Return carbon by respiration
Plants and animals release carbon dioxide during respiration, transferring carbon from organic molecules back to the atmosphere.
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
Which process returns carbon dioxide from an animal’s organic molecules?
What you will learn
- Trace carbon from organism organic material to carbon dioxide
- Recognise respiration as a carbon-cycle return process
Respiration returns carbon to carbon dioxide
Plants and animals respire. During respiration, carbon-containing organic molecules are broken down and carbon dioxide is produced and released. This returns carbon from organisms to the atmosphere. The same organism can also photosynthesise if it is a plant, so these are separate processes in the carbon cycle.
Do not confuse respiration with breathing or gas exchange. In this syllabus carbon-cycle route, respiration is the cellular process that produces carbon dioxide. Breathing moves gases; it is not the process that releases carbon from organic molecules.
Trace the direction correctly
Draw plant or animal organic material → carbon dioxide in air and label the arrow respiration. Decomposers also respire; their role is considered in the decomposition lesson.
A carbon-cycle diagram can show the process without measuring its rate. Do not assume that a plant releases more carbon dioxide than it fixes without data on both respiration and photosynthesis.
Sources for this lesson
Pause and explain
What is the carbon-cycle direction of respiration?
Worked example
A rabbit respires after eating plant material. Describe the carbon transfer.
Show the worked solution
- The rabbit contains carbon in organic molecules.
- Respiration produces carbon dioxide.
Answer Carbon in rabbit organic material is transferred to carbon dioxide, which is released to the atmosphere during 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.
Label the carbon movement from a plant’s organic material to air.
Give me a hint
Name the process that produces carbon dioxide.
Compare my reasoning
- Plant organic molecules contain carbon.
- Respiration releases carbon dioxide.
Plant organic material → carbon dioxide in air by respiration.
Look for these in your work
- Direction correct
- Respiration named
A student writes “animals breathe carbon dioxide into existence.” Improve the explanation.
Give me a hint
Distinguish breathing from cellular respiration.
Compare my reasoning
- Respiration produces carbon dioxide from organic molecules.
- Breathing moves gases into and out of the body.
Animals produce carbon dioxide by respiration as carbon-containing organic molecules are broken down; breathing moves that gas between the body and surroundings.
Look for these in your work
- Respiration mechanism identified
- Processes distinguished
A leaf photosynthesises in daylight and respires. Does daylight switch off its respiration? Explain.
Give me a hint
Treat photosynthesis and respiration as distinct processes.
Compare my reasoning
- Plants respire.
- Daylight allows photosynthesis but does not make respiration cease.
No. A plant continues to respire; in daylight it may also photosynthesise. The net carbon-dioxide exchange depends on the rates of both processes.
Look for these in your work
- Respiration retained
- Net exchange qualified
Common mistakes
- Calling respiration the same process as breathing
- Showing carbon dioxide entering an animal by respiration
- Claiming a plant has no respiration
What can you explain now?
Which process returns carbon dioxide from an animal’s organic molecules?
Compare with the explanation
Respiration
Respiration produces carbon dioxide from organic material.
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Make it stick
Draw and explain the carbon-cycle arrow for respiration in a plant or animal.
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.