Factors affecting the rate of transpiration

Overview

How light, wind, temperature and humidity affect water uptake, setting up and using a potometer, why uptake only estimates transpiration, calculating rates per minute, per hour, by volume and per unit of leaf area, and plotting graphs, lines of best fit, gradients, intercepts and y = mx + c.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Explain the effect of increasing light intensity on the rate of water uptake by a plant.

   Model answer

   Uptake increases, because the stomata open wider in bright light so more water vapour is lost from the leaves.

2. 2

   Explain why a plant takes up water faster on a windy day.

   Model answer

   Moving air sweeps water vapour away from the leaf surface, keeping the concentration gradient steep so water is lost and replaced more quickly.

3. 3

   Explain the effect of a rise in temperature on the rate of water uptake by a plant.

   Model answer

   Water molecules gain energy, so they evaporate from the leaf cells and diffuse out of the stomata faster, and more water is drawn up to replace them.

4. 4

   Explain why a plant kept in very humid air takes up water slowly.

   Model answer

   The air outside the leaf is already full of water vapour, so the concentration gradient is small and little water diffuses out.

5. 5

   Name the apparatus used to measure the rate of water uptake by a leafy shoot.

   Model answer

   A potometer.

6. 6

   Describe how a potometer is used to measure the rate of water uptake.

   Model answer

   The distance that an air bubble travels along the capillary tube in a measured time is recorded, showing how much water the shoot has taken up.

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