Author: Philip Colls
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Nutrient cycles booklet revision questions            NAME:
Q1. The diagram shows part of the nitrogen cycle.
 
(a)     Which one of the processes P, Q or R involves nitrification?
 
(1)
(b)     The diagram above includes one process in which microorganisms add ammonium ions to soil. Describe another process carried out by microorganisms which adds ammonium ions to soil.
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(c)     Denitrification requires anaerobic conditions. Ploughing aerates the soil.
Explain how ploughing would affect the fertility of the soil.
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(d)     One farming practice used to maintain high crop yields is crop rotation. This involves growing a different crop each year in the same field.
Suggest two ways in which crop rotation may lead to high crop yields.
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2. _________________________________________________________________
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(2)
(Total 7 marks)
Q2. The diagram shows part of the nitrogen cycle.
 
(a)     Name processes P and Q.
P ___________________________________________
Q ___________________________________________   (2)
(b)     It is estimated that, each year, a total of 3 × 10^9 tonnes of ammonia are converted to nitrate. Only 2 × 10^8 tonnes of ammonia are produced from nitrogen gas. Explain the difference in these figures.
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(c)     The conversion of ammonia to nitrate involves oxidation. What evidence in the diagram supports this?
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(1)
(Total 5 marks)
Q3.
Upwelling is a process where water moves from deeper parts of the sea to the surface. This water contains a lot of nutrients from the remains of dead organisms.
(a)     (i)      Nitrates and phosphates are two of these nutrients. They provide a source of nitrogen and phosphorus for cells.
Give a biological molecule that contains:
1. nitrogen_____________________________________________________
2. phosphorus__________________________________________________
(2)
(ii)     Describe the role of microorganisms in producing nitrates from the remains of dead organisms.
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(3)
(b)     Upwelling often results in high primary productivity in coastal waters.
Explain why some of the most productive fishing areas are found in coastal waters.
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(2) (Total 7 marks)
Q4.
In autumn when there is no crop, farm land may be used to grow mustard. The mustard absorbs nitrates which otherwise can leach out of the soil at this time of the year. The mustard is ploughed back into the soil just before sowing of the main crop in the spring.
(a)     Nitrogen compounds in the mustard plants are made available for the main crop after ploughing in spring. Describe the role of microorganisms in this process.
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(5)
(b)     Explain why it is important for the farmer to reduce the leaching of nitrates.
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(2)
(c)     Plants absorb a number of other nutrients from the soil including phosphates. Describe why phosphates are needed by a growing plant.
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(4)
(Total 11 marks)
Q5. The soybean is a leguminous plant. The effect of nitrate fertiliser and of the nitrogen-fixing bacterium, Rhizobium, on the growth of soybeans and on the growth of one species of grass was investigated. The soybeans and grass seeds were sown together in pots of soil in five different proportions. They were then treated with different combinations of nitrate fertiliser and Rhizobium bacteria, as follows:
Batch A: no Rhizobium, no nitrate fertiliser
Batch B: Rhizobium added, no nitrate fertiliser
Batch C: no Rhizobium, nitrate fertiliser added
Batch D: Rhizobium added, nitrate fertiliser added
          The dry masses of the soybean plants and of the grass were determined after 6 months of growth. The results are shown in the graphs.
 
(a)     Did Rhizobium bacteria have any effect on the growth of the grass? Give evidence from graphs C and D for your answer.
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 (1)
(b)     Can the soybean make use of nitrogen supplied in the form of nitrate fertiliser?
Give evidence from the graphs for your answer.
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(2)
(c)     Describe and explain the effect of Rhizobium bacteria on the growth of soybeans.
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(3)
(Total 6 marks)
Q6.
(a)     Explain how farming practices increase the productivity of agricultural crops.

(5)
(b)     Describe how the action of microorganisms in the soil produces a source of nitrates for crop plants.

(5)
(Total 10 marks)
Q7.
Nitrate from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may cause the death of fish in fresh water.

(Total 5 marks)
Q8. Urea from animal waste can be used as a fertiliser. Some bacteria in the soil secrete the enzyme urease which hydrolyses urea into ammonia. Some of this ammonia is released into the atmosphere. NBPT is an inhibitor of urease and can be added to urea fertiliser to reduce the loss of ammonia to the atmosphere.
(a)     A molecule of NBPT has a similar structure to a molecule of urea. Use this information to suggest how NBPT inhibits the enzyme urease.
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Scientists investigated the effect of NBPT on the release of ammonia from urea fertiliser added to the soil. A control experiment was carried out. This involved adding urea fertiliser only. The graph shows their results.
 
