Transpiration (AQA GCSE Biology Combined Science): Revision Notes
Transpiration
What is transpiration?
Transpiration is when water moves from a plant's roots to its leaves and then evaporates into the air. Think of it like the plant breathing out water vapour.
This process happens continuously when the plant is alive. Water enters through the roots and travels up through the plant to reach the leaves, where it escapes as water vapour.
Transpiration is essential for plant survival as it helps transport nutrients and maintains the plant's structure. Without this process, plants would not be able to move water and minerals from their roots to where they're needed most.
The transpiration stream
The transpiration stream is the continuous flow of water through a plant. Here's how it works:
- Water enters the roots - Root hair cells absorb water from soil by osmosis
- Water travels up the stem - Water moves through special tubes called xylem
- Water reaches the leaves - Water spreads through the leaf tissues
- Water evaporates - Water turns into vapour and escapes through tiny holes called stomata
The transpiration stream also carries dissolved mineral ions that the plant needs to grow. These minerals get absorbed by the roots and travel up with the water.
The transpiration stream serves a dual purpose: it not only helps cool the plant through evaporation but also acts as the plant's transport system for essential nutrients and minerals.
Stomata and how they work
Stomata are tiny openings found mainly on the bottom surface of leaves. Each stoma (single opening) is surrounded by two guard cells.
How stomata open and close
Guard cells can change shape to control whether stomata are open or closed:
- When guard cells take in water - They become swollen and curved, making the stoma open
- When guard cells lose water - They become limp and straight, making the stoma close
This happens because of osmosis - water moves into and out of the guard cells.
The opening and closing of stomata is like a sophisticated valve system that plants use to balance their need for gas exchange with their need to conserve water.
Why stomata position matters
Most stomata are on the lower surface of leaves because:
- Less direct sunlight hits the bottom of leaves
- This reduces water loss through evaporation
- The plant can still exchange gases for photosynthesis
The strategic positioning of stomata on the lower leaf surface is an evolutionary adaptation that helps plants survive in various environmental conditions by minimising water loss while maintaining essential gas exchange.
Water loss from different leaf surfaces
When scientists tested leaves with different surfaces covered, they discovered important patterns about where water loss occurs:
Experimental Results: Water Loss from Leaf Surfaces
Method: Scientists covered different parts of leaves and measured water loss over time.
Results:
- Upper surface only covered - Lost lots of water (40% mass loss)
- Lower surface only covered - Lost very little water (4% mass loss)
- Both surfaces covered - Lost almost no water (2% mass loss)
Conclusion: This proves that most water loss happens through stomata on the lower leaf surface.
Summary
Key Points to Remember:
- Transpiration moves water from roots → stems → leaves → air
- The transpiration stream carries water and minerals through the plant
- Stomata are tiny holes controlled by guard cells
- Most stomata are on the bottom of leaves to reduce water loss
- Guard cells swell with water to open stomata and shrink to close them
- The process relies on osmosis to control water movement