The Nitrogen Cycle

1. 1 State [1 mark]

   Name the ions that plants take in from the soil to make proteins.

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   Model answer

   Nitrate ions.

   Mark scheme

   • Nitrates (nitrate ions) — 1 mark
2. 2 Explain [2 marks]

   Explain why plants need nitrates.

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   Model answer

   Plants use nitrates to make amino acids. The amino acids are joined together to make proteins, which are needed for growth.

   Mark scheme

   • To make amino acids — 1 mark
   • Which make proteins for growth — 1 mark
3. 3 Name [3 marks]

   Name the type of bacteria that (a) change nitrogen gas into ammonia, (b) change ammonium compounds into nitrates and (c) change nitrates into nitrogen gas.

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   Model answer

   (a) Nitrogen-fixing bacteria. (b) Nitrifying bacteria. (c) Denitrifying bacteria.

   Mark scheme

   • (a) Nitrogen-fixing bacteria — 1 mark
   • (b) Nitrifying bacteria — 1 mark
   • (c) Denitrifying bacteria — 1 mark
4. 4 Describe [3 marks]

   Describe the role of decomposers in the nitrogen cycle.

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   Model answer

   Decomposers, which are bacteria and fungi, break down dead plants and animals, faeces and urine. They break down the proteins and urea in these. This releases ammonium compounds into the soil, which nitrifying bacteria can then change into nitrates.

   Mark scheme

   • Bacteria and fungi break down dead organisms and waste — 1 mark
   • They break down proteins and urea — 1 mark
   • This releases ammonium compounds into the soil — 1 mark
5. 5 Explain [3 marks]

   A field becomes waterlogged after a long period of heavy rain, and the crop grows poorly. Explain why.

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   Model answer

   Waterlogged soil has little oxygen, so there are fewer nitrifying bacteria, which need oxygen, to make nitrates. Denitrifying bacteria thrive in these conditions and change the nitrates into nitrogen gas. The soil therefore has fewer nitrates, so the plants cannot make enough protein and grow poorly.

   Mark scheme

   • Little oxygen, so fewer nitrifying bacteria — 1 mark
   • Denitrifying bacteria change nitrates into nitrogen gas — 1 mark
   • Fewer nitrates, so less protein is made and the crop grows poorly — 1 mark
6. 6 Explain [4 marks]

   A farmer grows wheat, then potatoes, then peas and then barley in the same field in a four year cycle. Explain how growing peas in the cycle helps the farmer.

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   Model answer

   Peas are legumes. They have root nodules that contain nitrogen-fixing bacteria, which change nitrogen gas from the air into nitrogen compounds. When the peas are ploughed in or their roots decay, the compounds are released and nitrifying bacteria change them into nitrates. This increases the nitrates for the next crop, so the farmer needs to add less fertiliser, which saves money.

   Mark scheme

   • Peas are legumes with nitrogen-fixing bacteria in root nodules — 1 mark
   • The bacteria change nitrogen from the air into nitrogen compounds — 1 mark
   • This increases the nitrates in the soil for the next crop — 1 mark
   • So less fertiliser is needed, which saves money — 1 mark
7. 7 Explain [3 marks]

   In a wild meadow the plants die and decay, and the soil stays fertile. In a field of wheat the farmer has to add fertiliser. Explain why.

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   Model answer

   In a meadow the dead plants decay where they fall, so decomposers return the nitrogen to the soil as ammonium compounds and nitrates for the next plants. When wheat is harvested the plants are taken away, so the nitrogen in their proteins is not returned to the soil. The farmer has to add fertiliser to replace the nitrates that were removed.

   Mark scheme

   • In a meadow the dead plants decay and the nitrogen returns to the soil — 1 mark
   • In the field the crop is harvested and the nitrogen is removed — 1 mark
   • Fertiliser replaces the nitrates that were removed — 1 mark
8. 8 Describe [6 marks]

   Describe how nitrogen from the air becomes part of the protein in a plant, and how the nitrogen returns to the air.

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   Model answer

   Nitrogen-fixing bacteria in the soil or in root nodules change nitrogen gas into ammonium compounds, and lightning can turn it into nitrates. Nitrifying bacteria change the ammonium compounds into nitrates. The plant roots take up the nitrates by active transport, and the plant uses them to make amino acids and then proteins. When the plant dies, decomposers break down the proteins and release ammonium compounds, which are again changed into nitrates. In waterlogged soil, denitrifying bacteria change nitrates into nitrogen gas, which returns to the air.

   Mark scheme

   • Nitrogen-fixing bacteria (or lightning) change nitrogen from the air into compounds — 1 mark
   • Nitrifying bacteria change ammonium compounds into nitrates — 1 mark
   • Roots take up the nitrates (by active transport) — 1 mark
   • The plant makes amino acids and then proteins — 1 mark
   • Decomposers break down dead plants and release ammonium compounds — 1 mark
   • Denitrifying bacteria change nitrates into nitrogen gas, which returns to the air — 1 mark

Quick check

1. 1

   What percentage of the air is nitrogen gas?

   1. A21 percent
   2. B0.04 percent
   3. C78 percent
   4. D50 percent
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   C: 78 percent

   About 78 percent of the air is nitrogen, but plants cannot use it directly.

2. 2

   Which bacteria change ammonium compounds into nitrates?

   1. ANitrifying bacteria
   2. BDenitrifying bacteria
   3. CNitrogen-fixing bacteria
   4. DDecomposer bacteria only
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   A: Nitrifying bacteria

   Nitrifying bacteria change ammonium compounds into nitrates that plants can take up.

3. 3

   Where do the nitrogen-fixing bacteria that help legumes live?

   1. AIn the leaves
   2. BIn the xylem
   3. CIn the stem
   4. DIn root nodules
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   D: In root nodules

   They live in nodules on the roots of legumes such as peas, beans and clover.

4. 4

   What do denitrifying bacteria do?

   1. AChange nitrogen gas into nitrates
   2. BChange nitrates into nitrogen gas
   3. CBreak down dead plants into carbon dioxide
   4. DChange ammonium compounds into nitrates
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   B: Change nitrates into nitrogen gas

   Denitrifying bacteria change nitrates into nitrogen gas, which reduces the fertility of the soil.

5. 5

   Why are legumes grown in a crop rotation?

   1. AThey make the soil more acidic
   2. BThey use up all the nitrates
   3. CThey add nitrogen compounds to the soil
   4. DThey kill all pests
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   C: They add nitrogen compounds to the soil

   Nitrogen-fixing bacteria in their root nodules add nitrogen compounds to the soil.

6. 6

   Which conditions favour denitrifying bacteria?

   1. AWaterlogged soil with little oxygen
   2. BWell aerated, dry soil
   3. CSoil with many legumes
   4. DSoil that is very hot and dry
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   A: Waterlogged soil with little oxygen

   Denitrifying bacteria work best in waterlogged soil with little oxygen.

7. 7

   What do plants make using the nitrates they take up?

   1. AStarch
   2. BCellulose
   3. CGlucose
   4. DAmino acids
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   D: Amino acids

   Plants use nitrates to make amino acids, which are joined to make proteins.

8. 8

   How does lightning add nitrates to the soil?

   1. AIt kills denitrifying bacteria
   2. BIt makes nitrogen oxides that dissolve in rain
   3. CIt makes decomposers work faster
   4. DIt adds ammonia to the air
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   B: It makes nitrogen oxides that dissolve in rain

   Lightning makes nitrogen and oxygen form nitrogen oxides, which dissolve in rain to form nitrates.