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

The scientists determined the dry mass of the roots and shoots separately. The reason for this was they were interested in the ratio of shoot to root growth of pea plants. It is the shoot of the pea plant that is harvested for commercial purposes. Explain why determination of dry mass was an appropriate method to use in this investigation.

Answer 2

1. Dry mass measures increase in organic material
2. Water content varies

Question 3

2 marks

Describe the role of saprobionts in the nitrogen cycle

Answer 3

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

Question 4

What are the 4 key stages in the nitrogen cycle?

Answer 4

Nitrogen fixation, ammonification, nitrification and denitrification

Question 5

Explain what nitrogen fixation is

Answer 5

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 6

What is nitrogen fixation

Answer 6

• 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 7

Explain what ammonification is

Answer 7

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

Question 8

Explain what nitrification is

Answer 8

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 9

Explain what denitrification is?

Answer 9

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

Question 10

What can inhibit nitrogen fixation

Answer 10

The presence of oxygen inhibits nitrogenase.
Inhibits nitrogen fixation process

Question 11

What is the phosphorus cycle?

Answer 11

This is the recycling of phosphate

Question 12

2 marks

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

Answer 12

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 13

3 marks

Why are mycorrhizae beneficial for plant growth

Answer 13

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 14

What does assimilated mean?

Answer 14

Absorbed and then used to make more complex molecules

Question 15

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

Answer 15

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 16

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

What are mycorrhizae

Answer 16

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 17

What is the second stage of the phosphorus cycle?

Answer 17

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

Question 18

How can phosphate ions get lost through organisms?

Answer 18

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 19

How can aquatic producers obtain phosphate ions?

Answer 19

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 20

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

Answer 20

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 21

How can harvesting remove nutrients?

Answer 21

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 22

What is an example of a natural fertiliser

Answer 22

Manure and compost

Question 23

What is an example of an artificial fertiliser?

Answer 23

Made out of chemicals containing the correct proportions of ions needed

Question 24

What is leaching?

Answer 24

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

Question 25

Why is leaching less likely to occur in natural fertiliser?

Answer 25

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

Question 26

Which ion is more likely to be leached and why?

Answer 26

Nitrates because they are less soluble in water

Question 27

Explain the 7 steps of eutrophication?

Answer 27

* 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 28

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

Answer 28

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 29

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

Answer 29

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 30

# 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 30

Ammonia

Question 31

# 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 31

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

Question 32

# 2 marks If scientists could transfer the gene that codes for nitrogenase to cereal plants, these cereal plants would be able to fix nitrogen. However, the scientists would expect these genetically engineered cereal plants to grow more slowly than cereal plants that get their nitrogen from fertiliser. **Explain** why they would grow more slowly.

Answer 32

1. ATP needed for nitrogen fixation 1. so less ATP available for growth

Question 33

Why would genetically modified cereal plants that fix nitrogen grow more slowly than plants given nitrogen fertilisers?

Answer 33

1. Because nitrogen fixation (N₂ → NH₃) using the enzyme nitrogenase requires a lot of ATP. 1. This means less ATP is available for growth processes like cell division and protein synthesis. 1. In contrast, fertiliser-grown plants get ready-made nitrogen (nitrates), so they don’t use ATP to fix nitrogen, and can grow faster.

Question 34

Why does nitrogen fixation (conversion of N₂ to NH₃) require ATP?

Answer 34

Because nitrogen gas (N₂) has a triple bond, which is very strong and stable. Breaking this bond to convert N₂ into ammonia (NH₃) needs a lot of energy. Nitrogenase uses ATP to power this reduction reaction. Around 16 ATP molecules are needed for each N₂ molecule fixed.

Question 35

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

Answer 35

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 36

# 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 36

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 37

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

Answer 37

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 38

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

Answer 38

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 39

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

Answer 39

* Ammonification: saprobionts break down dead organisms into ammonia, then converted to ammonia ions * Saprobionts use enzymes to decompose proteins/DNA/RNA/urea into ammonia * 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 40

# 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 40

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

Question 41

# 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 41

Protein, ATP, DNA, RNA

Question 42

# 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 42

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

Question 43

# 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 43

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

Question 44

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

Answer 44

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

Question 45

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

Answer 45

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 46

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 46

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

Question 47

# 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 47

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

Question 48

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

Answer 48

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 49

# 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 49

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 50

# 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 50

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 51

# 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 51

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 52

# 1 mark Suggest one source of phosphate in the lake

Answer 52

Fertilisers / detergents / slurry / manure / sewage / faeces

Question 53

# 2 marks Suggest how the relationship between alga and the fungus allows the lichen to survive on an inorganic surface

Answer 53

1. Algae photosynthesise- produce organic molecules such as carbohydrates, glucose etc. 2. Fungus absorbs water/ mineral ions such as nitrates, phosphate ions which is available to the alga- prevents dehydration of alga

Question 54

# 6 marks Describe how phosphorus is recycled in an ecosystem

Answer 54

1. Phosphate ions in rocks are released by weathering. 1. These phosphate ions are absorbed by plants through their roots. 1. Phosphate ions move through the food chain as animals eat plants and are then eaten by other animals. 1. When animals and plants die, saprobionts break down their organic matter, releasing phosphate ions into the soil. 1. Excretion and egestion by animals also add phosphate ions to the soil or water. 1. In aquatic systems, phosphate ions can settle and form sedimentary rocks, completing the cycle over long periods.

Question 55

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

Answer 55

1. Fertilisers added to soil 2. Contain nitrogen for proteins/DNA/RNA/ATP or phosphate for ATP/RNA/DNA 3. Selective breeding for high productivity crops 4. Ploughing allows nitrification, reduces dentrification 5. Crop rotation leads to pest reduction