Investigate carbon dioxide availability and gas release
Use matched solutions without confusing added hydrogencarbonate with a measured gas concentration.
Section B6.1 · Photosynthesis
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
- 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
What supplies carbon to photosynthetic carbohydrate?
What you will learn
- Use matched solutions without confusing added hydrogencarbonate with a measured gas concentration.
- Use the stated evidence to explain the method and its limits.
Make the intended variable explicit
An aquatic-plant study can compare carbon dioxide availability while holding light, temperature, plant material, collection duration and apparatus matched. A teacher-approved hydrogencarbonate solution can provide a carbon dioxide source; prepare matched total volumes and record the solution concentrations.
Added hydrogencarbonate concentration is an experimental treatment, not automatically an exact dissolved carbon dioxide concentration. Equilibria and pH affect availability, so describe what was actually prepared or measured and recognise additional solution effects.
Interpret repeated evidence
Collect gas or count bubbles consistently after adjustment, repeat each treatment and calculate the mean rate with the appropriate units. Keep temperature measured and light fixed; stronger light in the richer solution would create a second explanation.
A gas-release increase with higher carbon dioxide availability is the supplied response. A plateau shows that more of the treatment no longer increases the measured rate over that range; it does not show that carbon dioxide has become chemically unnecessary. Uncontrolled pH or unequal plant quantities can also distort the comparison.
Sources for this lesson
Pause and explain
Why does a richer-solution treatment also receiving brighter light create a problem?
Worked example
Matched treatments collect mean gas volumes of 0.8 and 1.6 cm³ in 4 min. State rates and the treatment response.
Show the worked solution
- 0.8 ÷ 4 = 0.2 cm³/min.
- 1.6 ÷ 4 = 0.4 cm³/min.
- The richer treatment has twice the observed collection rate, under the stated controls.
Answer 0.2 and 0.4 cm³/min; the measured release doubles in the richer treatment.
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 sample collects 1.5 cm³ gas in 5 min. Calculate the average collection rate.
Give me a hint
Use volume divided by minutes.
Compare my reasoning
- 1.5 ÷ 5 = 0.3.
- Keep cm³/min.
0.3 cm³/min.
Look for these in your work
- Correct rate.
- Correct units.
At three matched solution treatments, mean rates are 0.2, 0.4 and 0.4 cm³/min. Describe the supplied pattern without inventing unmeasured values.
Give me a hint
Compare increases and equality.
Compare my reasoning
- The first increase raises measured release.
- The next increase leaves it unchanged.
- The response reaches a plateau over the final tested interval.
The rate rises from 0.2 to 0.4 cm³/min, then remains at 0.4 cm³/min in the final tested interval.
Look for these in your work
- Use the stated data.
- Do not claim all future values are known.
Why should a report not rename “0.1% hydrogencarbonate solution” as “0.1% dissolved carbon dioxide” without measurement?
Give me a hint
Treatment identity and available gas are different quantities.
Compare my reasoning
- Hydrogencarbonate is the prepared solute.
- Dissolved carbon dioxide depends on solution conditions.
- Use the recorded treatment name and a measured gas value if available.
The prepared solute concentration is not an established dissolved carbon dioxide concentration.
Look for these in your work
- Identify the quantity mismatch.
- Use honest measurement labels.
Common mistakes
- Keep solution volume and plant quantity matched.
- Prepared hydrogencarbonate is not a direct carbon dioxide meter.
What can you explain now?
Why does a richer-solution treatment also receiving brighter light create a problem?
Compare with the explanation
Two possible rate-affecting variables changed.
The carbon-dioxide-only effect is not isolated.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe a controlled carbon dioxide comparison and one solution-related limitation.
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.