Investigating transpiration (AQA GCSE Biology Combined Science): Revision Notes
Investigating transpiration
What is transpiration investigation?
Scientists can measure how fast plants lose water through transpiration. This helps us understand how environmental conditions affect plants. We use a special piece of equipment called a potometer to do this.
Transpiration investigations are a key part of understanding plant physiology and how plants respond to their environment. This practical skill is essential for biology students.
How to use a potometer
A potometer measures how quickly a plant takes up water. Since plants lose water through transpiration, the rate of water uptake gives us the transpiration rate.
Equipment setup
- Plant shoot in water
- Capillary tube with measuring scale
- Air bubble in the tube
- Rubber stopper to seal the system
- Reservoir for adding more water
Method steps
Worked Example: Using a Potometer
Step 1: Mark where the air bubble starts on the scale
Step 2: Let the plant transpire for a set time (like 5 minutes)
Step 3: Measure how far the bubble has moved
Step 4: Work out the rate by dividing distance moved by time taken
The bubble moves because water is drawn up as the plant loses water through its leaves.
Environmental factors that increase transpiration
Understanding what affects transpiration helps predict when plants lose water fastest. Each environmental factor works by affecting either the plant's physiology or the physical process of diffusion.
Light intensity
- Effect: More light = faster transpiration
- Why: Light makes photosynthesis happen faster, so stomata open wider for gas exchange
- This lets more water vapour escape
Temperature
- Effect: Higher temperature = faster transpiration
- Why: Heat makes water molecules move faster, speeding up diffusion
- Photosynthesis also increases, opening stomata more
Air movement (wind)
- Effect: More air movement = faster transpiration
- Why: Moving air increases the concentration gradient around leaves
- This makes water vapour diffuse away faster
Common Mistake Alert: Students often forget that all three factors (light, temperature, wind) work by either opening stomata more OR speeding up the diffusion process. Understanding the mechanism is just as important as knowing the effect!
Working with transpiration data
When you collect data from potometer experiments, you can calculate rates and volumes. These calculations help you quantify exactly how environmental conditions affect plant water loss.
Worked Example: Calculating Transpiration Rate
If an air bubble moves 50mm in 5 minutes:
- Rate =
- Rate =
Volume calculations
You can work out how much water the plant used if you know the tube diameter:
- Volume =
- Where
Explaining results
Understanding Plastic Bag Experiments:
If you put a plant in a plastic bag, transpiration slows down because:
- Humidity increases inside the bag
- Water vapour can't escape easily
- This reduces the concentration gradient
- So diffusion becomes slower
Important points about potometers
Potometers are useful tools, but it's important to understand their limitations and assumptions.
Key Assumptions About Potometers:
- They measure water uptake, not direct water loss
- We assume most water uptake replaces water lost by transpiration
- Some water is used in photosynthesis, but this is usually a tiny amount
- The method works best with leafy shoots that transpire actively
Common exam questions
You might be asked to:
- Name environmental factors affecting transpiration
- Explain why each factor increases the rate
- Calculate rates from experimental data
- Explain results from bag experiments
Key Points to Remember:
- A potometer measures water uptake to estimate transpiration rate
- Light, temperature and air movement all increase transpiration
- Higher rates happen because stomata open more or diffusion speeds up
- You can calculate rates using: Rate =
- Plastic bags reduce transpiration by increasing humidity around leaves