3.5.4 - NUTRIENT CYCLES

Question 1

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 1

1. less nitrification OR less nitrifying bacteria OR nitrifying bacteria require aerobic conditions
2. less oxidation of ammonium to nitrite to nitrate

Question 2

suggest one way in which an increase in the uptake of phosphate could increase plant growth (1)

Answer 2

used to produce DNA/ATP/phospholipids in cells

Question 3

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 3

1. amino acids
2. carbohydrates
3. vitamins

Question 4

upwelling is a process where water moves from deeper parts of the sea to the surface. this water contains a lot of nutrients from the remains of dead organisms.

describe the role of microorganisms in producing nitrates from the remains of dead organisms (3)

Answer 4

1. saprobiotic microorganisms/bacteria/saprobionts breakdown remains into ammonia/ammonium
2. ammonium ions converted into nitrite then into nitrate
3. by nitrifying bacteria/nitrification

Question 5

upwelling is a process where water moves from deeper parts of the sea to the surface. this water contains a lot of nutrients from the remains of dead organisms.

upwelling often results in high primary production in coastal waters.

explain why some of the mostly productive fishing areas are found in coastal waters (2)

Answer 5

1. nitrate/phosphate absorbed by algae/plants/producers
2. more producers/food so more fish move to area/reproduce

Question 6

nitrate from fertiliser applied to crops may enter ponds and lakes.

explain how nitrate may cause the death of fish in fresh water (5)

Answer 6

1. growth of algae creates algal bloom which blocks light
2. reduced photosynthesis so submerged plants die
3. saprobiotic microorganisms/bacteria
4. aerobically respire
5. so less oxygen for fish to respire

Question 7

nitrates and phosphates provide a source of nitrogen and phosphorus for cells.

give a biological molecule that contains: (2)

1. nitrogen
2. phosphorous

Answer 7

1. amino acid/protein/enzyme/DNA/RNA/ATP/NAD
2. DNA/RNA/ATP

Question 8

describe the role of microorganisms in producing nitrates from the remains of dead organisms (3)

Answer 8

1. saprobiotic bacteria break down remains into ammonia
2. ammonia converted into nitrite then into nitrate
3. by nitrifying bacteria

Question 9

describe the role of saprobionts in the nitrogen cycle (2)

Answer 9

1. they decompose proteins/DNA/RNA/urea
2. producing/releasing ammonia/ammonium ions

Question 10

denitrification requires anaerobic conditions. ploughing aerates the soil. explain how ploughing would affect the fertility of the soil (2)

Answer 10

1. fertility increased as) more nitrate formed / less nitrate removed/broken down
2. less/no denitrification/denitrifying bacteria

Question 11

one farming practice used to maintain high crop yields is crop rotation. this involves growing a different crop each year in the same field.

suggest two ways in which crop rotation may lead to high crop yields. (2)

Answer 11

1. grow crops with nitrogen-fixing (bacteria);
2. different crops use different minerals / salts / nutrients / ions
3. different crops have different pests / pathogens / diseases.

Question 12

leguminous crop plants have nitrogen-fixing bacteria in nodules on their roots. on soils with a low concentration of nitrate ions, leguminous crops often grow better than other types of crop.

explain why. (2)

Answer 12

1. they convert nitrogen to ammonia
2. produce protein/amino acids/DNA/RNA

Question 13

applying very high concentrations of fertiliser to the soil can reduce plant growth.

use your knowledge of water potential to explain why. (2)

Answer 13

1. soil has lower water potential
2. osmosis from plant

Question 14

after harvesting, the remains of crop plants are often ploughed into the soil.

explain how microorganisms in the soil produce a source of nitrates from these remains (5)

Answer 14

1. protein/amino acids/DNA into ammonium compounds
2. by saprobionts
3. ammonium/ammonia into nitrite
4. nitrite into nitrate
5. by nitrifying bacteria