(b)     (i)      Describe how NBPT affected the loss of ammonia from urea fertiliser.
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(ii)     Suggest an explanation for the increase in mass of ammonia released over the first four days in the control experiment.
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(2)
(c)     Suggest how the addition of NBPT to urea fertiliser could result in increased growth of crop plants.
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Q9. Arbuscular mycorrhiza fungi (AMF) are fungi which grow on, and into, the roots of plants. AMF can increase the uptake of inorganic ions such as phosphate.
(a)     Suggest one way in which an increase in the uptake of phosphate could increase plant growth.
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(1)
(b)     Suggest one way in which AMF may benefit from their association with plants.
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(1)
(c)     Scientists investigated the effects of different AMF species on the productivity of the plant community of a prairie grassland ecosystem when growing in/on soil containing different phosphate concentrations.
The scientists set up identical plots of prairie grassland soil containing seeds of the plant species found in the ecosystem. The scientists added different AMF species and different concentrations of phosphate to particular plots. Control plots without AMF species were also set up. After 20 weeks the scientists determined the shoot biomass for each plot.
The results the scientists obtained are shown in the graph.
 
Explain why an increase in shoot biomass can be taken as a measurement of net primary productivity.
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(2)
(d)     Using the data from the graph in part (c), evaluate the effect on plant productivity of adding AMF species and adding phosphate to the soil.
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(e)     Using the e^x button on your calculator, determine the rate of shoot biomass production in grams per day for the control plot in soil with normal phosphate concentration.
Answer = ____________________ g day^−1   (2)    (Total 10 marks)
Q10. Farmers use artificial fertilisers to maintain or increase yield from grain-producing crop plants such as wheat.
(a)     Artificial fertiliser is used to replace mineral ions removed from the land when crops are harvested. One of the mineral ions is nitrate.
Give two examples of biological molecules containing nitrogen that would be removed when a crop is harvested.
1. _________________________________________________________________
2. _______________________________________________________________(2)
(b)     Scientists investigated changes in the use of artificial fertiliser in India between 1970 and 2005. They also investigated changes in the fertiliser response ratio. This ratio shows how many kg of grain are produced for each kg of fertiliser used. The graph shows their results in the form the scientists presented them. (A hectare is a unit of area commonly used in agriculture)
 
Use these data to calculate the difference in the mass of grain produced per hectare in 1970 compared with 2005.
Show your working.
Difference ____________________ kg hectare^−1
(2)
(c)     Use the data in the graph above to evaluate the use of artificial fertilisers on grain-producing crops in India.
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(2)
(Total 6 marks)
Q11.
Ammonia in soil is oxidised to nitrites and nitrates by species of nitrifying bacteria.
Scientists investigated whether two soils with a different pH contained different communities of nitrifying bacteria. These communities consist of all the nitrifying bacteria of different species in each soil. They took samples of soil from two sites, A and B.
They measured the pH of the samples and found that
•        the soil from site A had a pH of 6.9
•        the soil from site B had a pH of 4.3
The scientists measured the concentration of ammonia in soil samples over 20 days. Each sample contained the same concentration of ammonia at the start and had the same mass. They recorded the concentration of ammonia in
•        soil A with a pH of 6.9
•        soil B with a pH of 4.3
•        a mixture of equal masses of soils A and B with its pH adjusted to 6.9
Their results are shown in Figure 1.

Figure 1

(a)     The scientists used units of μg g^–1 for the concentration of ammonia in soil.
Suggest why, in this investigation, the scientists used these units.
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g^–1 ________________________________________________________________
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(2)
(b)     Calculate the difference in the rate of breakdown of ammonia per day between day 0 and day 2 in soil A and soil B.
Show your working and the units for your answer.
Difference in rate = ____________________ (2)
(c)     The scientists concluded that the soil mixture experiment showed there were different communities of bacteria in soils A and B. What evidence from Figure 1 supports their conclusions? Give reasons for your answer.
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The oxidation of ammonia by nitrifying bacteria involves the enzyme ammonia monooxygenase. Each species of nitrifying bacteria has its own specific amoA gene that codes for production of ammonia monooxygenase. In a second investigation, the scientists determined the expression of the amoA gene in two species of bacteria, S and T. Species S was from acid soil and species T was from soil with a neutral pH.
The scientists grew cultures of each species separately in soils of different pH. They determined the amount of mRNA from the amoA gene in each culture. Their results are shown in Figure 2.
Figure 2

