Nutrient cycle

Question 1

Explain how a high nitrate concentration increases the growth of algae.

Answer 1

more proteins / amino acids / more DNA / nucleotides / nucleotide derivative; increased cell division / number of cells formed;

Question 2

Suggest how increased growth of algae could lead to the death of the submerged plants

Answer 2

reduced light / shading; less photosynthesis;

Question 3

Explain how the decay of dead plants results in reduced oxygen concentration and increased nitrate production.

Answer 3

1 bacteria / fungi feed on dead matter saprobiotically
2 respiration uses up oxygen;
3 converts proteins to amino acids;
4 then to ammonium compounds;
5 nitrifying bacteria convert ammonium compounds;
6 via nitrates;

Question 4

Describe how the reduced oxygen concentration of the water will change the composition of the communities in the river.

Answer 4

lower species diversity / number of species;

species tolerant to low oxygen thrive / species requiring high oxygen die out;

Question 5

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.

Answer 5

proteins / amino acids broken down;
deamination / ammonification / release of ammonium compounds; conversion to nitrates;
by nitrifying bacteria / named bacterium;
nitrates absorbed into roots;

Question 6

Explain why it is important for the farmer to reduce the leaching of nitrates.

Answer 6

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;

Question 7

Plants absorb a number of other nutrients from the soil including phosphates. Describe why phosphates are needed by a growing plant.

Answer 7

production of phospholipids; in cell membranes;

synthesis of ATP; production of DNA; production of RNA; production of NADP;

Question 8

Suggest and explain two reasons why a poor supply of phosphate ions results in poor growth of plants (lines 1–2).

Answer 8

(Required to) make ATP / glucose phosphate, so less respiration / less energy for growth;
(Required to) make nucleotides, so less DNA / mRNA / tRNA for cell division / production of protein (for growth);

Question 9

The signal proteins secreted into the air by a plant being attacked by a pathogen act as stimuli leading to the expression of genes for defensive enzymes in other plants (lines 9–12).
Suggest how they lead to the expression of these genes.

Answer 9

Bind to receptor (on target plant);
Acts as / leads to production of a transcription factor;
stimulates transcription of genes

Question 10

Suggest and explain the advantage to tomato plants of transmitting signal proteins through mycorrhizal networks, rather than releasing them into the air (line 11–12 and lines 14–16)

Answer 10

Direct plant-to-plant transmission;

faster response

Question 11

A student who read this passage concluded that farmers should not use fertilisers to increase yields when growing tomato plants.
Evaluate his conclusion.

Answer 11

1. Fertilisers prevent development of mycorrhizae;
2. Mycorrhizae help plants to defend themselves (causing an increase in crop yield);
3. Mycorrhizae help plants to take up nitrates / phosphates (causing an increase in crop yield);
   Should use:
4. Fertilisers containing phosphate and nitrate increase gross primary production so increase yield;
5. Most soil is poor in phosphate so without fertiliser (tomato) plant might not get enough phosphate;

Question 12

Describe how the nitrogen-containing substances get into the lake.

Answer 12

dissolve (in soil water) / run-off / leaching; reject nitrogen dissolving.

Question 13

It takes longer for the nitrogen-containing substances to get into the lake when an organic fertiliser is used than when an inorganic fertiliser is used. Explain why it takes longer when an organic fertiliser is used.

Answer 13

insoluble / less soluble;

(molecules) require breaking down / slow release;

Question 14

(b) Describe how the presence of nitrates in a lake may eventually lead to the death of fish.

Answer 14

1. Growth of algae / surface plants / algal bloom blocks light;
2. Reduced / no photosynthesis so (submerged) plants die;
3. Saprobiotic (microorganisms / bacteria);
4. Accept: Saprobiont / saprophyte / saprotroph 3. Neutral: decomposer
5. Aerobically respire / use oxygen in respiration;
6. Less oxygen for fish to respire / aerobic organisms die;

Question 15

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 15

(Nitrogen) to ammonia / NH3 / ammonium; being converted to nitrite, nitrate etc
1. Do not disqualify mark for any references to ammonia
Produce protein / amino acids / named protein / DNA / RNA;

Question 16

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

Answer 16

Soil has low(er) water potential / plant / roots have higher water potential;

Osmosis from plant / diffusion of water from plant;

Question 17

A large lake is surrounded by fields. These fields are separated from each other by hedges. One hundred years ago the lake was a habitat for many plants, invertebrates and fish. Today the lake has no fish and few plants or invertebrates.
Explain how increased use of inorganic fertilisers on the fields may have led to these changes.

