3.3.4 transpiration

Question 1

potometer

Answer 1

device that can measure the rate of water uptake as a leafy stem transpires

Question 2

transpiration

Answer 2

loss of water vapour from aerial parts of a plant, mostly through stomata in leaves

Question 3

why is water loss through the upper surface of the leaf limited

Answer 3

due to waxy cuticle

Question 4

where does most water vapour leave

Answer 4

the stomata

Question 5

when is majority of water vapour lost & why

Answer 5

during the day as photosynthesis occurs in sunlight so stomata are open to allow gaseous exchange

Question 6

describe the typical pathway taken by most water leaving the leaf

Answer 6

1. water enters leaf via xylem & moves by osmosis into cells of spongy mesophyll - may also pass along cell walls via apoplast pathway
2. water evaporates from cell walls of spongy mesophyll
3. water vapour moves by diffusion out of leaf through open stomata - relies on difference in concentration of water vapour molecules in leaf compared to outside (water vapour potential gradient)

Question 7

why is transpiration essential for plant survival

Answer 7

as water vapour lost, it must be replaced which draws water up stem as transpiration stream

Question 8

importance of transpiration

Answer 8

• transports useful mineral ions up plant
• maintains cell turgidity
• supplies water for growth, cell elongation & photosynthesis
• supplies water that (as it evaporates) can keep plant cool when hot

Question 9

name the 5 environmental factors which affect transpiration

Answer 9

• light intensity
• temperature
• relative humidity
• air movement (wind)
• water availability

Question 10

how does light intensity affect transpiration

Answer 10

• in light, stomata open for gaseous exchange (photosynthesis)
• higher light intensity = increased transpiration rate

Question 11

how does temperature affect transpiration

Answer 11

higher temp. will increase transpiration rate in 3 ways:
1. increase evaporation rate from cell surfaces so water potential in leaf increases
2. increase diffusion rate through stomata as water molecules have more kinetic energy
3. decrease relative water vapour potential in air which allows more rapid diffusion of molecules out of leaf

Question 12

how does relative humidity affect transpiration

Answer 12

• higher relative humidity = decrease rate of water loss
• smaller water vapour potential gradient between air spaces in leaf & air outside

Question 13

how does air movement (wind) affect transpiration

Answer 13

• air moving outside leaf will carry water vapour away (which has just diffused out of leaf)
• maintain high water vapour potential gradient

Question 14

how does water availability affect transpiration

Answer 14

• little water in soil = plant cannot replace lost water
• insufficient water in soil = stomata close & leaves wilt

Question 15

(assuming all cells are turgid) how much water that’s taken up is lost via transpiration

Answer 15

95%

Question 16

precautions taken when using a potometer to ensure valid results

Answer 16

• set it up under water to ensure no air bubbles in apparatus
• ensure shoot is healthy
• cut stem under water to prevent air entering xylem
• cut stem at angle to provide large surface area in contact with water
• dry leaves

Question 17

components of potometer to measure transpiration

Answer 17

• air taken in through capillary tube (water inside)
• reservoir
• plant shoot
• water lost by leaves replaced by water in capillary tube
• movement of meniscus at end of water column can be measured

Question 18

how do you measure different environmental factors on transpiration

Answer 18

place potometer under different conditions & vary 1 factor at a time