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What this skill means
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.
This investigation is one of the most commonly examined practicals in IGCSE Biology. You need to understand how to set it up, what to measure, how to control variables, and how to evaluate the method: all of which are tested in Papers 5 and 6.
The Standard Method
Place a piece of aquatic plant (such as Elodea or Cabomba) in a beaker of water under a lamp. As the plant photosynthesises, it releases oxygen as bubbles. Count the number of bubbles produced in a set time (e.g. One minute) at different distances of the lamp from the plant.
Moving the lamp closer increases light intensity. The inverse square law means light intensity is proportional to 1/d², where d is the distance from the lamp to the plant.
Variables
Independent variable: light intensity (changed by altering the distance between the lamp and the plant).
Dependent variable: rate of photosynthesis, measured as the number of oxygen bubbles per minute (or volume of gas collected using a gas syringe for greater accuracy).
Controlled variables:
Temperature: use a water bath or a heat shield (glass tank of water) between the lamp and the plant to absorb heat from the lamp
CO₂ concentration: add sodium hydrogen carbonate to the water to provide a constant supply of CO₂
Same piece of plant, same length, same species
Time period for counting (e.g. Always one minute)
Sources of Error and Improvements
Errors:
Bubbles vary in size: counting bubbles does not give an accurate measure of volume of gas
Some oxygen dissolves in the water rather than forming visible bubbles
The lamp produces heat as well as light, which could change the temperature
Ambient light from the room may affect results
Improvements:
Use a gas syringe or graduated tube to measure volume of gas instead of counting bubbles
Place a heat shield (water-filled glass tank) between lamp and plant to absorb heat
Carry out the experiment in a darkened room to eliminate ambient light
Allow 2 minutes for the plant to acclimatise at each new distance before starting to count
Use this before submitting your answer
A complete answer is specific enough for another person to repeat or assess.
✓Use at least five different distances (e.g. 10, 20, 30, 40, 50 cm).
✓Add sodium hydrogen carbonate to the water for a constant CO₂ supply.
✓Use a heat shield between the lamp and the plant.
✓Count bubbles for the same time period at each distance.
✓Repeat each distance at least three times and calculate a mean.
✓Allow acclimatisation time (2 minutes) before counting at each new distance.
✓Record room temperature and keep it constant.
Try the skill in a question (3 marks)
A student counts the number of bubbles from an aquatic plant at different lamp distances. At 10 cm the count is 42 bubbles per minute. At 50 cm the count drops to 6 bubbles per minute. Explain this result. [3]
Show the answer and marking guidance
At 10 cm the light intensity is much higher than at 50 cm because light intensity decreases with the square of the distance
Higher light intensity increases the rate of photosynthesis because more light energy is available for the light-dependent reactions
More photosynthesis produces more oxygen, which is released as more bubbles per minute
Original practice content written for this site. It is not an official Cambridge question.