Transpiration

Transpiration is the loss of water vapour from the aerial parts of a plant, predominantly through the stomata of the leaves. While transpiration is an inevitable consequence of gas exchange, it plays a vital role in driving the movement of water through the plant. This lesson covers the factors that affect transpiration rate, the potometer practical, guard cell function and xerophyte adaptations, as required by Edexcel GCSE Biology (1BI0) Topic 6.

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What is Transpiration?

Transpiration is defined as the loss of water vapour from the surfaces of a plant, mainly through the stomata in the leaves.

The Process

1. Water evaporates from the surfaces of mesophyll cells inside the leaf
2. The water vapour collects in the air spaces in the spongy mesophyll
3. The water vapour diffuses out through the open stomata into the surrounding atmosphere
4. This diffusion occurs because the air inside the leaf is usually more humid (higher concentration of water vapour) than the air outside

Exam Tip: Transpiration is the loss of water vapour through the stomata — NOT the loss of liquid water. This is a common mistake. The water evaporates inside the leaf first, then the vapour diffuses out.

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Factors Affecting the Rate of Transpiration

Four main environmental factors affect how quickly a plant loses water through transpiration:

1. Temperature

• Higher temperature → faster transpiration
• Warmer air provides more kinetic energy to water molecules, so they evaporate faster from mesophyll cell surfaces
• Warmer air can also hold more water vapour, maintaining a steeper concentration gradient between the leaf interior and the outside air

2. Humidity

• Lower humidity → faster transpiration
• If the surrounding air is dry (low humidity), there is a large concentration gradient between the moist air inside the leaf and the dry air outside
• Water vapour diffuses out more quickly down this steeper gradient
• If the air is already humid (high humidity), the gradient is smaller and transpiration slows down

3. Wind Speed

• Higher wind speed → faster transpiration
• Wind blows away the layer of water vapour that accumulates just outside the stomata
• This maintains a steep concentration gradient between the inside and outside of the leaf
• In still air, a layer of humid air builds up around the leaf, reducing the gradient and slowing transpiration

4. Light Intensity

• Higher light intensity → faster transpiration
• Light triggers guard cells to open the stomata (for photosynthesis — CO₂ needs to enter)
• When stomata are open, water vapour can escape more easily
• In the dark, stomata close, and transpiration is greatly reduced

Summary Table

Factor	Change	Effect on transpiration rate	Explanation
Temperature	Increase	Increases	Faster evaporation; steeper concentration gradient
Humidity	Decrease	Increases	Steeper concentration gradient (drier outside air)
Wind speed	Increase	Increases	Removes humid air from leaf surface; maintains gradient
Light intensity	Increase	Increases	Stomata open wider; more water vapour escapes

Exam Tip: All four factors ultimately affect the concentration gradient of water vapour between the inside of the leaf and the outside air. If the gradient is steeper, diffusion (and therefore transpiration) is faster. This is the key underlying principle.

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Core Practical 6: Investigating Transpiration Using a Potometer

What is a Potometer?

A potometer is a piece of apparatus used to measure the rate of water uptake by a plant (or a leafy shoot). Since almost all water taken up by a plant is lost through transpiration, the rate of water uptake is a good approximation of the transpiration rate.

The Apparatus

A potometer consists of:

• A leafy shoot (e.g., from a shrub or tree)
• A capillary tube with a scale (marked in mm or cm)
• A reservoir of water to reset the experiment
• A connection between the shoot and the tube, sealed with petroleum jelly (Vaseline) to prevent leaks

An air bubble is introduced into the capillary tube. As the plant takes up water, the bubble moves along the tube. The distance the bubble moves in a given time is proportional to the rate of water uptake.

Method

1. Cut the shoot under water — this prevents air entering the xylem, which would block water flow
2. Assemble the potometer under water — connect the shoot to the capillary tube while submerged, ensuring no air bubbles are trapped in the connection
3. Seal all joints with petroleum jelly to prevent air leaks
4. Allow the plant to acclimatise for a few minutes in the conditions to be tested
5. Introduce an air bubble at the open end of the capillary tube (or let one form naturally)
6. Measure the distance the air bubble moves along the capillary tube in a set time (e.g., every 2 minutes for 10 minutes)
7. Reset by opening the reservoir tap to push the bubble back to the starting position
8. Repeat under different conditions (e.g., different wind speeds, temperatures, etc.)

Exam Tip: A potometer measures water UPTAKE, not transpiration directly. However, because almost all water taken up is transpired, it is a valid approximation. Examiners may ask you to explain this distinction.

Variables

Variable	Details
Independent	The environmental factor being tested (e.g., wind speed — use a fan at different settings)
Dependent	Rate of water uptake (distance moved by the bubble per unit time, e.g., mm/min)
Controlled	All other environmental factors (temperature, humidity, light), same plant/shoot, same potometer set-up

Investigating Different Conditions

Condition tested	How to create it
Wind	Use a fan at different speeds, or compare with no fan
Temperature	Place the potometer in water baths at different temperatures
Humidity	Place a plastic bag loosely over the shoot (high humidity) vs normal air
Light intensity	Use a lamp at different distances, or compare light vs dark

Calculating Rate

Rate of water uptake = distance moved by bubble (mm) ÷ time taken (min)

Units: mm/min or mm³/min (if you know the cross-sectional area of the capillary tube, you can calculate volume)

Sources of Error and Improvements

Error	Solution
Air leaks at joints	Seal carefully with petroleum jelly; check for leaks before starting
Shoot not cut under water	Always cut under water to prevent air locks in xylem
Temperature changes during experiment	Use consistent conditions; record temperature
Bubble moves too fast or too slow	Choose an appropriate capillary tube diameter
Plant not acclimatised	Allow 5 minutes to equilibrate before taking readings

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Guard Cells and Stomatal Opening (Detailed)

As covered in the leaf structure lesson, guard cells control stomatal opening. Here is a more detailed mechanism:

Opening Mechanism

1. In the light, guard cells photosynthesise and produce sugars
2. The accumulation of sugars (and the active uptake of potassium ions, K⁺) lowers the water potential inside the guard cells
3. Water enters the guard cells by osmosis from neighbouring epidermal cells
4. The guard cells become turgid (swollen)
5. Because the inner wall is thicker than the outer wall, the guard cells bend outwards when they expand
6. This bending creates a gap — the stoma opens

Closing Mechanism