Module 5- Nutrient cycle

Question 1

What are saprobionts?

Answer 1

They are a type of bacteria that break down dead organisms or waste material.
They are also involved in extracellular digestion.

Question 2

2 marks

Describe the role of saprobionts in the nitrogen cycle

Answer 2

1. They use enzymes to decompose proteins, DNA, RNA, ATP, urea
2. Producing ammonium ions

Question 3

What are the 4 key stages in the nitrogen cycle?

Answer 3

Nitrogen fixation, ammonification, nitrification and denitrification

Question 4

Explain what nitrogen fixation is

Answer 4

This is when mutualistic nitrogen fixing bacteria breaks down the nitrogen gas from the atmosphere into ammonia. This then gets broken down to form ammonium ions. Some bacteria can be found in root nodules of leguminous plants

Question 5

What is nitrogen fixation

Answer 5

• Atmospheric nitrogen gas is converted into nitrogen-containing compounds
• This biological nitrogen fixation is carried out by nitrogen-fixing bacteria
• The bacteria convert nitrogen into ammonia, which forms ammonium ions that can then be used by plants
• These nitrogen-fixing bacteria are found inside the root nodules of leguminous plants such as peas, beans and clover
• The bacteria have a symbiotic (mutually beneficial) relationship with these plants - the bacteria provide the plants with nitrogen-containing compounds and the plants provide the bacteria with organic compounds such as carbohydrates

Question 6

Explain what ammonification is

Answer 6

This is when saprobionts break down dead organisms or waste material into ammonia. These then get turned into ammonium ions.

Question 7

Explain what nitrification is

Answer 7

This is when nitrification converts ammonium ions in the soil into nitrites. Then nitrifying-bacteria will turn these into nitrates which can be used by the plant

Question 8

Explain what denitrification is?

Answer 8

This is when denitrifying bacteria breaks down nitrates into nitrogen gas. This happens under anaerobic conditions

Question 9

What can inhibit nitrogen fixation

Answer 9

The presence of oxygen inhibits nitrogenase.
Inhibits nitrogen fixation process

Question 10

What is the phosphorus cycle?

Answer 10

This is the recycling of phosphate

Question 11

2 marks

Suggest and explain why a poor supply of phosphate ions results in poor growth of plants

Answer 11

1. Required to make ATP, so less respiration for growth;
2. Required to make nucleotides, so less DNA for cell division for growth
3. Required to make RuBP / NADP, so less CO2 fixed
4. Required to make phospholipids for membranes
   5.Required to make nucleotides, so less RNA for production of protein for growth

Question 12

3 marks

Why are mycorrhizae beneficial for plant growth

Answer 12

1. The fungi increases surface area for water and mineral aborportion
2. Acts like a sponge so holds water and minerals around the roots
3. Makes plants more drought resistant and able to take up more inorganic ions

Question 13

What does assimilated mean?

Answer 13

Absorbed and then used to make more complex molecules

Question 14

The first stage of the phosphate cycle is weathering. What is this?

Answer 14

This is when rocks are broken down which causes the release of phosphate ions into the soil. These are then taken up by planta through the mycorrhizae.

Question 15

ways the UK is actually trying to reduce absolute and relative poverty

What are mycorrhizae

Answer 15

This is a form of symbiotic relationship which is when long thin filaments attach to the roots of plants - known as hyphae which increase the rate of absorption of water and ions

Question 16

What is the second stage of the phosphorus cycle?

Answer 16

The phosphate ions are transferred through the food chain as animals eat the plants and are in turn eaten by other animals.

Question 17

How can phosphate ions get lost through organisms?

Answer 17

They can be lost through excretion or when organisms die, saprobionts will break dowm the organisms causing the release of phosphate ions into the soil

Question 18

How can aquatic producers obtain phosphate ions?

Answer 18

Weathering of rocks releases phosphate ions into the sea, lake and rivers. These phosphate ions can be taken up by aquatic producers and passed along the food chain

Question 19

An example of a natural fertiliser and how this can be used to give phosphate back into soil and rocks.

Answer 19

Bird faeces is known as guano and contains a high proportion of phosphate ions back into the soil which can therefore be used as a natural fertiliser

Question 20

How can harvesting remove nutrients?

