Organisms Exchange Substances with their Environment: Mass Transport in Plants - Transpiration

Question 1

What is transpiration?

Answer 1

• Transpiration is the evaporation of water from a plant through an open stomata
• Water leave mesophyll cells by osmosis and accumulate intercellular spaces in a leaf
• When stomata open, water diffuse out the leaf down a water potential gradient

Question 2

What factors affect transpiration rate?

Answer 2

• Light intensity
• Temperature
• Humidity
• Wind

Question 3

How does light intensity affect transpiration rate?

Answer 3

• The higher the light intensity, the faster the transpiration rate
• Stomata open when it is light to let in CO2 for photosynthesis
• Stomata close when it’s dark, so there’s little transpiration

Question 4

How does temperature affect transpiration rate?

Answer 4

• The higher the temperature, the faster the transpiration rate
• Warmer water molecules have more energy so they evaporate from cells inside the leaf faster
• This increases water potential gradient between inside and outside of leaf, making water diffuse out of leaf faster

Question 5

How does humidity affect transpiration rate?

Answer 5

• The lower the humidity, the faster the transpiration rate
• If air around plant is dry, the water potential gradient between leaf and air is increased, increasing transpiration rate

Question 6

How does wind affect transpiration rate?

Answer 6

• The windier it is, the faster the transpiration rate
• Lots of air movement blows away water molecules from around stomata
• This increases water potential gradient, increasing transpiration rate

Question 7

What is a potometer?

Answer 7

• Apparatus used to estimate transpiration rate
• It measures water uptake by a plant, but is assumed it is directly related to water loss by leaves
• Can be used to estimate how different factors affect transpiration rate

Question 8

How do you set up a potometer?

Answer 8

• Cut shoot underwater to prevent air from entering xylem
• Cut at slant to increase SA for water uptake
• Assemble potometer under water and insert shoot with apparatus under water, so no air enters
• Remove apparatus from water but keep end of capillary tube submerged in beaker of water

Question 9

What do you do once potometer is assembled?

Answer 9

• Dry leaves, allow time for shoot to acclimatise (adjust) and shut tap
• Remove end of capillary tube from beaker of water until one air bubble is formed, then put end of tube back in water
• Record starting position of air bubble
• Start stopwatch and record distance moved by bubble per unit time - rate of air bubble movement is estimate of transpiration rate