Question 15

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

Answer 15

1. protein/amino acids/DNA into ammonium compounds
2. by saprobionts
3. ammonium/ammonia into nitrite
4. nitrite into nitrate
5. by nitrifying bacteria
6. nitrogen to ammonia

Question 16

name the process by which some bacteria oxidise ammonia to nitrate (1)

Answer 16

nitrification

Question 17

reeds have hollow, air-filled tissue in their stems which supplies oxygen to their roots.

explain how this enables the roots to take up nitrogen-containing substances (2)

Answer 17

1. uptake by roots involves active transport
2. requires ATP/aerobic respiration

Question 18

an increase in nitrate concentration in the water entering the lake could affect algae and fish in the lake.

explain how (3)

Answer 18

1. increase in algae blocks light
2. decomposers/saprobionts break down dead plant material
3. saprobionts use up oxygen in respiration causing fish to die

Question 19

the over-application of fertiliser increases the rate of leaching.

explain the consequences of leaching of fertiliser into ponds and lakes (3)

Answer 19

1. increases algae/algal bloom causes light to be blocked out
2. plants cannot photosynthesise so they die
3. saprobionts breakdown dead organisms, using up oxygen

Question 20

nitrogenase catalyses the reduction of nitrogen during nitrogen fixation. the reaction requires 16 molecules of ATP for each molecule of nitrogen that is reduced.

nitrogen gas is the usual substrate for this enzyme.

name the product (1)

Answer 20

ammonia/ammonium

Question 21

describe the role of bacteria in making the nitrogen in dead leaves available to growing plants (5)

Answer 21

1. saprobionts
2. break down DNA/proteins
3. ammonia produced
4. ammonia converted to nitrite to nitrate
5. nitrifying bacteria
6. oxidation

Question 22

describe how nitrogen-containing substances get into a lake (1)

Answer 22

dissolve/leaching

Question 23

nitrifying bacteria are one kind of bacteria that are important in the nitrogen cycle, nitrogen-fixing bacteria are another kind.

describe the part played by nitrogen-fixing bacteria in the nitrogen cycle (2)

Answer 23

1. convert nitrogen (gas) into ammonia
2. they add usable nitrogen to an ecosystem

Question 24

name the type of bacteria which convert nitrogen in the air into ammonium compounds (1)

Answer 24

nitrogen-fixing bacteria

Question 25

name the type of bacteria which convert nitrites into nitrates (1)

Answer 25

nitrifying bacteria

Question 26

purification ponds can be used in warm climates to break down sewage. the ponds are around 1m deep and contain bacteria and green algae.

explain the advantage of having both algae and bacteria in a purification pond (4)

Answer 26

1. breakdown of organic matter by enzymes from bacteria
2. nitrates/ammonia used by algae to make amino acids/proteins
3. algae photosynthesise
4. bacterial respiration uses O2
5. respiration allows for reproduction of bacteria

Question 27

purification ponds can be used in warm climates to break down sewage. the ponds are around 1m deep and contain bacteria and green algae.

purification ponds only work efficiently when they are shallow and warm.

explain why (4)

Answer 27

1. sufficient light penetration for photosynthesis of algae
2. warm leads to faster enzyme activity
3. faster bacterial respiration
4. faster photosynthesis
5. increased reproduction of bacteria

Question 28

explain how a high nitrate concentration increases the growth of algae (2)

Answer 28

1. more proteins/amino acids/DNA
2. increased cell division/no of cells formed

Question 29

suggest how increased growth of algae could lead to death of the submerged plants (2)

Answer 29

1. reduced light/shading
2. less photosynthesis

Question 30

explain how the decay of dead plants results in reduced oxygen concentration and increased nitrate production (6)

Answer 30

1. saprobionts feed on dead matter
2. respiration uses up oxygen
3. converts proteins to amino acids
4. then to ammonium compounds
5. nitrifying bacteria convert ammonium compounds
6. via nitrates

Question 31

describe how the reduced oxygen concentration of the water will change the composition of the communities in the river (2)

Answer 31

1. lower species diversity/number of species
2. species requiring high oxygen die out