Transpiration (AQA GCSE Biology): Revision Notes
Transpiration
What is transpiration?
Transpiration is how plants lose water. Water moves from the roots up to the leaves, where it turns into water vapour and escapes into the air. This process happens constantly in living plants and is essential for their survival.
Think of transpiration like sweating in humans - it's how plants cool down and get rid of excess water. Just as humans sweat to regulate body temperature, plants use transpiration to maintain their internal water balance and temperature control.
The water doesn't just disappear randomly though. It follows a specific path through the plant, creating what we call the transpiration stream.
The transpiration stream
The transpiration stream describes how water flows through a plant from bottom to top. This process works like a one-way conveyor belt that never stops moving.
Here's how it works step by step:
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Water enters the roots: Water from the soil enters the plant through tiny root hair cells. This happens by osmosis, where water naturally moves from where there's lots of it (in the soil) to where there's less (inside the plant).
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Water travels up the stem: The water then moves up through special tubes called xylem vessels. These act like tiny straws running from the roots to the leaves.
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Water reaches the leaves: Once in the leaves, water travels through the leaf structure and reaches the surface.
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Water evaporates: Finally, water changes from liquid to gas and escapes through tiny openings called stomata.
What makes this system really clever is that it doesn't just transport water. The transpiration stream also carries important dissolved mineral ions that plants need to stay healthy. These minerals get picked up from the soil and delivered throughout the plant like a natural delivery system.
Stomata and how they work
Stomata are tiny holes found mainly on the undersides of leaves. You can think of them as the plant's breathing pores. Most water loss happens through these openings, making them crucial for controlling transpiration.
Each stoma (that's the singular form) is surrounded by two special cells called guard cells. These cells work like automatic doors, opening and closing the stoma depending on what the plant needs.
When stomata open:
- Guard cells take in water by osmosis
- The cells become swollen and curved
- This creates a gap (the stoma opens)
- Water vapour can escape, and gases can be exchanged
When stomata close:
- Guard cells lose water
- The cells become flatter and less curved
- The gap closes (stoma shuts)
- Less water is lost
This opening and closing system helps plants control how much water they lose. During hot, dry conditions, plants can close their stomata to prevent too much water loss - a crucial survival mechanism that prevents dehydration.
How we can study transpiration
Scientists often study transpiration by doing experiments with leaves. One common method involves cutting leaves and covering different surfaces with grease to see how this affects water loss.
Worked Example: Leaf Surface Experiment
Take four leaves and treat them differently:
- Leaf A: No grease (normal leaf)
- Leaf B: Grease on top surface only
- Leaf C: Grease on bottom surface only
- Leaf D: Grease on both surfaces
Results: Leaves with no grease or grease only on the top surface lose the most water. This proves that most transpiration happens through stomata on the bottom surface of leaves. Leaves with grease on the bottom surface (where most stomata are) lose much less water.
This experimental approach demonstrates that the bottom surface of leaves is where most water loss occurs, confirming the location and function of stomata.
Factors that affect transpiration
Several environmental factors can speed up or slow down transpiration:
- Temperature: Higher temperatures make water evaporate faster
- Humidity: Dry air increases transpiration rate
- Wind: Moving air carries away water vapour, increasing transpiration
- Light: Bright light causes stomata to open more, increasing water loss
Understanding these factors helps us predict when plants will lose more or less water. This knowledge is particularly important for farmers and gardeners who need to plan irrigation schedules and protect plants during extreme weather conditions.
Key Points to Remember:
- Transpiration is the loss of water from plants, mainly through leaves
- The transpiration stream carries water and minerals from roots to leaves through xylem vessels
- Most water loss happens through stomata on the bottom surface of leaves
- Guard cells control whether stomata are open or closed
- Environmental factors like temperature, humidity, wind and light affect how fast transpiration occurs