5- Nutrient Cycles =)

Question 1

Describe how the action of microorganisms in the soil produces a source of nitrates for crop plants. (5)

Answer 1

• Saprobionts feed on nitrogen-containing substances + release ammonia;
• ammonia converted into nitrite;
• Nitrite is converted into nitrate;
• by nitrifying bacteria;
• Nitrogen is converted to ammonia;
• by nitrogen-fixing bacteria in soil

Question 2

Pea plants respire aerobically, producing ATP which can be used for amino acid synthesis.
Describe the role of oxygen in aerobic respiration. (2)

Answer 2

• final e- acceptor
• used to form water

Question 3

The bacteria respire aerobically. This produces hydrogen and ATP used in nitrogen fixation. The hydrogen comes from reduced NAD.
Explain how the regeneration of NAD in this way allows ATP production to continue. (2)

Answer 3

• glycolysis can occur
• NAD can accept ↑ H+ ions

Question 4

The scientists concluded that an increase in phosphate in the embryo was linked to growth of the embryo.
Suggest two reasons why an increase in phosphate can be linked to growth of the embryo. (2)

Answer 4

1. make DNA
2. make ATP
3. make RNA
4. make mem
5. for phosphorylation

Question 5

Arbuscular mycorrhiza fungi (AMF) are fungi which grow on and into the roots of plants. AMF can increase the uptake of inorganic ions such as phosphate.
Suggest one way in which AMF may benefit from their association with plants. (1)

Answer 5

Example of a carbon-containing biological
compound e.g. carbohydrate/ amino acid/vitamin

allow sugars

Question 6

Explain why an increase in shoot biomass can be taken as a measurement of net primary productivity. (2)

Answer 6

1. Represents dry mass / mass of carbon
2. Represents gross production minus respiratory
   losses

Question 7

Explain why determination of dry mass was an appropriate method to use in this investigation. (2)

Answer 7

1. Dry mass measures ↑ in organic material
2. Water content varies

Question 8

Freshwater marshes have one of the highest rates of gross primary production (GPP) and net primary production (NPP) of all ecosystems.
Carbon use efficiency (CUE) is the ratio of NPP:GPP. Freshwater marshes have a high CUE.
Use your knowledge of NPP to explain why freshwater marshes have a high CUE and the advantage of this.
Do not refer to abiotic factors in your answer. (2)

Answer 8

• low resp.
• ↑ growth –> ↑ biomass

Question 9

Freshwater marsh soils are normally waterlogged. This creates anaerobic conditions.
Use your knowledge of the nitrogen cycle to suggest why these soils contain relatively high concentrations of ammonium compounds and low concentrations of nitrite ions and nitrate ions. (2)

Answer 9

1. ↓ nitrification (aerobic)
2. ↓ ammonium oxidised to nitrites + nitrates
3. ↑ denitrification
4. ↑ nitrates reduced to N2 gas

Question 10

The scientists used units of μg g–1 for the concentration of ammonia in soil.
Suggest why, in this investigation, the scientists used these units. (2)

Answer 10

μg
- v. little ammonia in soil
- avoids use of decimal places/ standard form
/
g–1
- allows comparison betw. diff. samples

Question 11

Give two examples of biological molecules containing nitrogen that would be removed when a crop is harvested. (2)

Answer 11

1. DNA
2. a.a./ proteins
3. RNA
4. ATP
5. Chlorophyll
6. NAD

Question 12

Nitrogenase catalyses the reduction of nitrogen during nitrogen fixation. The reaction requires 16 molecules of ATP for each molecule of nitrogen that is reduced.
When ammonia inhibits nitrogenase activity, nitrogen-fixing bacteria may benefit.
Explain how. (2)

Answer 12

• ↑ ATP available
• for rep/ growth