Transpiration and Its Measurement

Overview

Recall questions on why transpiration is unavoidable, how light, temperature, humidity and air movement change its rate, what a potometer actually measures, and comparing shoots of different sizes.

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

   Define transpiration.

   Model answer

   It is the loss of water vapour from a plant, mainly by evaporation from the mesophyll cell walls followed by diffusion out through the stomata.

2. 2

   Why is transpiration an unavoidable consequence of gas exchange in a leaf?

   Model answer

   The stomata have to be open to let carbon dioxide diffuse in for photosynthesis, and water vapour inevitably diffuses out through the same open pores.

3. 3

   What effect does increasing light intensity have on the rate of transpiration, and why?

   Model answer

   The rate increases, because the stomata open more widely in the light so there is a greater area through which water vapour can diffuse out.

4. 4

   What effect does increasing temperature have on the rate of transpiration, and why?

   Model answer

   The rate increases, because water molecules gain kinetic energy so evaporation is faster, and warmer air can hold more water vapour so the gradient out of the leaf is steeper.

5. 5

   What effect does increasing humidity have on the rate of transpiration, and why?

   Model answer

   The rate decreases, because the air outside already holds a lot of water vapour so the water potential gradient between the air spaces and the atmosphere is shallower.

6. 6

   What effect does air movement have on the rate of transpiration, and why?

   Model answer

   Wind increases the rate, because it blows away the humid layer of air held next to the leaf surface and so keeps the diffusion gradient steep.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.