Explain light and dark net gas exchange
Compare photosynthesis and respiration rather than assuming one switches the other off.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does a plant respire in bright light?
What you will learn
- Compare photosynthesis and respiration rather than assuming one switches the other off.
- Use the stated evidence to explain the method and its limits.
Read the balance
An aquatic plant respires in light and dark. In darkness, photosynthesis lacks its light-energy supply while respiration can continue releasing carbon dioxide. In the supplied working indicator comparison, that can move the colour toward yellow.
In suitable bright light, photosynthetic carbon dioxide uptake can exceed respiratory release, giving a net carbon dioxide decrease and a purple indicator result. In dim light the processes may balance or respiration may exceed uptake; light by itself does not guarantee purple.
Avoid a zero-process inference
An unchanged red comparison can mean no detectable net carbon dioxide change. It does not demonstrate that neither photosynthesis nor respiration occurred: equal opposite changes can balance. Dead material, a failed indicator or insufficient time can also yield no observable change, so the controls and specimen condition matter.
For a supplied balance calculation, net carbon dioxide removal equals photosynthetic uptake minus respiratory release. Arbitrary teaching units illustrate the balance; a colour name alone does not provide those numerical rates. Positive net removal and positive net release must be labelled with their direction.
Sources for this lesson
Pause and explain
What can an unchanged working indicator show?
Worked example
A supplied lit-plant model takes up 5 units CO₂/min by photosynthesis and releases 2 units CO₂/min by respiration. State net exchange.
Show the worked solution
- Both processes occur.
- Subtract 5 − 2 = 3.
- Label the result as net carbon dioxide removal.
Answer Net removal is 3 units CO₂/min in the supplied model.
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.
In the supplied model, uptake is 4 and release is 1 unit CO₂/min. State net exchange.
Give me a hint
Subtract opposing changes and label direction.
Compare my reasoning
- 4 − 1 = 3.
- Uptake exceeds release.
Net removal of 3 units CO₂/min.
Look for these in your work
- Correct subtraction.
- Correct direction.
In dim light, photosynthetic uptake is 2 and respiratory release is 3 units CO₂/min. Predict the net direction and the supplied indicator tendency.
Give me a hint
Release exceeds uptake.
Compare my reasoning
- 3 − 2 = 1 unit/min net release.
- Carbon dioxide rises.
- The supplied key tends toward yellow.
Net release of 1 unit CO₂/min; a tendency toward yellow under working indicator conditions.
Look for these in your work
- Compare both rates.
- Connect higher carbon dioxide to the key.
A healthy plant tube stays red in dim light while controls work. Explain why “the plant did nothing” is too strong.
Give me a hint
Consider balanced opposite processes.
Compare my reasoning
- Photosynthetic uptake and respiratory release can balance.
- The indicator observes the net change.
- Numerical rates or other checks would be needed to distinguish alternatives.
Active photosynthesis and respiration may balance, leaving no detectable net colour change.
Look for these in your work
- Use a net-zero explanation.
- Retain alternative possibilities and measurement limits.
Common mistakes
- Purple does not mean respiration has stopped.
- Red unchanged colour does not prove both processes are absent.
What can you explain now?
What can an unchanged working indicator show?
Compare with the explanation
No detectable net carbon dioxide change.
Opposite process rates may balance.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain bright-light, dark and balance outcomes without switching respiration off.
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.