Limiting Factors Practical

Aim:  To investigate the effect of light intensity on photosynthesis.


Independent variable - the light intensity (how close the light is).

Dependant variable - the number of oxygen bubbles given off (the rate of photosynthesis).

Controlled variables – species of pondweed, the size of the pondweed, the volume of water used, temperature of the water, pH of the water, the amount of time bubbles were counted for, the amount of time the plant was given to adjust to new light position.


  1. 1. Set up your apparatus as shown on the right.

  2. Position the light 80cm from the pondweed and allow 2 minutes for the pondweed to adjust.

  3. Count the number of bubbles released over a one-minute period.

  4. Move the lamp 20cms closer and repeat steps 1 and 2.

  5. Continue at further intervals of 20 cm up to 0cm.

  6. Record your results a table and use these to plot a graph.

Table to show how light intensity affects the rate of photosynthesis
Graph to show how light intensity affects the rate of photosynthesis


More oxygen bubbles are produced when the light is closer to the beaker. So a greater light intensity gives a greater rate of photosynthesis.


Fair test: only change one factor and keep all the others the same – so you only change the position of the lamp.

Three improvements: 

Use a gas syringe to collect the oxygen and measure volume. 

Use a blue lamp instead of a white lamp – blue light increases vegetative growth. 

Use a sodium hydrogencarbonate solution instead of water to increase the availability of carbon dioxide.

If you are using tap water, leave the water to stand overnight to remove the chlorine as chlorine will reduce the rate of photosynthesis.  

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All images sourced from unless otherwise stated