5.4 Nutrient Cycles

Question 1

What is decomposition?

Answer 1

The chemical process of hydrolysis of complex biological molecules to smaller inorganic molecules or ions

Question 2

Explain the significance of nitrogen to living organisms

Answer 2

Plant roots uptake nitrates via active transport and use them to make biological compounds, e.g. amino acids, NADP, nucleic acids

Question 3

What are saprobionts?

Answer 3

Organisms that secrete digestive enzymes onto the dead remains of other organisms, hydrolyse the biological molecules in these dead remains and then absorb some of the products of this digestion, the rest remains in the surroundings and can be absorbed by other organisms

Question 4

What happens to nutrients once they are used?

Answer 4

Nutrients are recycled within natural ecosystems

Question 5

What are the four stages of the nitrogen cycle?

Answer 5

1. Ammonification
2. Nitrification
3. Denitrification
4. Nitrogen fixation

Question 6

Why can’t organisms use nitrogen directly from the atmosphere?

Answer 6

N2 is very stable due to the strong covalent triple bond

Question 7

Describe ammonification

Answer 7

Ammonification is where microbes known as saprobionts break down organic matter into ammonia in a two stage process:
1. proteins are broken down into amino acids with the use of extracellular protease enzymes
2. these are then broken down further to remove amino groups with use of deaminase enzymes

Question 8

Outline the role of bacteria in ammonification

Answer 8

1. Saprobionts feed on and decompose organic waste containing nitrogen
2. NH3 released
3. NH3 dissolves in water in soil to form NH4+

Question 9

How do saprobionts use the products of decomposition?

Answer 9

Respiration

Question 10

Describe nitrification

Answer 10

Nitrification is where nitrifying bacteria convert ammonia to nitrate ions, NO2-, in an oxidation reaction. Nitrites are then oxidised by nitrfying bacteria into nitrates, NO3-

Question 11

How do plants take up ammonia?

Answer 11

Most plants take up nitrate ions, produdced in nitrification, through their roots

Question 12

Describe denitrification

Answer 12

Denitrification is where nitrates, NO3-, are converted to nitrogen gas, N2, by the denitryfing bacteria.

Question 13

How can denitrification be prevented?

Answer 13

Denitrification is wasteful and can be prevented from occurring by soil being well drained and aerated

Question 14

How is ammonia made through nitrogen fixation?

Answer 14

1. High energy of lightning breaks N2 into N
2. N reacts with oxygen to form NO2-
3. NO2- dissolves in water to form NO3-

Question 15

Describe nitrogen fixation

Answer 15

Nitrogen fixation is where nitrogen gas is fixed into other compounds by bacteria with nitrogen fixing ability. They do so by reducing nitrogen gas to ammonia which then dissolves into ammonium ions

Question 16

Where do nitrogen fixing bacteria live?

Answer 16

Nitrogen fixing bacteria live in root nodules of leguminous plants

Question 17

Which process describes the conversion of nitrogen containing compounds in plants to united

Answer 17

Decomposition

Question 18

How is phosphate released?

Answer 18

Phosphate is released from sedimentary rocks as a result of weathering, as well as through the decay of bones, shells and the excreta of some birds

Question 19

Name the general stages in the phosphorous cycle

Answer 19

1. Weathering
2. runoff
3. assimilation
4. decomposition
5. uplift

Question 20

Why is the phosphorous cycle a slow process?

Answer 20

Most phosphorous is store as PO4,3- in rocks, there is also no gas phase - so there is no atmospheric cycle

Question 21

What happens during weathering and runoff?

Answer 21

Phosphate compounds from sedimentary rocks leach into surface water and soil

Question 22

Explain the significance of phosphorous to living organisms

Answer 22

Plants convert inorganic phosphate into biological molecules, e.g. DNA, ATP..
phosphorous is passed to consumers via feeding

Question 23

What happens during uplift?

Answer 23

Sedimentary layers from the oceans (formed by the bodies of aquatic organisms) are brought up to land over many years

Question 24

How does mining affect the phosphorous cycle?

Answer 24

Speeds up uplift

Question 25

Why are mycorrhizae important?

Answer 25

They are important in facilitating the uptake of water and inorganic ions by plants

Question 26

Outline the role of mycorrhizae

Answer 26

Mutualistic relationship between plant and fungus increases the surface area of root system = increases uptake of water and mineral ions

Question 27

What are the advantages of mycorrhizae?

Answer 27

1. increase the surface area for nutrient absorption as well as water
2. increased nutrient absorption of plant, therefore faster growth

Question 28

Why are natural and artificial fertilisers used?

Answer 28

Natural and artificial fertilisers are used to replace the nitrates and phosphates lost by harvesting plants and removing livestock

Question 29

What is another name for fertilisers and state the purpose of using it

Answer 29

Natural - organic : decaying organic matter and animal waste
Artificial - Inorganic : minerals from rocks, usually containing NPK
Fertiliser increases the gross productivity for a higher yield

Question 30

At a certain point, using more fertiliser no longer increases crop yield. Why?

Answer 30

A factor unrelated to the concentration of mineral ions limits the rate of photosynthesis, so rate of growth cannot increase any further

Question 31

What does NPK fertiliser stand for?

Answer 31

NPK
N - nitrate
P - phosphorous
K - potassium

Question 32

How do fertilisers increase productivity?

Answer 32

Nitrates are needed for:
1. DNA and proteins, both for plant growth
2. Likely that plants develop earlier and grow taller
3. Increased rate of photosynthesis
4. Increases crop productivity

Question 33

What are the main consequences of fertiliser use?

Answer 33

1. Reduces species diversity
2. leaching
3. eutrophication

Question 34

How does fertiliser reduce species diversity?

Answer 34

1. NPK fertilisers favour the growth of grasses, nettles and rapidly growing species , which then take away the mineral ions and sunlight
2. Meadows (species rich) often have low levels of nitrogen in the soil

Question 35

Describe leaching

Answer 35

The process by which mineral ions, dissolving in rainwater and are carried from the soil to end up in rivers and lakes

Question 36

Describe eutrophication

Answer 36

1. An increase in nitrate ions causes algae to grow on the water surface
2. The algae then reproduces, causing a thicker layer called an algal bloom
3. The dense surface layer absorbs light and prevents it from penetrating lower
4. The lack of light then stops plants from photosynthesising, causing them to eventually die
5. This then causes an increase of bacteria using the dead organisms as food
6. The bacteria then reproduce, causing a lack of oxygen as they need to respire
7. The lack of oxygen then causes anaerobic organisms, such as fish to die

Question 37

How can risk of eutrophication be reduced?

Answer 37

1. Sewage treatment marshes on farms
2. pumping nutrient-enriched sediment out of water
3. Using phosphate-free detergent