Investigate light intensity and photosynthetic gas release
Measure a defined rate while controlling heat and plant material.
Section B6.1 · Photosynthesis
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
Which factor should be kept matched while light is varied?
What you will learn
- Measure a defined rate while controlling heat and plant material.
- Use the stated evidence to explain the method and its limits.
Change one factor and allow adjustment
A teacher-supervised aquatic-plant investigation can vary measured light intensity while keeping carbon dioxide availability, water temperature, plant material, apparatus and collection time matched. Allow the plant to adjust at each setting before recording repeated observations.
Lamp distance is a useful setting, but distance is not itself a calibrated light-intensity measurement. Ambient light and lamp geometry affect the relationship. A light meter strengthens an intensity comparison; moving a warm lamp can also heat the water, which needs measuring and control.
Describe the measured response
Collecting gas volume over a stated interval gives a rate in cm³/min. Repeated bubble counts can provide a rough proxy, but unequal bubble sizes mean bubbles/min is not automatically proportional to volume or an exact total photosynthesis rate.
Plot measured intensity on the horizontal axis and the mean gas-release rate on the vertical axis. A rise followed by a plateau describes the supplied evidence; do not extrapolate unlimited increases beyond the measurements. Other lesson sequences distinguish the measured net release from the plant’s total photosynthetic production.
Sources for this lesson
Pause and explain
Which graph axis placement fits this investigation?
Worked example
At two measured light settings, matched plants release 1.2 and 2.4 cm³ gas in 4 min. Compare average collection rates.
Show the worked solution
- Calculate 1.2 ÷ 4 = 0.3 cm³/min.
- Calculate 2.4 ÷ 4 = 0.6 cm³/min.
- Compare measured release: twice the first rate.
Answer The average gas-release rates are 0.3 and 0.6 cm³/min, a twofold increase.
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 plant releases 1.8 cm³ gas in 3 min. Calculate the average gas-release rate.
Give me a hint
Divide volume by time.
Compare my reasoning
- 1.8 ÷ 3 = 0.6.
- Use cm³/min.
0.6 cm³/min.
Look for these in your work
- Correct direction of division.
- Correct units.
Repeated one-minute counts at the same light setting are 16, 18 and 20 bubbles. Calculate the mean count rate.
Give me a hint
Add the three counts and divide by three.
Compare my reasoning
- 16 + 18 + 20 = 54.
- 54 ÷ 3 = 18.
18 bubbles/min; a count proxy rather than a calibrated gas-volume rate.
Look for these in your work
- Mean of all repeats.
- Retain proxy units.
A lamp is moved closer and the rate rises, but water temperature also rises by 8 °C. What can and cannot be concluded?
Give me a hint
Two factors changed.
Compare my reasoning
- The measured gas-release rate increased.
- Light and temperature both differ.
- Control temperature before attributing the change to light alone.
The observed increase is real in the supplied data, but a light-only cause is not isolated.
Look for these in your work
- Separate observation and cause.
- Specify temperature control.
Common mistakes
- A nearer lamp can change temperature as well as light.
- Bubble counts are a proxy with size-related limits.
What can you explain now?
Which graph axis placement fits this investigation?
Compare with the explanation
Measured light intensity on x; mean gas-release rate on y.
The changed factor and measured response are separated.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Plan the intensity comparison with a rate, units, repeats and temperature control.
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.