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Key concept
The rate of photosynthesis can be measured by counting oxygen bubbles produced by pondweed per minute, or by measuring the volume of oxygen collected. Light intensity, CO₂ concentration and temperature can be varied as independent variables.
This is one of the most commonly examined practicals in IGCSE Biology. You need to describe the method, identify variables, interpret graphs, and explain results in terms of limiting factors. This guide covers everything for Papers 2, 4 and 6.
The Pondweed Experiment
Method:
Place a piece of aquatic plant (Elodea / pondweed) in a beaker of water.
Add sodium hydrogen carbonate (NaHCO₃) to the water to provide a constant supply of CO₂.
Place a lamp at a measured distance from the plant.
Wait 2 minutes for the plant to acclimatise.
Count the number of oxygen bubbles released per minute, or collect the gas and measure its volume.
Repeat at different distances (to vary light intensity) or with different concentrations of NaHCO₃ (to vary CO₂).
Measuring rate: Rate of photosynthesis = number of bubbles per minute OR volume of gas collected per minute.
Controlling Variables
Variable type
Examples
Independent variable (changed)
Light intensity (change lamp distance), CO₂ concentration (change NaHCO₃ amount), temperature (use water bath)
Dependent variable (measured)
Rate of photosynthesis (bubbles per minute or volume of O₂)
Controlled variables
Temperature, CO₂ concentration (if not IV), volume of water, same piece of pondweed, same counting time
Light intensity and distance: Light intensity is inversely proportional to the square of the distance from the lamp (inverse square law). Moving the lamp from 10 cm to 20 cm reduces light intensity to one-quarter, not one-half.
Interpreting Results
The results graph typically shows:
Rate increases as light intensity (or CO₂) increases: more light energy drives more photosynthesis.
Rate then levels off (plateaus): another factor becomes the limiting factor.
If light intensity is the independent variable and rate plateaus, ask: "What is now limiting?" → It could be CO₂ concentration or temperature.
Improving the experiment:
Repeat each measurement 3 times and calculate a mean: increases reliability.
Use a gas syringe instead of counting bubbles: more accurate volume measurement.
Use a coloured filter between the lamp and the plant: to eliminate heat (infrared) from the lamp.
Try a focused question (3 marks)
A student counts oxygen bubbles from pondweed at different light intensities. At low intensities, the count increases. At high intensities, the count stays the same. Explain the plateau. (3 marks)
Show the answer and marking guidance
At low light intensities, light is the limiting factor: increasing it increases the rate.
At high light intensities, another factor becomes the limiting factor (e.g. CO₂ concentration or temperature).
Even though light intensity increases further, the rate cannot increase because the other factor limits the reaction.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Forgetting to add NaHCO₃: without it, CO₂ becomes limiting very quickly.
Not allowing acclimatisation time: the plant needs time to adjust to new conditions before counting.
Counting bubbles as a measure of "oxygen": bubble size varies; collecting and measuring volume is more accurate.
Practical connection
Investigating the Rate of Photosynthesis
The rate of photosynthesis can be measured by counting oxygen bubbles produced by an aquatic plant (e.g. Elodea) per minute at different light intensities, temperatures, or CO₂ concentrations.