(d)     In which species was the number of copies of mRNA more affected by changes in soil pH from 4.9 to 7.5? Use a calculation to support your answer.
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(e)     This method allowed the scientists to estimate the expression of the amoA gene in each culture but not the growth of the bacterial population in each culture. Explain why.
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(f)     The scientists set up their cultures in sterile glass bottles.
Suggest one suitable method for sterilising the bottles and explain why it was necessary to sterilise them.
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(2) (Total 15 marks)

Mark schemes
Q1.
(a)     R.
1
(b)     1.      Protein / amino acids broken down (to ammonium ions / ammonia);
Accept: nucleic acids / RNA / DNA / urea / any named nitrogen containing compound as an alternative to protein / amino acids
Accept: saprophytes / saprotrophs
2.      By saprobionts / saprobiotic (microorganisms).
Neutral: decomposers
Reject: answers where incorrect type of bacteria given as saprobionts e.g. Nitrogen fixing bacteria
2
(c)     1.      (Fertility increased as) more nitrate formed / less nitrate removed / broken down;
Accept: Nitrate remains
2.      Less / no denitrification / process P is decreased / fewer denitrifying bacteria.
Accept: more nitrification / more nitrifying bacteria / process R is increased
2
(d)     1.      Grow crops / plants with nitrogen-fixing (bacteria);
Accept: grow legumes / named example e.g. peas, beans, clover
Accept: fallow year
Accept: use different amounts of ions / nutrients
2.      (Different crops use) different minerals / salts / nutrients / ions (from the soil);
3.      (Different crops have) different pests / pathogens / diseases.
2 max
[7]
Q2.
(a)     P – denitrification;
Q – Nitrogen fixation;
2
(b)     Ammonia formed by decay / decomposition / putrefying / ammonifying / 
by action of decomposers / saprobionts;
On nitrogenous waste / urea or nitrogenous compounds (e.g. proteins,
amino acids, DNA, ATP);
2
(c)     Oxygen added / hydrogen removed;
Ignore references to electron loss
1
[5]
Q3.
(a)     (i)      1.      Amino acid / protein / enzyme / urea / nucleic acid / 
         chlorophyll / DNA / RNA / / ATP / ADP / AMP / NAD / NADP;
2.      DNA / RNA / nucleic acid / ATP / ADP / AMP / NADP / TP / GP / RuBP / phospholipids;
1. and 2. Accept any named equivalent examples e.g. nucleotides.
Neutral: ammonia / nitrite / nitrate / phosphate.
2
(ii)     1.      Saprobiotic (microorganisms / bacteria) break down remains / dead material / protein / DNA into ammonia / ammonium;
Accept: saprobionts / saprophytes / saprotrophs
Neutral: decomposer
2.      Ammonia / ammonium ions into nitrite and then into nitrate;
Allow correct chemical symbols.
Accept: correct answers which use incorrect bacteria e.g. nitrogen-fixing but then reject m.p. 3.
3.      (By) Nitrifying bacteria / nitrification;
3
(b)     1.      Nitrate / phosphate / named ion / nutrients for growth of / absorbed / used by plants / algae / producers;
2.      More producers / consumers / food so more fish / fish reproduce more / fish grow more / fish move to area;
Must have idea of more plants related to some increase in fish.
2
[7]
Q4.
(a)     proteins / amino acids broken down;
deamination / ammonification / release of ammonium compounds;
conversion to nitrates;
by nitrifying bacteria / named bacterium;
nitrates absorbed into roots;
5
(b)     fewer nitrates in the soil for the next crop / plants grow less well
because of lack of nitrates;
requiring application of more fertiliser / economic reason for using less fertiliser / valid environmental reason explained e.g. nitrates leaching into water / eutrophication / explanation / health related e.g drinking water;
2
(c)     production of phospholipids;
in cell membranes;
synthesis of ATP;
production of DNA;
production of RNA;
production of NADP;
4 max
[11]
Q5.
(a)     No - very little increase / no increase in yield of grass when Rhizobium 
added / no difference between C and D;
1