Answer 20

When plants and animals are harvested. This prevents them from decomposing in the fields and therefore nitrogen and phosphate cannot be returned back into the soil. Furthermore, animal excretion is removed from the soil.

Question 21

What is an example of a natural fertiliser

Answer 21

Manure and compost

Question 22

What is an example of an artificial fertiliser?

Answer 22

Made out of chemicals containing the correct proportions of ions needed

Question 23

What is leaching?

Answer 23

This is when water-soluble compounds in the soil are washed away into near by waterways

Question 24

Why is leaching less likely to occur in natural fertiliser?

Answer 24

Natural fertiliser has to be decomposed by saprobionts and therefore uptake by plants is more controlled then artifical fertilisers

Question 25

Which ion is more likely to be leached and why?

Answer 25

Nitrates because they are less soluble in water

Question 26

Explain the 7 steps of eutrophication?

Answer 26

* Minerals are leached from fertilised fields which stimulate rapid growth of algae. * Large amounts of algae block the light * The plants die because they cannot receive light for photosynthesis * Increase in saprobionts- break down dead plant materials * Saprobionts feed on dead matter and reduce oxygen conc because they aerobically respire. * Fish and other organisms die because there isnt enough oxygen * Plants unable to photosynthesise so less oxygen produced

Question 27

Why may too much artificial fertiliser cause a reduction in crop yield?

Answer 27

The water-soluble minerals inside the fertiliser will create a lower water potential in the soil around the plant. This will cause water to move out into the soil from the plant, killing it

Question 28

# 1 mark Give one advantage of using natural fertiliser produced in the digester rather than an artifical fertiliser

Answer 28

1. Improves drainage/ increases organic content of soil 2. Contains wider range of elements 3. Production of artificial fertiliser energy-consuming 4. Less leaching

Question 29

# 1 marks 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.

Answer 29

Ammonia

Question 30

# 2 marks Azotobacter is a nitrogen-fixing bacterium. It produces the enzyme nitrogenase. The enzyme only works in the absence of oxygen. Azotobacter has a very high rate of aerobic respiration compared with bacteria that do not fix nitrogen. Suggest two advantages of the very high rate of aerobic respiration.

Answer 30

1. Provides ATP for nitrogen fixation--> formation of ammonia 2. Nitrogenase produced quicker 3. Uses oxygen so nitrogenase works

Question 31

# 3 marks An increase in nitrate concentration in the water entering the lake could affect algae and fish in the lake. Explain how.

Answer 31

1. Algal bloom: increase in algae blocks light 2. Saprobionts break down dead plant materials 3. Saprobionts use up oxygen in respiration causing fish to die

Question 32

# 5 marks Much of Indonesia is covered with forest. Large areas of forest have been cleared and planted with oil-palm trees to be used in the production of fuel. In these forests, nitrogen in dead leaves is made available to growing plants by the action of bacteria. Describe the role of bacteria in making the nitrogen in dead leaves available to growing plants.

Answer 32

1. Saprobionts 2. Digest proteins/ DNA 3. Extracellular digestion (release of enzymes) 4. Ammonia produced 5. Ammonia converted to nitrite to nitrate 6. Niitrification 7. Oxidation

Question 33

# 5 marks Explain how farming practices increase the productivity of agricultural crops.

Answer 33

1. Fertilisers added to soil e.g. manure, bone meal etc. 2. Fertilisers containing nitrate, nitrogen for proteins,phosphate, DNA 3. Selective breeding / genetic modification of crops 4. Ploughing allows nitrification and decreases denitrification 5. Benefit of crop rotation in terms of soil nutrients / fertility / pest reduction

Question 34

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

Answer 34

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

Question 35

# 5 marks Describe how the action of microorganisms in the soil produces a source of nitrates for crop plants. | DETAILED

Answer 35

* Ammonification: saprobionts break down dead organisms into ammonia, then converted to ammonia ions * Produces proteins, RNA, DNA, amino acids, ATP (saprobionts use enzymes to decompose proteins/DNA/RNA/urea) * Nitrification: Converts ammonium ions in the soil into nitrites * Nitrites converted into nitrates * Using nitrifying- bacteria * Nitrogen fixation: Mutualistic nitrogen- fixing bacteria breaks nitrogen gas from the atmosphere into ammonia. This is then broken down to ammonium ions. * Bacteria is found in root nodules of leguminous plants

Question 36

# 2 marks 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.

Answer 36

1. Nitrogen to ammonia 2. Produce protein, amino acids, DNA, RNA

Question 37

# 2 marks Mammals and fish remove nitrogenous waste from their bodies in different forms. Name two polymers present in mammals and fish that contain nitrogen.