Answer 17

1. growth of algae / plants stimulated / increased;
2. death of algae / plants;
   3 MORE bacteria / decomposers / decomposition;
3. respiration;
4. decomposers / bacteria remove oxygen;
5. animals die (because of lack of oxygen);

Question 18

Explain the advantage of having both algae and bacteria in a purification pond.

Answer 18

breakdown of organic matter / sewage by enzymes from bacteria;
nitrates / ammonia used by algae to make amino acids / proteins;
algae photosynthesise;
bacterial respiration uses O2 / produces CO2 for algae; (respiration) allows for reproduction / growth of bacteria;

Question 19

Purification ponds only work efficiently when they are shallow and warm. Explain why.

Answer 19

sufficient light penetration for photosynthesis (of algae); warm leads to faster enzyme activity;
faster bacterial respiration / decomposition;
faster photosynthesis;
increased growth / reproduction of bacteria / algae;

Question 20

Explain how including leguminous plants in a crop rotation reduces the need to use artificial fertilisers

Answer 20

contain nitrogen-fixing bacteria in roots / nodules (so don’t need fertiliser); nitrogen containing compounds added to the soil
when plant dies / after harvest of crop;

Question 21

Application of very high concentrations of fertiliser to the soil causes plants to wilt. Explain why.

Answer 21

low(er) / more negative water potential in soil (than in the plant);
prevents roots from taking up water (from the soil) / plants still lose water by transpiration;
plants lose water to soil by osmosis;

Question 22

Crops use light energy to produce photosynthetic products.

Describe how crop plants use light energy during the light-dependent reaction.

Answer 22

1.Excites electrons / electrons removed (from chlorophyll);
2. Electrons move along carriers/electron transfer chain releasing energy;
Accept: movement of H+/protons across membrane releases energy.
3. Energy used to join ADP and Pi to form ATP;
4. Photolysis of water produces protons, electrons and oxygen;
5. NADP reduced by electrons / electrons and protons / hydrogen;
Accept: NADP to NADPH (or equivalent) by addition of electrons/hydrogen.

Question 23

After harvesting, the remains of crop plants are often ploughed into the soil.
Explain how microorganisms in the soil produce a source of nitrates from these remains.

Answer 23

1. Protein/amino acids/DNA into ammonium compounds /
2. By saprobionts;
   Accept: saprophytes.
3. Ammonium/ammonia into nitrite;
4. Nitrite into nitrate;
5. By nitrifying bacteria/microorganisms;

Question 24

Other than spreading fertilisers, describe and explain how one farming practice results in addition of nitrogen-containing compounds to a field.

Answer 24

growing legumes / named legume;
ploughed in / allowed to decompose / nitrogen-fixing (bacteria in nodules);
OR
allow cattle / named species / (farm) animals (to graze); add dung / urine;

Question 25

Describe and explain how one farming practice results in the removal of nitrogen- containing compounds from a field.

Answer 25

bare soil / fallow in winter / hedge removal; leaching (of nitrates) / soil erosion;
OR

(farm) animals eat plants
(in field); (then) animals removed;

Question 26

In the presence of oxygen, respiration yields more ATP per molecule of glucose than it does in the absence of oxygen. Explain why.

Answer 26

Correct statement in the context of aerobic respiration or
anaerobic respiration concerning:
Oxygen as terminal hydrogen / electron acceptor allowing operation of electron transport chain / oxidative phosphorylation;
Fate of pyruvate;
Significance of ATP formed in glycolysis;

Question 27

Heterocysts are thick-walled cells that do not contain chlorophyll. Suggest how the features of the heterocysts improve the efficiency of the process of nitrogen fixation.

Answer 27

Thick walls exclude oxygen;
Produced by photosynthetic cells (of fern and Anabaena); Contain no chlorophyll so do not photosynthesise;
Do not produce oxygen;
Oxygen would inhibit nitrogen fixation process;

Question 28

In China, the fern is cultivated and ploughed into fields to act as an organic fertiliser. Explain how ploughing the fern plants into the soil results in an improvement in the growth of the rice crop grown in these fields.

Answer 28

Decomposers / bacteria / fungi / saprobionts (in fields);
Convert protein / organic nitrogen (in cells of fern) into
ammonium ions (allow ammonia);
Ammonium ions (ammonia) converted to nitrite, then converted to nitrate;

Question 29

Explain the roles of the decomposers and the nitrifying bacteria in converting nitrogen in organic compounds in the sewage into a soluble, inorganic form.