(b)     Yes: increased yield with nitrates;
Correct reference to result in graph C c.f. graph A / use of correct 
numbers (from C + A)
e.g. greater yield of soyabean in C than in A / 
       greater yield of soyabean with nitrate than without if no Rhizobium;
2
(c)     Forms mutualistic / symbiotic union with soyabean / forms root nodules / 
mutual benefits ( / described);
makes ammonia / ammonium;  (Nitrates – CANCEL)
Helps produce organic-N / amino acids / protein;
max 3
[6]
Q6.
(a)     1.      Fertilisers / minerals / named ion (added to soil);
Accept any named examples of natural fertilisers for mark point 1 e.g. manure, bone meal etc. Ignore named elements
2.      Role of named nutrient or element e.g. nitrate / nitrogen for proteins / phosphate / phosphorus for ATP / DNA;
Accept fertilisers / minerals / named nutrient / element removes limiting factor for mark point 2
3.      Selective breeding / genetic modification (of crops);
Accept idea of choosing particular variety of crop for mark point 5
4.      Ploughing / aeration allows nitrification / decreases denitrification;
5.      Benefit of crop rotation in terms of soil nutrients / fertility / pest reduction;
5
(b)     1.      Protein / amino acids / DNA into ammonium compounds / ammonia;
Accept any named nitrogen containing compound e.g. urea for mark point 1
2.      By saprobionts;
Accept saprophytes for mark point 2
3.      Ammonium / ammonia into nitrite;
Accept marks for conversion i.e. mark points 1, 3, 4 and 6 even if incorrect type of bacteria named as being involved
4.      Nitrite into nitrate;
However, reject marks for type of bacteria i.e. mark points 2, 5 and 7 if linked to incorrect process e.g. nitrite converted to nitrate by saprobionts
5.      By nitrifying bacteria / microorganisms;
6.      Nitrogen to ammonia / ammonium;
Award one mark for ammonia / ammonium into nitrate if neither mark point 3 or 4 awarded
7.      By nitrogen-fixing bacteria / microorganisms in soil;
Ignore reference to nitrogen-fixing bacteria in root nodules. If not specified, assume nitrogen-fixing bacteria are in the soil
5 max
[10]
Q7.
1.      Growth of algae / surface plants / algal bloom blocks light;
2.      Reduced / no photosynthesis so (submerged) plants die;
3.      Saprobiotic (microorganisms / bacteria);
3. Accept: Saprobiont / saprophyte / saprotroph
3. Neutral: decomposer
4.      Aerobically respire / use oxygen in respiration;
5.      Less oxygen for fish to respire / aerobic organisms die;
[5]
Q8.
(a)     Complementary to / fits / binds to active site;
Competitive / competes / ‘prevents’ enzyme-substrate complexes / ‘prevents’ urea attaching;
Max one mark if candidate suggests that active site / enzyme is damaged destroyed or useless.
Allow inhibitor ‘prevents’ or ‘stops’ urea / substrate attaching unless candidate clearly indicates this is permanent.
Ignore reference to inhibitor forming an enzyme / substrate complex.
2
(b)     (i)      Reduces loss of ammonia up to day8 / 9;
1
(ii)     Increase in urease / temperature;
More enzyme-substrate complexes;
More bacteria;
2 max
(c)     Less urea / ammonia lost (from soil) / less urea broken down;
Urea / ammonia converted to nitrite / nitrate;
Used to produce protein / amino acids / DNA / bases / nucleotides;
Reference to incorrect bacteria (e.g. denitrifying) producing nitrite / nitrate negates second marking point.
3
[8]
Q9.
(a)     Used to produce named phosphate compound in cells;
e.g. ATP / ADP / phospholipids / DNA / RNA / RuBP / TP /GP etc.
1
(b)     Example of a carbon-containing biological compound e.g. carbohydrate / amino acid / vitamin;
Accept: sugars / organic (compounds).
Ignore: products of photosynthesis.
Ignore: starch.
1
(c)     1.      Represents dry mass / mass of carbon;
2.      Represents gross production minus respiratory losses;
2.      Accept: NPP = GPP −R.
2.      Accept: Chemical energy minus respiratory losses.
1 and 2. Chemical energy store minus respiratory losses = 2 marks.
2
(d)     1.      For the control an increase in phosphate increases (plant) growth;
2.      For Entrophospora an increase in phosphate reduces (plant) growth;