Answer 37

Protein, ATP, DNA, RNA

Question 38

# 2 marks Applying very high concentrations of fertiliser to the soil can reduce plant growth. Use your knowledge of water potential to explain why.

Answer 38

1. Soil has a lower water potential 2. Plant takes up less water by osmosis.

Question 39

# 2 marks Azotobacter is a nitrogen-fixing bacterium. It produces the enzyme nitrogenase. The enzyme only works in the absence of oxygen. Azotobacter has a very high rate of aerobic respiration compared with bacteria that do not fix nitrogen. Suggest two advantages of the very high rate of aerobic respiration.

Answer 39

1. Provides ATP for nitrogen fixation--> formation of ammonia 2. Nitrogenase produces quicker 3. Uses O2 so nitrogenase works

Question 40

# 2 marks Denitrification requires anaerobic conditions. Ploughing aerates the soil. Explain how ploughing would affect the fertility of the soil. ___________________________________________________________________

Answer 40

1. Fertility increase as more nitrate formed 2. Less denitrification

Question 41

# 6 marks Explain how decay of dead plants results in reduced oxygen concentraion and increased nitrate production

Answer 41

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 ions 6. via nitrates

Question 42

Much of Indonesia is covered with forest. Large areas of forest have been cleared and planted with oil-palm trees to be used in the production of fuel. CIn these forests, nitrogen in dead leaves is made available to growing plants by the action of bacteria. Describe the role of bacteria in making the nitrogen in dead leaves available to growing plants

Answer 42

Saprbionts Break down proteins Extracellular digestion Ammonia produced Nitrfying bacteria convert anmonia to nitrites to nitrates Oxidation

Question 43

# 2 marks Nitrogenase catalyses the reduction of nitrogen during nitrogen fixation. The reaction requires 16 molecules of ATP for each molecule of nitrogen that is reduced. When ammonia inhibits nitrogenase activity, nitrogen-fixing bacteria may benefit. Explain how

Answer 43

1. Less ATP used 2. ATP can be used for growth, synthesis and replication

Question 44

# 2 marks The tomato seeds were planted into sterilised soil. Suggest two reasons why the soil was sterilised.

Answer 44

1. Kill/remove/no (other) mycorrhizae/fungi 2. Kill/remove/no nitrogen-fixing /nitrifying/denitrifying/saprobiotic bacteria 3. Kill/remove/no pathogens 4. Kill/remove/no pests 5. Kill/remove/no competitors 6. Kill/remove/no seeds/spores 7. Is a controlled variable

Question 45

# 2 marks The soil used in this investigation had the recommended concentration of fertiliser. Suggest two reasons why the soil contained the recommended concentration offertiliser.

Answer 45

1. So mycorrhizae is the only variable 2. Affects crop production/growth/mass 3. Concentration of fertiliser would affect water potential of soil 4. Does not affect pH of soil 5. To provide(usual farming conditions

Question 46

# 2 marks 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.

Answer 46

1. Less nitrification 2. Less conversion of ammonium (ions) to nitrites and to nitrates 3. More denitrification 4. So more nitrates converted to nitrogen gas

Question 47

# 3 marks One environmental issue arising from the use of fertilisers is eutrophication. Eutrophication can cause water to become cloudy. You are given samples of water from three different rivers. **Describe** how you would obtain a quantitative measurement of their cloudiness

Answer 47

1. Use of colorimeter 2. Measure the absorbance/transmission 3. Example of how method can be standardised eg same volume of water, zeroing colorimeter, same wavelength of light, shaking the sample;

Question 48

# 1 mark Suggest one source of phosphate in the lake

Answer 48

Fertilisers / detergents / slurry / manure / sewage / faeces