Potometers Flashcards
How can you measure the rate of water uptake
By using a potometer
What does a bubble potometer consist of
- A fine capillary tube filled with water
- The tube is connected to a plant which is being cut at the stem
- the tube is connected to a syringe filled with water
- We use a needle to place a small air bubble at the end of the capillary tube
What happens as water evaporates from the leaves in a potometer
Water is drawn into the stem. This causes the air bubble to move towards the plant.
How can you measure the rate of transpiration using a potometer
By measuring how far the air bubble moves in a given time we can calculate the rate of water uptake in the plant. We can then use pi x r squared x length and divide by time to calculate the rate of transpiration.
How can you reset a potometer
By adding more water from the syringe
What is a negative of a bubble potometer
They only measure the water uptake into the plant. And not all of the water will take part in transpiration.
What is the water used for in a potometer
A small amount of the water takes part in photosynthesis and some being involved in turgidity. However, the majority of water taken in by the plant will take part in transpiration.
What is something we have to do when cutting plants
When we take our cutting from the plant, air will be sucked into the xylem vessels. And these air gaps would prevent water being taken up the stem. So to correct this we place the stem of our plant in water and cut off at least 1 cm allowing for water to flow into the xylem avoiding any air gaps.
What do we do once the plant has been cut
We then place the potometer under the water and insert the cut end, avoiding any air gaps. We handle the plant carefully to avoid damage and to prevent from getting onto the underside of leaves where the stomata are found.
What do we allow the plant to adapt to before the experiment
We also need to allow the plant to adapt to its surroundings for 10 minutes before starting experiments.
What is a mass potometer
Is when we place the plant in its pot on a balance
As the plant loses water through transpiration, the total mass decreases
What do we have to prevent with the mass potometer
With this potometer we have to prevent evaporation of water from the soil.
Otherwise this would contribute to mass loss, giving a false reading for transpiration. This is by covering the plant in a polyethene bag.
What are advantages of mass potometer
- Directly measures the rate of transpiration rather than the rate of water uptake
- This is much less disruptive to the plant as it does not involve cutting the cells.
