Carbon-cycle pathways
Trace carbon through photosynthesis, feeding, respiration, decomposition, fossil-fuel formation and combustion.
This original StudyForA sequence supports Cambridge IGCSE Environmental Management 0680 Topics 1–4 for exams in 2027, 2028 and 2029. Named local examples are chosen by the learner and centre; verify current conditions and local context with reliable sources. It is not a Cambridge paper, official grade or complete course.
Before you begin
- Use the 2027–2029 syllabus and check your exam year and centre's entry arrangements. Topics 1–4 are taught on this page; Topics 5–7 are still being built.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a pyramid of numbers show?
What you will learn
- Trace carbon through photosynthesis, feeding, respiration, decomposition, fossil-fuel formation and combustion.
- Use evidence from an unfamiliar case to explain a cause-and-effect link and justify a suitable response.
Explain aerobic respiration
Aerobic respiration breaks down glucose in the presence of oxygen to release energy, producing carbon dioxide and water as waste products. The word equation is: glucose + oxygen → carbon dioxide + water.
Organisms use released energy for life processes. Respiration releases carbon dioxide, linking energy transfer with the carbon cycle.
Trace carbon through living systems
Photosynthesis moves carbon dioxide from the atmosphere into glucose in plants. Feeding transfers carbon compounds between organisms; respiration returns carbon dioxide to the atmosphere.
Decomposition returns carbon from dead organisms and waste to the environment. Over long periods, some buried organic material may form fossil fuels. Combustion of fossil fuels releases carbon dioxide back into the atmosphere.
Use the cycle to explain a change
The carbon cycle is a set of linked transfers, not a one-way path. A change to one process, such as increased combustion or reduced forest photosynthesis, can change the balance of carbon transfers.
When describing a diagram, name the store or material, process and direction of movement. Do not use an arrow without explaining what is transferred.
Sources for this lesson
Pause and explain
Which process releases carbon dioxide from fossil fuels?
Worked example
A diagram shows atmospheric carbon entering a plant, then a herbivore, then dead material, and later fossil fuel. Name the processes for the transfers and the process that returns carbon to the air when fuel is used.
Show the worked solution
- Photosynthesis transfers carbon dioxide into plant glucose.
- Feeding transfers carbon compounds to the herbivore; decomposition acts on dead material, and some buried carbon can contribute to fossil-fuel formation over long periods.
- Combustion releases carbon dioxide back into the atmosphere; respiration also returns carbon dioxide.
Answer The pathway includes photosynthesis, feeding, decomposition and fossil-fuel formation. Combustion returns carbon dioxide to the atmosphere; respiration also releases it.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A dead leaf is decomposed, while nearby plants remove carbon dioxide from the air. Explain the carbon transfers and state how combustion differs.
Give me a hint
Connect plant growth and decay to carbon movement, then compare burning fossil fuels.
Compare my reasoning
- Photosynthesis moves carbon dioxide from the atmosphere into plant material.
- Decomposition returns carbon from dead organisms and waste to the environment.
- Combustion of fossil fuels releases stored carbon as carbon dioxide to the atmosphere.
Photosynthesis takes carbon into plant material, decomposition returns carbon from dead matter, and combustion releases fossil-fuel carbon dioxide.
Look for these in your work
- Photosynthesis direction correct
- Decomposition explained
- Combustion distinguished
Place photosynthesis, feeding, respiration, decomposition, fossil-fuel formation and combustion in a coherent carbon-cycle explanation.
Give me a hint
For each process, say whether carbon enters organisms, moves between them or returns to the atmosphere.
Compare my reasoning
- Photosynthesis moves carbon dioxide into plant material; feeding transfers carbon to consumers.
- Respiration returns carbon dioxide, while decomposition returns carbon from dead material and waste.
- Over long periods some buried organic material forms fossil fuels; combustion releases carbon dioxide.
The processes form linked transfers: photosynthesis, feeding, respiration, decomposition, fossil-fuel formation and combustion all move carbon among atmosphere, organisms and stored material.
Look for these in your work
- All six processes included
- Direction of transfer explained
- Long time for fossil formation noted
A region clears forest and burns more fossil fuel. Explain how each change can affect carbon transfers and state one measurement that could help evaluate the trend.
Give me a hint
One change affects photosynthetic removal; the other adds carbon through combustion.
Compare my reasoning
- Forest clearing reduces the amount of photosynthesis removing carbon dioxide from the atmosphere.
- Burning fossil fuels transfers stored carbon to atmospheric carbon dioxide through combustion.
- A trend could be investigated using land-cover and atmospheric carbon-dioxide records over time, while considering other sources and sinks.
The changes can reduce carbon removal and increase carbon release. Long-term land-cover and atmospheric data help evaluate the trend but do not isolate every source by themselves.
Look for these in your work
- Removal and release distinguished
- Both changes linked to a process
- Evidence and limitation stated
Common mistakes
- Saying energy cycles in the same way as carbon
- Treating all pyramids of numbers as upright
- Forgetting respiration as a carbon transfer
- Describing fossil-fuel formation as immediate
What can you explain now?
Which process transfers carbon dioxide from air into plant glucose?
Compare with the explanation
Photosynthesis
The correct answer is “Photosynthesis”. The alternatives “Combustion” and “Predation” do not match the syllabus concept. Trace carbon through photosynthesis, feeding, respiration, decomposition, fossil-fuel formation and combustion.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain the role of respiration and trace the six syllabus processes in the carbon cycle.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.