Fix carbon dioxide by photosynthesis
Photosynthesis transfers carbon from atmospheric carbon dioxide into organic molecules in producers.
Section B15.3 · Carbon cycle
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and 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 happens to carbon dioxide carbon during photosynthesis?
What you will learn
- Explain the carbon transfer during photosynthesis
- Distinguish incorporation of carbon from release of carbon dioxide
Carbon enters organic material
During photosynthesis, a producer takes in carbon dioxide and uses it to make organic molecules. Carbon atoms from carbon dioxide become part of substances such as carbohydrates in plant material. This is the carbon-cycle route from the atmosphere into living biomass.
The word “fixation” may be used for incorporating carbon into organic molecules, but the required process here is photosynthesis. A plant also respires; photosynthesis does not mean it permanently removes all carbon dioxide or stops carbon cycling.
Label the transfer direction
For the carbon route, draw carbon dioxide in air → plant organic material and label the arrow photosynthesis. Keep the carbon atoms as matter; light supplies energy for the process but is not the source of the carbon atoms.
Avoid saying photosynthesis creates carbon atoms or energy. It rearranges matter into organic molecules and transfers energy into chemical stores.
Sources for this lesson
Pause and explain
Which arrow label fits carbon dioxide entering a plant organic molecule?
Worked example
A leaf takes in carbon dioxide during photosynthesis. Where can the carbon atoms be found after they enter organic molecules?
Show the worked solution
- Carbon dioxide contains carbon atoms.
- Photosynthesis incorporates them into plant organic material.
Answer The carbon atoms from carbon dioxide become part of organic molecules in the plant, such as carbohydrates.
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.
Write the carbon-cycle arrow from air to a green plant, including the process and destination.
Give me a hint
Name the gas and where its carbon goes.
Compare my reasoning
- Carbon dioxide is in the air.
- Photosynthesis incorporates its carbon into organic plant material.
Carbon dioxide in air → plant organic material by photosynthesis.
Look for these in your work
- Source is carbon dioxide
- Destination is plant organic material
Why is “light turns into carbon in a leaf” incorrect?
Give me a hint
Separate the energy input from the matter source.
Compare my reasoning
- Light supplies energy for photosynthesis.
- Carbon atoms come from carbon dioxide.
Light provides energy, while carbon atoms in plant organic molecules come from carbon dioxide; light does not turn into carbon.
Look for these in your work
- Energy source separated
- Carbon source identified
A plant is placed in light and absorbs carbon dioxide while photosynthesising. What can you conclude about carbon incorporation, and what cannot be inferred about its net carbon balance without more data?
Give me a hint
Photosynthesis incorporates carbon; respiration also releases carbon dioxide.
Compare my reasoning
- Carbon enters organic material in photosynthesis.
- The plant also respires.
- Rates are needed to compare net exchange.
Photosynthesis incorporates carbon dioxide carbon into organic molecules. Without comparing photosynthesis and respiration rates over the interval, the net carbon balance cannot be determined.
Look for these in your work
- Photosynthetic incorporation stated
- Net-balance limit stated
Common mistakes
- Saying sunlight supplies carbon atoms
- Saying plants only photosynthesise and do not respire
- Treating carbon fixation as creation of matter
What can you explain now?
What happens to carbon dioxide carbon during photosynthesis?
Compare with the explanation
It becomes part of organic molecules in the producer
Photosynthesis transfers carbon into plant material.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain where the carbon atoms go when a producer photosynthesises, and identify the separate role of light.
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.