Lesson 3.5.4.1

3.5.4.1 Carbon and nitrogen cycles Quiz: AQA Biology, Unit 5

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.5.4.1, Carbon and nitrogen cycles: 20 multiple choice questions for the AQA Biology (7402), Unit 5: Energy transfers in and between organisms, written with Revision Ninja.

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The 20 questions

  1. Which process removes carbon dioxide from the atmosphere in the carbon cycle?

    • Denitrification
    • Photosynthesis
    • Respiration
    • Combustion

  2. Which process converts atmospheric nitrogen gas into compounds that organisms can use?

    • Decomposition by fungi only
    • Nitrogen fixation
    • Ammonification
    • Denitrification

  3. Which process converts nitrates into nitrogen gas?

    • Nitrification
    • Ammonification
    • Denitrification
    • Nitrogen fixation

  4. What is ammonification?

    • Bacteria converting nitrate into nitrogen gas
    • Decomposers converting nitrogen-containing compounds, such as proteins and urea, into ammonia
    • Plants absorbing nitrogen as nitrogen gas
    • Conversion of ammonia into nitrates in the soil

  5. What is nitrification?

    • Fixation of nitrogen gas in root nodules
    • Breakdown of glucose to ethanol
    • Oxidation of ammonium ions to nitrite and then to nitrate
    • Reduction of nitrate to nitrogen gas

  6. What role do saprobionts play in nutrient cycles?

    • They produce oxygen in photosynthesis
    • They fix nitrogen gas from the atmosphere
    • They store carbon as fossil fuel
    • They decompose dead matter and release mineral ions back into the soil

  7. How do mycorrhizae help plants?

    • They fix carbon dioxide into sugars in the leaves
    • They make plants resistant to all herbivores
    • They convert nitrate directly into oxygen
    • They increase the uptake of water and inorganic ions by plant roots

  8. A waterlogged soil has very little oxygen. Which nitrogen process is most likely to increase?

    • Nitrogen fixation by aerobic bacteria
    • Denitrification
    • Aerobic decomposition
    • Nitrification

  9. Which sequence correctly shows nitrogen moving from organic matter back to nitrogen gas?

    • Denitrification, then nitrification, then ammonification
    • Nitrogen fixation, then denitrification, then ammonification
    • Ammonification, then nitrification, then denitrification
    • Nitrification, then ammonification, then denitrification

  10. Why is nitrate important for plants?

    • It is needed for the light-dependent reaction only
    • It is used to make amino acids and therefore proteins
    • It is stored in the xylem as oxygen
    • It is the main energy store used in respiration

  11. Which process releases carbon dioxide back to the atmosphere from dead organic matter?

    • Nitrogen fixation
    • Respiration by saprobionts
    • Photosynthesis by producers
    • Denitrification

  12. Which feature is shared by the carbon cycle and the nitrogen cycle?

    • Both involve living organisms that recycle elements through the ecosystem
    • Neither involves decomposers
    • Both occur only in the sea
    • Both depend on the Krebs cycle to fix nitrogen

  13. Which combustion process adds carbon dioxide to the atmosphere?

    • Nitrogen fixation in legume nodules
    • Photolysis in chloroplasts
    • Denitrification in waterlogged soil
    • Burning of fossil fuels

  14. Which best describes nitrogen fixation?

    • Conversion of nitrate into nitrogen gas
    • Oxidation of ammonia to nitrite
    • Release of ammonia from urea by decomposers
    • Conversion of atmospheric nitrogen gas into ammonia or related compounds usable by organisms

  15. If decomposers stopped working in an ecosystem, what would be the most likely result?

    • Photosynthesis would stop at once in every plant
    • Fossil fuels would form more quickly
    • Nutrients would remain locked in dead matter, and nitrates and carbon dioxide would not be released for plant use
    • Nutrients would be released faster into the atmosphere

  16. A plot with mycorrhizal fungi shows higher phosphate uptake by plants than a plot without them. What is the best explanation?

    • The fungi make phosphate in the leaves by photosynthesis
    • The fungi remove carbon dioxide from the soil
    • The fungi increase the surface area for uptake of inorganic ions such as phosphate
    • The fungi stop nitrogen fixation in the roots

  17. Which statement best explains why nutrients are described as recycled rather than used up?

    • Nutrients are created by photosynthesis from light energy
    • Nutrients are constantly removed from the ecosystem as oxygen
    • Nutrients are never used by living organisms
    • Decomposers and other organisms return elements to the soil or atmosphere, so they can be taken up again

  18. Which statement about the carbon cycle is correct?

    • Carbon leaves the ecosystem permanently in every reaction
    • Carbon moves between the atmosphere, living organisms and dead matter
    • Carbon is found only in the nitrogen cycle
    • Carbon is fixed only by bacteria

  19. Why are bacteria important in the nitrogen cycle?

    • They replace the need for fertilisers entirely
    • They are the only producers in ecosystems
    • They carry out nitrogen fixation, nitrification and denitrification
    • They make oxygen in photosynthesis

  20. A plant grows in soil where no nitrogen-fixing bacteria are present. Which nitrogen source must it rely on?

    • Carbon dioxide converted to nitrates by the roots
    • Nitrogen gas taken directly from the air by the leaves
    • Nitrates and ammonium ions from the soil
    • Oxygen released by decomposers

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