Potometer/ Transpiration Flashcards
True or False
Potometers directly measure rate of transpiration.
- False.
- Potometer measures rate of uptake of water from a plant which is proportional to transpiration as it is impossible to measure transpiration directly.
When we cut a plant for our potometer experiment, how does this have to be done and why?
- Due to cohesion-tension theory creating a negative pressure (ie. PULLS water UP!) in the xylem.
- If it was cut in air, air would be drawn into xylem.
- This would break the continuous water column and prevent transpiration. By cutting underwater, only water is drawn into the xylem.
How is the cut plant attached
to the potometer and why?
- Using ruber seals/ petroleum jelly.
- To make equipment airtight.
What must we do to set up potometer, in terms of with water?
- Fill potometer with water and remove all air bubbles.
- Then, introduce one air bubble.
What do we look at in potometer experiment to measure rate of transpiration?
- Look at distance air bubble moved.
- You can work out, from this, the volume of water in the tube that evaporated.
- Volume is divided by time it took to lose that volume of water to get a rate.
How can the apparatus be reset in this experiment?
- Move the air bubble back to the 0 mark.
Why must all the joints in the apparatus be covered in petroleum jelly?
- Petroleum jelly is waterproof therefore, it prevents any water leaking out - ensuring that all water can only leave by evaporation out of the stomata on the plant.
Common Maths Q
If the air bubble moved 15.28mm in 1 minute through capilary tube (0.5mm in radius) what would the rate of transpiration be?
- Calculate volume of cylinder:pix r²xl
- pi x 0.5² x 15.28mm.
- 12mm min⁻¹
What variables would have to be controlled if you were to perform potometer experiment on 2 different plant species?
- Surface area of the leaves (number/ size of leaves.)
How does wind affect the rate of transpiration and why?
- Positive correlation.
- More wind will blow away humid air containing water vapour, maintaining the water potential gradient.
How does humidity affect the rate of transpiration and why?
- Negative correlation.
- More water vapour in air makes water potential more positive outside the leaf, reduces the water potential gradient.
How does temperature affect the rate of transpiration and why?
- Positive correlation.
- More heat means more kinetic energy, faster moving molecules - more evaporation.
How does light intensity affect the rate of transpiration and why?
- Positive correlation.
- More light causes more stomata to open = larger surface area for evaporation.
What 4 factors affect the rate of transpiration?
- Light intensity.
- Temperature
- Wind
- Humidity.
What is “transpiration?”
- Transpiration is the loss of water vapour from the stomata by evaporation.
Give 2 reasons why the rate of water uptake by plant might not be the same as rate of transpiration (2 marks)
- Water used in photosynthesis.
- Water used in hydrolysis.
