3.5.4 Nutrient Cycling (Unit 5 Energy Transfers)

Question 1

What is a saprobiont?

Answer 1

They feed on the remains of dead plants and animals. They secrete enzymes and digest food externally, then absorb the nutrients they need.

Question 2

What is a symbiotic relationship

Answer 2

A relationship where two organisms of different species interact.

Question 3

What do plants and animals use nitrogen for?

Answer 3

Making proteins and nucleic acids.

Question 4

What is nitrogen fixation?

Answer 4

When nitrogen gas in the atmosphere is converted into ammonia/ammonium ions

Question 5

What are Rhizobium?

Answer 5

Bacteria found in root nodules of leguminous plants. They turn nitrogen into ammonia.

Question 6

What is ammonification?

Answer 6

When nitrogen containing compounds from decaying organisms/waste are turned into ammonia. This ammonia then forms ammonium ions.

Question 7

Define nitrification

Answer 7

Ammonium ions in the soil are oxidised into nitrites and then nitrates

Question 8

Name the type of bacteria that converts nitrites into nitrates.

Answer 8

Nitrifying bacteria

Question 9

Name a type of bacteria that converts ammonium ions into nitrites.

Answer 9

Saprobionts

Question 10

What term is used to describe the conversion of nitrates in the soil into nitrogen gas.

Answer 10

Denitrification

Question 11

Why does denitrification occur in waterlogged soils?

Answer 11

Because denitrifying bacteria need anaerobic conditions to convert nitrates into nitrogen gas.

Question 12

How does nitrogen get into an ecosystem?

Answer 12

Nitrogen fixing, fertiliser, waste material decomposition and lightning

Question 13

Name 4 things plants and animals need phosphorus for?

Answer 13

1. phospholipids
2. DNA
3. RNA
4. ATP

Question 14

How does phosphate get into the soil?

Answer 14

Phosphate ions in rocks are released into the soil by weathering.

Question 15

When fertilisers are applied to fields next to a lake, nitrogen-containing substances from the fertilisers get into the lake.

(a) (i) Describe how the nitrogen-containing substances get into the lake.

.

Answer 15

dissolve (in soil water) / run-off / leaching

Question 16

Describe how the presence of nitrates in a lake may eventually lead to the death of fish

Answer 16

Excess nitrates runoff into bodies of water
Excess growth of algae/Algal bloom forms on the surface of water
Reduced light so aquatic plants die less photosynthesis
Saprobionts respire aerobically while decomposing dead matter
Less oxygen for fish and other organisms to respire so they die

Question 17

It is estimated that, each year, a total of 3 × 109 tonnes of ammonia are converted to nitrate. Only 2 × 108 tonnes of ammonia are produced from nitrogen gas. Explain the difference in these figures.

Answer 17

1. Ammonia formed by decay of nitrogenous waste by decomposition.
2. by action of decomposers / saprobionts;

Question 18

Describe the role of microorganisms in producing nitrates from the remains of dead organisms.

Answer 18

1. Saprobiotic (microorganisms / bacteria) break down remains into ammonia / ammonium;
2. Ammonia / ammonium ions into nitrite and then into nitrate;
3. (By) Nitrifying bacteria / nitrification;

Question 19

Describe the phosphorous cycle

Answer 19

Plants absorb phosphorous form the soil
Consumers eat the plants and absorb phosphorous
Dead and waste (faeces etc.) material is decomposed releasing phosphorous into the soil
Runoff from farm fertiliser means excess phosphorous enters bodies of water (lakes/rivers etc.)
Phosphorous sediment in water is uplifted forming rocks on the surface
Weathering releases phosphorous from the rock into the water and soil
Leeching of phosphorous from soil/weathered rock causes phosphorous to enter the water

Question 20

Describe how the nitrogen cycle has been optimised

Answer 20

Using crop rotation to plant leguminous plants – these will replenish nitrates in the soil
Using crop rotation to replenish soil nutrients
Ploughing aerates soil to ensure more oxidised ammonia  nitrates in nitrification
Preventing waterlogging reduces anaerobic conditions so less denitrification occurs
Selective breeding can be used to optimise growing conditions
Fertilisers can be added to increase concentration of minerals e.g. nitrates

Question 21

Describe the nitrogen cycle in full detail

Answer 21

Nitrogen gas in the air is converted into ammonia in the soil by nitrogen fixing bacteria
Some nitrogen fixing bacteria in leguminous plant root nodules have a mutualistic relationship with plants and convert nitrogen gas to ammonia then nitrates directly

Nitrates in the soil are absorbed by plant roots and converted to nitrogen containing compounds e.g. amino acids and DNA
Nitrogen containing compounds in plants may be absorbed when eaten by consumers

Proteins from waste and dead material are broken down/hydrolysed to ammonia in soil by enzymes released by saprobionts during ammonification

Ammonia in the soil is oxidised to nitrites, then nitrates, by nitrifying bacteria in the soil in aerobic conditions. These nitrates can be absorbed by the plants

If the soil is waterlogged, the lack of oxygen leads to denitrification where nitrates are converted back to gaseous nitrogen by denitrifying bacteria