potometer

Question 1

Describe how to set up a potometer

Answer 1

1. Cut a shoot underwater at a slant →
   prevent air entering xylem
2. Assemble potometer with capillary tube
   end submerged in a beaker of water
3. Insert shoot underwater
4. Ensure apparatus is watertight / airtight
5. Dry leaves and allow time for shoot to
   acclimatise
6. Shut tap to reservoir
7. Form an air bubble - quickly remove end
   of capillary tube from water

Question 2

Describe how a potometer can be
used to measure the rate of transpiration

Answer 2

1. Record position of air bubble
2. Record distance moved in a certain amount of time (eg. 1 minute)
3. Calculate volume of water uptake in a given time:
   ○ Use radius of capillary tube to calculate cross-sectional area of water (πr2)

○ Multiply this by distance moved by bubble
4. Calculate rate of water uptake - divide volume by time taken

Question 3

Suggest limitations in using a potometer to measure rate of transpiration

Answer 3

● Rate of water uptake might not be same as rate of transpiration
○ Water used for support / turgidity
○ Water used in photosynthesis and produced during respiration
● Rate of movement through shoot in potometer may not be same as
rate of movement through shoot of whole plant
○ Shoot in potometer has no roots whereas a plant does
○ Xylem cells very narrow