Investigate limiting factors in whole plants
Plan whole-plant or aquatic-plant tests of light intensity, carbon dioxide concentration and temperature.
Section 13.2 · Limiting factors
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- Chloroplasts contain photosynthetic pigments, and enzymes catalyse metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
If testing carbon dioxide concentration, which factor should be varied systematically?
What you will learn
- Describe whole-plant measurements for photosynthesis rate
- Control conditions and evaluate indirect gas-rate measures
Build the explanation
Whole plants, including aquatic plants, can be used to investigate how light intensity, carbon dioxide concentration or temperature affects photosynthesis rate. Measure oxygen production or carbon dioxide uptake over a fixed time using a suitable gas collector or sensor, then calculate a rate. Change one factor at a time, keep plant species and size, water supply, light spectrum and other conditions consistent, and allow the sample to equilibrate. Repeat each condition and use a control where possible.
Apply it carefully
Counting bubbles is a convenient proxy but bubble sizes vary, so it is less reliable than collected gas volume or a calibrated sensor. A rise in oxygen output is an indirect measure and net gas exchange also reflects respiration. Prevent lamp heating when changing light distance, and distinguish the measured range from a universal optimum.
Pause and explain
Why can counting bubbles give a less reliable gas-rate estimate than collecting volume?
Worked example
A plant releases 9 cm³ oxygen in 3 minutes. Calculate its mean oxygen-release rate.
Show the worked solution
- Divide gas volume by elapsed time.
- 9 ÷ 3 = 3 cm³ per minute.
Answer 3 cm³ min⁻¹.
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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.
Outline a whole-plant method to test how carbon dioxide concentration affects photosynthesis rate.
Give me a hint
Include the carbon dioxide treatment, gas measurement, controls and repeats.
Compare my reasoning
- Expose matched plants to different CO₂ concentrations at the same light and temperature.
- Measure oxygen volume or CO₂ exchange over fixed time, calculate rate and repeat conditions.
Use matched whole plants or aquatic shoots with different controlled carbon dioxide concentrations. Keep light intensity, temperature, species, shoot size and time constant. Measure oxygen production or carbon dioxide uptake with a suitable sensor or collected gas volume, calculate rate and repeat each concentration. Use a control and supervised handling.
Look for these in your work
- CO₂ varied
- Other factors controlled
- Direct gas measurement described
- Rate and repeats stated
An aquatic plant produces 18 cm³ oxygen in 6 minutes at one light intensity and 30 cm³ in 6 minutes at another. Calculate both rates and compare them.
Give me a hint
Divide each volume by the same time.
Compare my reasoning
- 18 ÷ 6 = 3 cm³/min.
- 30 ÷ 6 = 5 cm³/min; the second measured rate is 2 cm³/min higher.
The first rate is 3 cm³ min⁻¹ and the second is 5 cm³ min⁻¹. The second measured rate is 2 cm³ min⁻¹ greater under the tested conditions.
Look for these in your work
- Both calculations correct
- Units included
- Comparison bounded to conditions
A student moves a lamp closer to increase intensity and observes more bubbles. Explain one confound and one measurement limitation.
Give me a hint
Consider lamp heating and variable bubble volume.
Compare my reasoning
- The closer lamp may warm the plant or water, changing temperature.
- Bubble counts do not give a precise gas volume because bubble size varies.
Moving the lamp closer may also raise temperature, so light intensity is not isolated unless temperature is controlled. Bubble counts are an imprecise rate measure because bubble volume varies; a gas syringe, collector or sensor can measure volume more directly.
Look for these in your work
- Temperature confound identified
- Bubble-size limitation stated
- Improved measurement suggested
Common mistakes
- Changing light distance without controlling temperature
- Treating bubbles as equal volumes
- Calling net oxygen output a direct carbohydrate measurement
What can you explain now?
If testing carbon dioxide concentration, which factor should be varied systematically?
Compare with the explanation
Carbon dioxide concentration
Other conditions such as light and temperature should be controlled.
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Make it stick
How can a whole-plant experiment compare photosynthesis rates fairly and reliably?
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.