrogen-fixing bacteria free-lying in the soil could which convert NH3 into NH4+ by dissolving in water

Question 5

Describe Nitrification:

Answer 5

• nitrifying bacteria converts NH3/NH4+ (made from nitrogen-fixing bacteria) into nitrates in a 2 step process:
  1. oxidation of NH4+ -> NO2- (nitrite ions)
  2. oxidation of NO2- -> NO3- (nitrate ions)
• then plants can take up these nitrates via active transport, which returns nitrogen to the biotic phase

Question 6

Describe Denitrification:

Answer 6

• loss of nitrates from the soil (near plants) to the atmosphere (Nitrogen gas), due to anaerobic conditions
  e.g waterlogged soil

Question 7

Describe Decomposition/Ammonification:

Answer 7

• when organisms die, excrete urea or egest faeces saprobiotic microorganisms convert organic nitrogen compounds back into NH3/NH4+
• this returns nitrogen to the abiotic phase
• these saprobionts are digested eternally via extracellular digestion
• the products of this digestion are absorbed via saprobionts, but some ions/molecules are released back into the environment

Question 8

Describe The Phosphorous Cycle:

Answer 8

• saprobionts digest dead animals/plant/algae/waste material via extracellular digestion
• saprobionts absorb products of digestion, and release phosphorous ions into soil/lakes/rivers/oceans where they dissolve
  1.
  
  • some of the dissolved phosphorous ions (in roots) is absorbed by plants + algae, to form growth biological molecules such as phospholipids, ATP, nucleic acids
  • phosphorous ions are then passed into animals that are herbivores, so phosphate-containing compounds are digested then absorbed

2. • some of the dissolved phosphorous ions in oceans can form rocks
   • these phosphates in sedimentary rocks are only returned to the cycle if erosion occurs

Question 9

What is Myocorrhizae?

Answer 9

• fungi that grows in association with plant roots, improving uptake of water + inorganic ions in a mutualistic relationship

Question 10

How does Myocorrihizae work in a mutualistic relationship?

Answer 10

• the fungus grows in and around the roots, producing a large system of threads/hyphae
• the plant provides the fungus with sugars, and the fungus large S.A hyphae provides benefits, such as the greater ability to absorb water + minerals, due to larger S.A

Question 11

Why are Fertilisers needed?

Answer 11

• nitrates and phosphates are 2 important mineral ions that are often limiting factors in plant and algae growth
• increased food production means that nitrates + phosphates are continually being lost in soil, due to crop harvesting
• feeding livestock, which are then removed, decreasing nitrates + phosphates
• plants do not decompose, so minerals aren’t recycled

Question 12

What are the 3 environmental consequences of using artificial fertilisers?

Answer 12

• leaching
• eutrophication
• reduced species diversity

Question 13

Describe Leaching:

Answer 13

• excess ions e.g nitrates + phosphates from fertiliser use, can be washed out of the soil by rain
• leached ions then find their way into streams/rivers/lakes
• this leads to eutrophication

Question 14

Describe Eutrophication:

Answer 14

• nitrates are generally a limiting factor of plant growth due to low quantities in rivers/lakes

1. an algal bloom occurs (due to increases nitrates)
2. a dense layer of algae forms on the surface of water, absorbing light and preventing it from going to lower depths, so plants below the surface cannot photosynthesise, leading to death
3. increased intraspecific competition between algae results in death of algae too
4. when algae and plants die, they are decomposed by bacteria and other saprobionts, whose population would suddenly increase
5. the increase in aerobic respiration by bacteria uses up oxygen
6. the dissolved oxygen in water decreases, causing many species of invertebrae and fish to die