Practical - Using a Potometer (OCR Gateway GCSE Biology: Combined Science)

Revision Note

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Biology Lead

Measuring Transpiration

  • There are several environmental conditions that have an impact on the rate of transpiration or water uptake
    • Air movement
    • Humidity
    • Light intensity
    • Temperature
  • The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low

Factors Affecting the Rate of Transpiration Table

Transpiration factors table, downloadable IGCSE & GCSE Biology revision notes

Investigating the effect of environmental factors on the rate of transpiration

  • We can investigate the effect of different environmental conditions (such as temperature, humidity, light intensity and wind movement) on the rate of transpiration using a piece of apparatus called a potometer
  • There are 2 types of potometer:
    • A mass potometer measures a change in mass of a plant as a measure of the amount of water that has evaporated from the leaves and stem
    • A bubble potometer measures the uptake of water by a stem as a measure of the amount of water that is being lost by evaporation consequently pulling water up through the stem to replace it

Mass potometer or bubble potometer 1, downloadable IGCSE & GCSE Biology revision notes Mass-potometer-or-bubble-potometer-2_1, downloadable IGCSE & GCSE Biology revision notes

There are 2 different types of potometer that could be used to investigate the effect of environmental conditions on transpiration


  • Potometer (bubble or mass potometer)
  • Timer
  • Lamp
  • Ruler
  • Plant


  • Cut a shoot underwater
    • To prevent air entering the xylem and place in tube
  • Set up the apparatus as shown in the diagram and make sure it is airtight, using Vaseline to seal any gaps
  • Dry the leaves of the shoot
    • Wet leaves will affect the results
  • Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
  • Set up a lamp 10 cm from the leaf
  • Allow the plant to adapt to the new environment for 5 minutes
  • Record the starting location of the air bubble
  • Leave for 30 minutes
  • Record the end location of the air bubble
  • Change the light intensity
  • Reset the bubble by opening the tap below the reservoir
  • Repeat the experiment
  • Calculate the rate of transpiration by dividing the distance the bubble travelled by the time period
    • The further the bubble travels in the same time period, the greater the rate of transpiration

Rate of transpiration calculation_1, downloadable IGCSE & GCSE Biology revision notes

Calculating the rate of transpiration using a bubble potometer

Investigating transpiration rates using a potometer, IGCSE & GCSE Biology revision notes

Investigating transpiration rates using a potometer

  • Other environmental factors can be investigated in the following ways:
    • Airflow: Set up a fan or hairdryer to blow air over the plant (this investigation can be extended by putting the fan at different distances from the plant or at different fan-speed settings)
    • Humidity: Spray water in a plastic bag and enclose the plant within the bag
    • Temperature: Change the temperature of room (e.g. cold room or warm room)


  • As light intensity increases, the rate of transpiration increases
  • This is shown by the bubble moving a greater distance in the 30 minute time period when the lamp was placed closer to the leaf
  • Transpiration rate increases with light intensity because more stomata tend to be open in bright light in order to maximise photosynthesis
  • The more stomata that are open, the more water can be lost by evaporation and diffusion through the stomatal pores


  • The potometer equipment has a leak
    • Solution: Ensure that all equipment fits together rightly around the rubber bungs and assemble underwater to help produce a good seal
  • The plant cutting has a blockage
    • Solution: Cut the stem underwater and assemble equipment underwater to minimise opportunities for air bubbles to enter the xylem
  • The potometer has shown no change during the experiment
    • Solution: Use the plant cuttings as soon as they have been cut, as transpiration rates may slow down when the cuttings are no longer fresh

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Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.