Practical - Using a Potometer (OCR GCSE Biology A (Gateway))

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

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

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

Apparatus

• Potometer (bubble or mass potometer)

• Timer

• Lamp

• Ruler

• Plant

Method

• 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

Calculating the rate of transpiration using a bubble potometer

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)

Results

• 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

Limitations

• 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