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IBDP Biology: Factors affecting rate of transpiration & Potometers

Factors affecting rate of transpiration & Potometers

· biology,IBDP BIOLOGY,ib biology,Transpiration,potometers

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

Factors affecting rate of transpiration

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.

Potometers

  • 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.
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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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