Answer 29

decomposers convert (nitrogen in organic compounds) into ammonia / ammonium; suitable example of “organic nitrogen” - protein / urea / amino acid etc. (e.g. linked to process); 
nitrifying bacteria / correctly named convert ammonium to nitrate; via nitrite;

Question 30

Nitrifying bacteria are one kind of bacteria that are important in the nitrogen cycle; nitrogen-fixing bacteria are another kind. Describe the part played by nitrogen-fixing bacteria in the nitrogen cycle.

Answer 30

convert nitrogen (gas) into ammonium / ammonia / amino acids; add usable / available nitrogen to an ecosystem / eq.;

Question 31

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

Provides ATP for the reaction / nitrogen fixation / reduction of nitrogen / formation of ammonia;
Accept: ATP or energy
Enzyme / nitrogenase produced quicker / more enzyme produced;
Ignore references to temperature
Uses / removes oxygen (so nitrogenase works);
Use of oxygen must be in the correct context

Question 32

The legume plants have nodules containing nitrogen-fixing bacteria on their roots. Explain how nitrogen-fixing bacteria could increase the growth of the maize.

Answer 32

Nitrogen-fixing bacteria convert nitrogen (in the air) into ammonium compounds (in the soil) which are converted into nitrates / nitrification occurs;

Maize uses nitrates (in soil) for amino acid / protein / ATP / nucleotide production;

Question 33

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

Answer 33

(Fertility increased as) more nitrate formed / less nitrate removed / broken down;

Less / no denitrification / process P is decreased / fewer denitrifying bacteria.

Question 34

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

Answer 34

1. Grow crops / plants with nitrogen-fixing (bacteria);
2. (Different crops use) different minerals / salts / nutrients / ions (from the soil);
3. (Different crops have) different pests / pathogens / diseases.

Question 35

Describe the role of saprobionts in the nitrogen cycle.

Answer 35

(They use enzymes to) decompose proteins/DNA/RNA/urea;

Producing/releasing ammonia/ammonium compounds/ammonium ions;

Question 36

Explain how farming practices increase the productivity of agricultural crops.

Answer 36

1. Fertilisers / minerals / named ion (added to soil);
2. Role of named nutrient or element e.g. nitrate / nitrogen for proteins / phosphate / phosphorus for ATP / DNA;
3. Selective breeding / genetic modification (of crops);
4. Ploughing / aeration allows nitrification / decreases
5. Benefit of crop rotation in terms of soil nutrients / fertility / pest reduction;

Question 37

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

Answer 37

1. Protein / amino acids / DNA into ammonium compounds / ammonia; (Nitrogen) to ammonia / NH3 / ammonium;
   By saprobionts;
2. Ammonium / ammonia into nitrite;
3. Nitrite into nitrate;
4. By nitrifying bacteria / microorganisms;
5. Nitrogen to ammonia / ammonium;

Question 38

Describe the role of microorganisms in producing nitrates from the remains of dead organisms.

Answer 38

Saprobiotic (microorganisms / bacteria) break down remains / dead material / protein / DNA into ammonia / ammonium;
Ammonia / ammonium ions into nitrite and then into nitrate;
(By) Nitrifying bacteria / nitrification;

Question 39

After harvesting the crop, the farmer digs the unwanted stems and roots into the soil. Explain how the nutrients contained in these plant parts become available for use by other organisms.

Answer 39

(mesquite) proteins / amino acids (ploughed) into soil / nodules ploughed in and (decomposers) bacteria / fungi feed on these;
excrete ammonia;
nitrifying bacteria convert these to nitrites / nitrates;
absorbed by roots of grasses and increase their growth;

Question 40

Explain how ploughing the mesquite into the soil makes it more fertile.

Answer 40

proteins / amino acids broken down;
deamination / ammonification / release of ammonium compounds; conversion to nitrates;
by nitrifying bacteria / named bacterium;
nitrates absorbed into roots;

Question 41

Explain why it is important for the farmer to reduce the leaching of nitrates.

Answer 41

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;

Question 42

Plants absorb a number of other nutrients from the soil including phosphates. Describe why phosphates are needed by a growing plant.

Answer 42

production of phospholipids; in cell membranes;

synthesis of ATP; production of DNA; production of RNA; production of NADP;