Nutrient cycles

Question 1

State a feature of all nutrient cycles

Answer 1

-They contain microorganisms such as bacteria and fungi
-Many of the microorganisms are saprobionts

Question 2

What is a saprobiont?

Answer 2

-A microorganism such as bacteria or fungi that decompose dead plant and animal material
-this returns nutrients to the soil

Question 3

What is meant by the term ‘extracellular digestion’

Answer 3

-saprobionts secrete enzymes
-this digests dead animal and plant material
-they absorb the nutrients they require
-also known as saprobiotic nutrition

Question 4

What are ‘mycorrhizae’?

Answer 4

-fungal associations
-between plant roots and beneficial fungi
-this is a symbiotic relationship

Question 5

What is the role of ‘mycorrhizae’?

Answer 5

-The fungi are composed of long, thin strands known as hyphae
-these connect to the plant roots and increase the SA
-this allows for greater water and mineral absorption

Question 6

Explain how the presence of mycorrhizae results in a symbiotic (mutualistic) relationship

Answer 6

-The store of water and mineral ions in the mycorrhizae can be used by the plant in times of drought
-The fungi obtain organic compounds such as carbohydrates from the plant

Question 7

State the percentage of the atmosphere that is composed of nitrogen

Answer 7

78%

Question 8

State why nitrogen is essential for plants and animals

Answer 8

-For growth
As it is needed to create:
-Amino acids/ proteins
-Nucleic acids (DNA and RNA)
-ATP

Question 9

Explain why plants are unable to absorb atmospheric nitrogen

Answer 9

-Atmospheric nitrogen is unreactive
-Require bacteria to convert it into nitrogen-containing compounds

Question 10

State the four processes involved in the nitrogen cycle

Answer 10

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

Question 11

Describe the process of nitrogen fixation

Answer 11

-Nitrogen-fixing bacteria such as Rhizobium break the triple bond between two nitrogen atoms in atmospheric nitrogen
-They convert this nitrogen into ammonium ions

Question 12

State an example of nitrogen-fixing bacteria

Answer 12

Rhizobium

Question 13

Where are nitrogen-fixing bacteria found?

Answer 13

-Free living in the soil
-In the root nodules of leguminous plants e.g. clover (symbiotic relationship)

Question 14

Describe the process of ammonification

Answer 14

-Saprobionts decompose proteins, urea and DNA
-from animal waste into ammonium ions

Question 15

Describe the process of nitrification

Answer 15

-Nitrifying bacteria convert ammonium ions to nitrate and nitrite ions
-A two-stage oxidation reaction

Question 16

Describe the process of denitrification

Answer 16

-Anaerobic denitrifying bacteria converts nitrates in the soil back to atmospheric nitrogen gas
-Anaerobic denitrifying bacteria use nitrates from the soil to respire and they produce atmospheric nitrogen

Question 17

Why is phosphorus essential for plants an animals?

Answer 17

To create:
-DNA
-RNA
-ATP
-Phospholipid bilayers

Question 18

Where is phosphorus found?

Answer 18

-In rocks
-Dissolved in oceans
-In the form of phosphate ions (PO4^3-)

Question 19

Phosphate ions that is dissolved in water and in soil is assimilated by plants. What does the term ‘assimilated’ mean?

Answer 19

-Plants absorb the phosphate
-use the phosphorus to make more complex biological molecules

Question 20

Describe the phosphorus cycle

Answer 20

1. Weathering of rocks releases phosphorus ions into the soil
2. Plants absorb phosphate via the roots and mycorrhizae increase the rate of assimilation of phosphorus
3. Phosphate ions are transferred through the food chain as animals consume plants
4. Phosphate ions are lost by animals through waste
5. Saprobionts decompose dead animal material and this releases phosphate ions into the soil and these are taken up by plants by assimilation
6. Weathering of rocks releases phosphate ions into lakes, rivers and seas and this is taken up by aquatic producers such as algae and this is passed along the food chain by consumers

Question 21

State the role of magnesium in plants

Answer 21

Needed to make chlorophyll

Question 22

State the two types of fertilisers

Answer 22

-Organic (natural)
-Inorganic (artificial)

Question 23

Explain why fertilisers are added to the soil

Answer 23

-Replace phosphate and nitrate ions
-lost when plants are harvested and removed from nutrient cycles
-to be used as crops

Question 24

What are organic fertilisers?

Answer 24

-Dead and decaying plant and animal matter
-includes manure, composted vegetables and crop residues

Question 25

What are inorganic fertilisers?

Answer 25

-Mined from rocks -Converted to a suitable form -Blended to suit a particular crop -Creates pollution to mine from rock and is artificial so people may disagree / not want it on crops

Question 26

Compare organic and inorganic fertilisers

Answer 26

Organic + Often cheaper (free if the farmer owns animals) - Exact mineral proportions cannot be controlled Inorganic + Exact proportions of minerals can be measured + More water-soluble, so more ions dissolve in water surrounding the soil and therefore more nitrates and phosphates are absorbed by plants - As they are more water soluble, larger quantities are washed away by rainfall, which creates environmental impacts

Question 27

What is meant by the term 'leaching'?

Answer 27

When water-soluble compounds found in the soil are washed away by rain into nearby ponds or rivers

Question 28

Describe the process of eutrophication

Answer 28

1. Mineral ions leached from fertilised soils stimulate the growth of algae on ponds or rivers 2. Excessive growth of algae on the surface blocks out light energy from reaching aquatic plants below the surface 3. The plants are unable to photosynthesise and they decompose 4. Bacteria feed on decaying plant matter and the increased number of bacteria reduce the oxygen concentration by aerobic respiration 5. Fish and aquatic organisms die due to a lack of dissolved oxygen

Question 29

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. State two reasons why.

Answer 29

-Nitrogen-fixing bacteria can fix atmospheric nitrogen into ammonium (NH4+) ions -used to make proteins and amino acids

Question 30

Nitrogenase catalyses the reduction of nitrogen during nitrogen fixation. Nitrogen gas is the usual substrate for this enzyme. Name the product.

Answer 30

Ammonium ions (NH4+)

Question 31

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

Answer 31

-Surface runoff when there is excess rainfall - Leaching

Question 32

Name the type of bacteria which convert: a) nitrogen in the air into ammonium compounds b) nitrites into nitrates

Answer 32

a) Nitrogen-fixing bacteria b) Nitrifying bacteria

Question 33

State two reasons why it is important for a farmer to reduce the leaching of nitrates

Answer 33

-Can result in eutrophication causing death of aquatic plants and animals -Leaching into water sources can lead to health concerns with drinking water -Can result in nitrate deficiencies in the soil which will affect crop yields -Costs money to add more fertilisers which have been leached

Question 34

What conditions are anaerobic denitrifying bacteria present in great numbers?

Answer 34

When soils become waterlogged

Question 35

State why farmers use good drainage systems

Answer 35

-Anaerobic denitrifying bacteria are present in great numbers in waterlogged soils -without drainage systems more nitrates in the soil would be converted back to atmospheric nitrogen -less would be available for crops -yield would decrease

Question 36

Explain why farmers plough the soil

Answer 36

-Increases O2 concentration in the soil -Less anaerobic denitrifying bacteria present in the soil (less anaerobic respiration) -Less denitrification -Less nitrates converted back to atmospheric nitrogen -More nitrates available in soil so more plant growth

Question 37

State two ways in which farmers increase the efficiency of energy transfers

Answer 37

-Shorten the food chain -> less energy lost between tropic levels -Crop rotation -> replenished concentration of nutrients in the soil