Map carbon through a cycle
The carbon cycle traces carbon through carbon dioxide and organic material by photosynthesis, feeding, respiration, decomposition, fossil-fuel formation and combustion.
Section B18.3 · Carbon cycle
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Use the food-web pathway and distinguish matter cycling from energy flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is being transferred around the carbon cycle?
What you will learn
- Name the carbon-cycle processes required by the syllabus
- Trace carbon atoms between the atmosphere and organisms
Follow carbon atoms, not energy
Carbon atoms move between carbon dioxide in the air and organic molecules in organisms. Photosynthesis incorporates carbon dioxide into plant organic material. Feeding transfers carbon-containing molecules between organisms. Respiration and decomposition return carbon dioxide to the air; fossil-fuel formation stores carbon over long timescales, and combustion releases carbon dioxide.
A cycle diagram uses arrows to show transfers of matter. It does not mean the same carbon atom moves through every route or that each route takes the same time. The syllabus limits this account to photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion.
Use a complete labelled route
Start with carbon dioxide in air, follow photosynthesis into plants and feeding to animals, then return carbon through respiration or decomposition. A separate route begins when some buried organic material forms fossil fuels over geological time; combustion transfers carbon dioxide back to the air.
Do not draw an arrow that turns energy into carbon or suggests carbon is created or destroyed. Identify the carbon-containing material at each stage and name the process that transfers it.
Sources for this lesson
Pause and explain
Which sequence correctly traces carbon from air into an animal?
Worked example
Trace a carbon atom from atmospheric carbon dioxide into a grazing animal and back to the atmosphere.
Show the worked solution
- Photosynthesis transfers carbon dioxide into plant organic material.
- Feeding transfers plant carbon to the animal.
- Respiration transfers carbon dioxide back to air.
Answer Atmospheric carbon dioxide → plant organic material by photosynthesis → grazing animal by feeding → carbon dioxide in air by respiration.
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.
Use photosynthesis, feeding and respiration to trace carbon from air through a plant to an herbivore and back to air.
Give me a hint
Write the carbon-containing material and process at each arrow.
Compare my reasoning
- Carbon dioxide is taken up in photosynthesis.
- Plant organic material is eaten by the herbivore.
- Respiration returns carbon dioxide.
Carbon dioxide in air → plant organic material by photosynthesis → herbivore organic material by feeding → carbon dioxide in air by respiration.
Look for these in your work
- All three named processes
- Direction follows carbon
A learner labels an arrow from a plant to a rabbit “energy created”. Correct the label for a carbon-cycle diagram.
Give me a hint
Name the material transferred and the process.
Compare my reasoning
- The rabbit eats plant material.
- Carbon-containing organic molecules transfer by feeding.
Label it “carbon-containing organic material transferred by feeding”; the arrow does not show energy being created.
Look for these in your work
- Carbon matter named
- Feeding process named
A cycle diagram has no timescale. Can you conclude fossil-fuel formation is as fast as photosynthesis? Explain.
Give me a hint
Do not infer rate from a cycle drawing.
Compare my reasoning
- The diagram shows processes and transfers.
- Fossil-fuel formation takes place over geological timescales.
No. A cycle diagram does not show equal rates; fossil-fuel formation occurs over geological timescales, unlike the much faster biological transfers.
Look for these in your work
- No unsupported rate conclusion
- Timescale distinguished
Common mistakes
- Calling the cycle an energy cycle
- Leaving the process off an arrow
- Implying all carbon follows one route or one timescale
What can you explain now?
What is being transferred around the carbon cycle?
Compare with the explanation
Carbon atoms in carbon dioxide and organic material
The cycle tracks matter through compounds.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
List the six carbon-cycle processes required here and trace one complete route for carbon from air through organisms back to air.
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.