3.5.4 Nutrient Cycles

Question 1

How are nitrogen
compounds in animals made
available for plants? (8)

Answer 1

1. Proteins/amino acids broken down;
2. by saprophytes/decomposers;
3. deamination/ammonium compounds/ammonia formed;
4. Ammonia converted to nitrite;
5. by nitrifying bacteria;
6. Nitrite converted to nitrate;
7. Nitrate can be absorbed by roots;
8. Nitrogen fixation forms ammonium compounds;

Question 2

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

Answer 2

1. Protein / amino acids / DNA into ammonium compounds / ammonia;
2. By saprobionts;
3. Ammonium / ammonia into nitrite;
4. Nitrite into nitrate;
5. By nitrifying bacteria / microorganisms;
6. Nitrogen to ammonia / ammonium;
7. By nitrogen-fixing bacteria / microorganisms in soil;

Question 3

Describe the phosphorus cycle. (10)

Answer 3

1. Phosphate ions in rocks released to soil by erosion/weathering;
2. Phosphate ions taken into plants by roots/root hair cells;
3. Phosphate ions assimilated into DNA/RNA/phospholipids/NADP/RuBP;
4. Rate of absorption increased by mycorrhizae;
5. Phosphate ions transferred through food chain;
6. Some phosphate ions lost by excretion/when plants and animals die;
7. Phosphorus containing compounds are decomposed by saprobionts/fungi;
8. Weathering of rocks releases phosphate ions into seas and taken up by aquatic producers/algae;
9. Phosphate ions passed along food chain to birds;
10. Guano returns phosphate ions to soils

Question 4

Describe the importance of
phosphorylation
(2)

Answer 4

1. Makes substrates more reactive;
2. Lowers activation energy for the reaction

Question 5

Describe
eutrophication (6)

Answer 5

1. Growth of algae / algal bloom blocks light;
2. Reduced / no photosynthesis so (submerged) plants die;
3. Saprobiotic (microorganisms / bacteria);
4. Aerobically respire / use oxygen in respiration;
5. Less oxygen for fish / aerobic organisms to respire;
6. (So) they die;

Question 6

Describe the function of saprobionts
releasing carbon dioxide (6)

Answer 6

1. Microorganisms are saprobionts/saprophytes;
2. Secrete enzymes (onto dead tissue) / extracellular digestion;
3. Absorb products of digestion/smaller molecules/named relevant substance;
4. Synthesis of structural compounds/named compound;.
5. Respiration provides energy for growth
6. Respiration (by microorganisms) produces carbon dioxide;

Question 7

Evaluate use of fertilisers for
mycorrhizas (4)

Answer 7

(Should not use fertilisers because)
1. Fertilisers prevent development of mycorrhizae;
2. Mycorrhizae help plants to take up nitrates/phosphates (causing an increase in crop yield);
(Should use fertilisers because)
3. Fertilisers containing phosphate and nitrate increase gross primary production so increase yield;
4. Most soil is poor in phosphate so without fertiliser so plant might not get enough phosphate;