Factors that affect the rate of transpiration
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the structure and function of root hair cells, xylem and stomata, as well as the process of osmosis and active transport.
What is the transpiration stream?
The movement of water and dissolved mineral ions up the plant from the roots to the leaves.
What does xylem tissue transport, and how is it adapted?
It transports water and mineral ions and is made of hollow, lignified tubes to allow one-way water flow.
What is the role of stomata in plants?
They control gas exchange and water loss by opening and closing.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the factors that affect the rate of transpiration, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
What is Transpiration?
Transpiration is the loss of water vapour from a plant’s leaves through the stomata, driven by evaporation of water from mesophyll cells and its diffusion through the stomata into the air.
Water is pulled up the plant via the transpiration stream in the xylem.
Factors that Affect the Rate of Transpiration
|
Factor |
Effect |
Why? |
|
Temperature |
↑ Temp = ↑ transpiration rate |
Warmer air increases evaporation of water from leaf surfaces. |
|
Humidity |
↓ Humidity = ↑ transpiration rate |
Dry air creates a steeper water vapour concentration gradient, increasing diffusion rate. |
|
Air movement |
↑ Wind = ↑ transpiration rate |
Wind removes humid air from leaf surface, creating a steeper water vapour concentration gradient, increasing diffusion rate. |
|
Light intensity |
↑ Light = ↑ transpiration rate |
More light means more photosynthesis so the stomata open to allow in CO2 so there is more water loss. |
Stomatal Opening & Photosynthesis
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Stomata open in light to allow carbon dioxide in for photosynthesis.
-
Guard cells control the opening and closing of stomata
-
When water moves into guard cells, by osmosis, guard cells become turgid, and bend outwards to open the stomata.
-
At night, there is less light so less photosynthesis. Guard cells close stomata to conserve water.
Key Terms
-
Transpiration: Loss of water vapour from leaves.
-
Transpiration stream: movement of water from roots to leaves, through the xylem.
No answer provided.
Exam Tip
In explain questions, always state how the factor changes water loss and why (e.g. increased temperature leads to increased evaporation of water).
No answer provided.
Practice Question
Explain why the rate of transpiration increases on a hot, windy day. (3 marks)
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Higher temperature increases evaporation of water from the leaf surface.
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Wind blows away moist air, maintaining a steep diffusion gradient.
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More water vapour diffuses out of the stomata.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!