practical: measuring transpiration

• what do u use to measure transpiration rate

  potometer
• what are the 2 types of potometers

  • bubble potometer
  • mass potometer
• how can we tell there is a greater rate of transpiration

  further distanced travelled by bubble = faster rate of transpiration
• calculate rate of transpiration

  rate of transpiration = distance travelled by bubble/ time
• how many steps


  10
• method (10)


  1. cut shoot underwater at an angle to prevent air from getting into xylem
  2. place potometer + shoot underwater so no air can enter
  3. ensure apparatus is air +water tight
  4. dry leaves to ensure only water lost by transpiration is measured+allow them to acclmatise
  5. remove end of capillary tube from beaker of water to introduce bubble+then place it back
  6. record starting position of bubble
  7. start stopwatch+record dstance moved by bubble per unit of time
  8. reset air bubble using tap
  9. repeat experiment after changing factor being investigated
  10. keep conditions constant eg:temp/ humidity
• role of mass potometer

  measures change in mass of plant as we measure amount of water which has evaporated from leaves+ stem
• bubble potometer


  measures uptake of water by stem as we measure amount of water loss by evaporation. (+ in turn pulling water up through stem)
• how can u measure the affect of other environmental factors

  • use experiment before as a control
  • vary the environmental condition (eg:temp / humidity)
  • repeat experiment+compare results to control experiment to notice how change has affected transpiration rate
• env factor: light intensity

  • use a lamp to increase light intensity that hits lamp, which increases transpiration rate
• how to decrease light intensity

  place potometer in cupboard to decrease light intensity, whch decreases transpiration rate
• env factor: temperature


  • place potometer in a warmer room (than control experiment) to increase temp, which increases transpiration rate
  • place potometer in a colder room (then control experiment) to decrease temp, which decreases transpiration rate
• env factor: humidity 


  • spray water inside a clear plastic bag and seal it around a plant to increase humidity, which decreases transpiration rate
• env factor: wind speed

  • use fan/ blowdryer to increase wind speed around plant, which increases transpiration rate
• precautions for setting up apparatus (4)

  • cut stem underwater to prevent air from entering xylem
  • dry leaves to ensure only water lost through transpiration is measured
  • keep environmental factors constant (eg:light intensity/ humidity)
  • ensure apparatus is airtight+watertight
• why is a capillary tube used rather than an extension of a glass tube

  as it needs to be thin so even small amounts of water loss can cause a noticeable movement of air bubbles
• what measurements would you need in order to calculate rate of water uptake by shoot

  • distance moved by bubble (m)
  • time taken (mins)
• what is a possible reason to why rate of water uptake might exceed rate of water loss in daylight

  as water may be used for photosynthesis
• black plastic bag is placed over shoot+around stem. name 2 environmental factors could alter result?

  • light intensity- as plant cant access sunlight due to bag black-> as light intensity decreases, stomata closes as wont need to open to allow co2 in for photosynthesis as it cant occur in dark. therefore no water can diffuse out stomata
  • humidity- as plant sealed with bag, it traps moisture from leaves-> as humidity increases, transpiration rate decreases as there a large amount of water in air, meaning conc gradient decreases where less water can diffuse out, decreasing transpiration
• if rate of water loss from leafy shoot exceeds rate of uptake, leaves will wilt. explain why warmth and breeze will each favour wilting

  • warmth: = high temp which increases transpiration rate. as water water particles have more kinetic energy, they move around faster+allow water to diffuse out stomata+evaporate
  • breeze: = wind which increases transpiration rate. as wind speed increases, water particles which have diffused out of stomata (surrounding leaf) are quickly blown away, leading to a high conc gradient between inside+outside of leaf, therfore more water vapour diffuses out