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IGCSE · Practice questions · The carbon and nitrogen cycles

Atoms go round.

Six original Cambridge-style questions on the carbon cycle and the nitrogen cycle, including fossil fuels, combustion, root nodules, waterlogged soil and the roles of microorganisms.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

In, along, back out.

01
Recall
[4 marks]

Describe how a carbon atom in a carbon dioxide molecule in the air could become part of a rabbit and then return to the air.

  • Carbon dioxide is taken in by grass and used in photosynthesis to make glucose (and other carbon compounds). ✓
  • The rabbit eats the grass (feeding) and digests and absorbs the carbon compounds. ✓
  • The rabbit respires, releasing the carbon as carbon dioxide into the air. ✓
  • Or: when the rabbit dies, decomposers feed on its body and release the carbon dioxide by respiration. ✓
02
Explain
[3 marks]

Explain how coal was formed from plants that lived millions of years ago, and why burning coal increases the carbon dioxide in the air.

  • Dead plants were buried in conditions with little oxygen (for example waterlogged swamps), so decomposers could not break them down fully. ✓
  • Over millions of years, heat and pressure turned the material into coal, locking the carbon away. ✓
  • Combustion of coal releases this stored carbon as carbon dioxide, faster than photosynthesis can remove it. ✓
03
Data
[3 marks]

The carbon dioxide concentration inside a closed glasshouse full of plants was measured over 24 hours. It fell during the day and rose during the night. Explain these changes.

  • During the day the rate of photosynthesis is greater than the rate of respiration, so carbon dioxide is removed from the air. ✓
  • At night there is no light, so no photosynthesis takes place. ✓
  • Plants (and soil decomposers) continue to respire, so carbon dioxide is released and the concentration rises. ✓
04
Recall
[4 marks]

Extended. State the role of microorganisms in each of these processes: (a) decomposition, (b) nitrification, (c) nitrogen fixation, (d) denitrification.

  • (a) Decomposers break down proteins in dead organisms and urea in waste, releasing ammonium ions. ✓
  • (b) Nitrifying bacteria convert ammonium ions into nitrate ions. ✓
  • (c) Nitrogen-fixing bacteria (in the soil or in root nodules) convert nitrogen gas into ammonium compounds. ✓
  • (d) Denitrifying bacteria convert nitrate ions into nitrogen gas. ✓
05
Explain
[4 marks]

Extended. Describe how a nitrogen atom in a protein in a dead leaf can become part of a protein in a cow.

  • Decomposers break down the protein in the leaf, releasing ammonium ions into the soil. ✓
  • Nitrifying bacteria convert the ammonium ions into nitrate ions. ✓
  • Grass absorbs nitrate ions through its root hairs and uses them to make amino acids and then proteins. ✓
  • The cow eats the grass, digests the protein into amino acids, absorbs them and builds its own proteins. ✓
06
Explain
[3 marks]

Extended. Crops grow poorly in a field that is often waterlogged. Use your knowledge of the nitrogen cycle to explain why, and suggest how draining the field helps.

  • Waterlogged soil has little oxygen, so denitrifying bacteria thrive and convert nitrates into nitrogen gas, which is lost to the air. ✓
  • Nitrifying bacteria need oxygen, so less ammonium is converted into nitrate. ✓
  • Draining the field lets air into the soil, so nitrification increases, denitrification decreases and more nitrate is available to the crop (roots can also respire for active uptake). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.