3.      Scutellospora reduces (plant) growth (compared to control);
4.      Entrophospora and Glomus increases (plant) growth (compared to control);
5.      No SD / statistical test to determine significance;
6.      Only 20 weeks of growth;
7.      Underground / root growth not known;
5.      Accept: no error bars.
7.      Accept: only shows shoot growth.
4 max
(e)     1.      Answer in range 0.07 to 0.09 = 2 marks;
2.      Answer in range 9.97 to 12.2
OR
Shows division by 140 or 20 × 7 = 1 mark;
2
[10]
Q10.
(a)     Two suitable examples;
Examples
1.      amino acid / protein / polypeptide / peptide;
2.      nucleic acid / nucleotide / base;
3.      DNA;
4.      RNA;
5.      ATP / ADP;
6.      NAD / NADP (reduced or not);
7.      Cyclic AMP / cAMP;
8.      Chlorophyll;
List rule applies
Reject for either point nitrates / nitrites / ammonia / ammonium / urea
4.      Accept pre-mRNA / mRNA / rRNA / tRNA
2 max
(b)     Correct answer in the range 90 to 133.2 scores 2 marks;
1 mark for answers where yield calculated correctly for 1970 OR 2005;
(1970 in range) 170.8 to 176.4
OR
(2005 in range) 266.4 to 304.0;
Accept positive or negative values
2
(c)     1.      Using more but getting less response over time;
2.      The graph shows correlation but doesn’t prove changes in yield due to fertiliser / but there could be other factors;
3.      Becomes less cost effective with time;
Idea of over time is important
1.      accept fertiliser becomes less effective over time
1.      Accept use of figures from graph
1.      Accept the idea of less grain / crop over time
2.      Ignore whether correlation is positive or negative
2 max
[6]
Q11.
(a)     1.      (μg because) very little ammonia (in soil);
2.      (μg because) avoids use of (lots of) decimal places (in their results) / avoids the use of powers of 10 / avoids the use of standard form;
Accept makes numbers more manageable
Accept makes easier to plot graph
3.      (g^–1) to allow comparisons (between samples);
2 max
(b)     Answer between 4.5 and 4.6 μg g^–1 day^–1;;
Award 1 mark for correct number but wrong / no units
Ignore plus or minus signs
Accept ‘per gram’ AND / OR ‘per day’
2
(c)     1.      pH 4.3 / B has fastest rate of breakdown (of ammonia);
2.      A + B / mixture at pH 6.9 slowest / slower (than A or B);
Not just ref. to A and then B on their own
3.      Suggests (community / bacteria at) pH 4.3 / B doesn’t work (well) at pH 6.9 / pH of mixture;
Accept converse that only (community / bacteria at) pH 6.9 / A is working in the mixture
3
(d)     (Species S because) no mark
1.      Species S change of 990,000 (per gram of soil);
Award MP1 and 2 OR MP3 and 4
Accept standard forms 9.9 × 10^5 and 9.9 × 10^3 for either
Accept for 1 mark for 100 times greater in correct context with no other calculations shown
2.      Species T change of 9,900 (per gram of soil);
Accept standard forms 9.9 × 10^5 and 9.9 × 10^3 for either
OR
(Species T because) no mark
3.      Species S has 99% change;
4.      Species T has 9900% change;
2
(e)     1.      They didn’t count bacteria / cells / population(s);
Ignore ref. to other factors / other named factors affecting growth
2.      Copies / number of mRNA related to amount of enzyme / amoA produced / translated;
3.      Don’t know how much mRNA / amoA produced by each cell;
Accept some bacteria produce more mRNA / amoA than others
4.      Don’t know if amoA (mRNA / enzyme) is linked to cell division / growth (of population);
Amount of amoA does not show cell division / growth
Reject references to mitosis / meiosis
4
(f)      1.      Suitable method;
eg in boiling water / steam / autoclave / wash in disinfectant / wash in alcohol
Ignore heat unqualified
Ignore flaming of bottle
Accept radiation
2.      (Reason) to remove / kill other bacteria / organisms that might break down ammonia;
Ignore ref to removing bacteria that ‘affect the result’
Accept other bacteria producing amoA
Accept other bacteria compete with / kill bacteria that produce amoA
Ignore contamination unqualified
2
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