Investigating transpiration Revision Notes for AQA GCSE Biology

Investigating transpiration

What is investigating transpiration?

Environmental factors affect how fast plants lose water through their leaves. This process is called transpiration. Scientists can measure the transpiration rate to understand how different conditions affect plants.

You can measure transpiration rate using special equipment called a potometer. This device tracks how much water a plant uses over time.

infoNote

A potometer doesn't directly measure water loss from leaves - instead, it measures water uptake by the plant. Scientists assume that water uptake equals transpiration rate under normal conditions.

How to measure transpiration rate

A potometer measures transpiration by tracking an air bubble moving through a tube. Here's the step-by-step process:

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Worked Example: Using a Potometer

Step 1: Record the starting position

Note where the bubble sits in the capillary tube using the scale
This is your starting measurement

Step 2: Record the end position

Let the plant transpire for a set amount of time
Note the new bubble position on the scale
The bubble moves as water is pulled up by the plant

Step 3: Calculate the rate

Work out how far the bubble moved
This shows the rate of water uptake
Water uptake equals the rate of transpiration

The equipment includes a reservoir for pushing air bubbles, a rubber stopper, and a capillary tube with a scale.

Factors affecting transpiration rate

Three main environmental factors increase transpiration rate when they increase:

Light intensity

Effect: Transpiration rate increases
Why: More light means more photosynthesis happens
The stomata (tiny pores in leaves) open wider for gas exchange
Open stomata allow more water to escape

Temperature

Effect: Transpiration rate increases
Why: Heat makes water molecules move faster
Faster movement increases the rate of diffusion
More water vapour escapes from the leaves

Air movement (wind)

Effect: Transpiration rate increases
Why: Moving air creates a steeper concentration gradient
Wind blows away water vapour from around the leaves
This increases the rate of diffusion out of the plant

chatImportant

All three factors work in the same way - they all help water escape from leaves more quickly by affecting the rate of diffusion.

Understanding the results

When you put a plant in different conditions, you can see how transpiration changes:

In still air: Lower transpiration rate because water vapour builds up around leaves In moving air: Higher transpiration rate because water vapour gets blown away

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Worked Example: Comparing Results

Example data:

Outside bag (moving air): Bubble moved 50mm in 5 minutes = 10mm/min
Inside bag (still air): Bubble moved 20mm in 5 minutes = 4mm/min

This shows that moving air increases the transpiration rate by 2.5 times compared to still air.

Calculating water volume

You can work out exactly how much water the plant used. If you know the diameter of the capillary tube, you can use this calculation:

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Worked Example: Water Volume Calculation

Formula: $\text{Volume} = \text{distance} \times \pi r^2$

Where radius $(r) = \frac{\text{diameter}}{2}$

Example: If the capillary tube has a diameter of 2mm and the bubble moved 30mm:

Radius = $\frac{2}{2} = 1\text{mm}$
Volume = $30 \times \pi \times 1^2 = 30\pi \approx 94.2\text{mm}^3$

This tells you the actual volume of water in mm³ that the plant absorbed.

Why plants wilt

Plants wilt when they lose water faster than they can absorb it through their roots. This is more likely to happen when:

The weather is hot (increases temperature factor)
The weather is windy (increases air movement factor)
It's sunny (increases light intensity factor)

chatImportant

All three factors work together to increase water loss, making the plant more likely to wilt. This is why plants often wilt on hot, sunny, windy days.

Key takeaways

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Key Points to Remember:

Potometers measure transpiration by tracking air bubble movement in a tube
Three factors increase transpiration: light intensity, temperature, and air movement
All three factors work the same way - they help water escape from leaves more quickly
Plants wilt faster in hot, windy, sunny weather because transpiration rate increases
You can calculate exact water volumes using the tube diameter and distance measurements

Investigating transpiration (AQA GCSE Biology): Revision Notes