In this topic of IBDP Biology, we will learn about Factors affecting rate of transpiration & Potometers.

Environmental factors affect the rate of transpiration:

Temperature:

• Increases kinetic energy of water molecules.
• When it comes to IBDP Biology, this leads to increased evaporation from spongy cells and increased diffusion out of the leaf.
• Rate of transpiration increases

Light:

• Increasing light intensity increases rate of transpiration:
  • Stimulates stomata to open for photosynthesis.
    • Enabling more water vapour to difuse out of the leaf
  • It also heats up the leaf
    • This increase rate of evaporation and diffusion.

Humidity:

• When there is high humidity, there is a lower water vapour concentration gradient from air spaces out of the stomata.
• As humidity increases, rate of transpiration decreases

Air movements:

• When it comes to IBDP Biology, Water diffuse out of the leaf through the stomata down a concentration gradient.
• In still air, rate of diffusion is slow.
• Water vapour tends to accumulate in ‘shells’ of decreasing concentration around each stomata.
• If these are disturbed, water vapour concentration gradients are increased.
• Therefore, as air movements increase, transpiration rate increases.

• When it comes to IBDP Biology, The rate of transpiration can be measured in the lab using a potometer.
•  A potometer actually measures the rate of water uptake by the cut stem, not the rate of transpiration; and these two are not always the same.
• The potometer can be used to investigate how various environmental factors affect the rate of transpiration.
• Potometers come in a variety of designs, but all follow the same basic principle:
  • A length of capillary tube. A bubble is introduced to the capillary; as water is taken up by the plant, the bubble moves. By marking regular gradations on the tube, it is possible to measure water uptake.
  • A reservoir. A funnel with a tap or a syringe
  • This can be used to reset the bubble between meaurements.
  • The shoot must be held in contact with the water.
  • A water tight seal around the leafy shoot using a rubber bung greased with petroleum jelly suffices or rubber tubing.

Preparation

• Immerse the whole of the potometer into the sink.
  • This removes any air bubbles.
• Cut the shoot's stem underwater.
  • This prevents the xylem from taking up any air.
• Put it into the bung; grease the bung with plenty of petroleum jelly (Vaseline).

This prevents air getting in

• Leave the end of the capillary tube out of the water until an air bubble forms then put the end into a beaker of water.

Use

1. Set up the conditions of the experiment. Alterations to lighting (placing the plant in bright light or shadow), wind (directing a fan at the plant), and humidity (placing the plant in a humid chamber) are typical independent variables.
2. Control all other variables.
3. Let the bubble reach a "zero" point in the tube.
4. Measure the distance moved by the bubble over a specific time period.
5. Calculate the water uptake in millimeters per minute.
6. Repeat several times and calculate a mean – this improves reliablility.
7. Carry out for each level of the independent variable.

That's the end of this topic.

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