1. Transport in flowering plants
Transpiration

2. Lesson Objectives By the end of this lesson you should be able to:
1) Define the term transpiration and explain that transpiration is a consequence of gaseous exchange in plants.
2) Describe the factors that affect transpiration rate

3. Get this right
Water potential= a measure of the tendency of water to move from one place to another.
A dilute solution has a water potential than a concentrated one.
Water moves from a solution of higher water potential to a solution of lower water potential
higher

4. Capillary action- cohesion
Each molecule of water consists of 1 oxygen and 2 hydrogen atoms
Oxygen carries
charge
Hydrogen carries charge
Charges results in forces of attraction between water molecules: force of cohesion
negative
positive

6. Capillary action- adhesion
Water also sticks to the inner surface of xylem vessels
The wall of the xylem just above the water surface attracts water molecules upwards: force of adhesion

7. Higher water potential
Transpiration- How?
Definition: The loss of water vapour from the aerial parts of a plant
Stomata on the surface of leaves open for gaseous exchange
Air surrounding the leaves are “drier”
Water molecules exit the leaves through the stomata
Higher water potential
Lower water potential
‘drier’ apply knowledge of water potential. Air has lower water potential. Aerial= parts of the plant above the ground

8. Transpiration- the process
Stomata open for gaseous exchange
More water vapour molecules in-between the spongy mesophyll cells than the outside air
Water molecules diffuses out of the leaves through the stomata
Loss of water creates water potential difference between xylem and mesophyll cells
Water moves out of xylem vessels through osmosis
Chain of water molecules pulled upwards with the assistance of the forces of cohesion and adhesion

9. Factors affecting the rate of transpiration
Rate of transpiration will be fast if
Rate of transpiration will be slow if
Note to teachers: manipulate the various factors to demonstrate to students the effects of humidity, temperature, light intensity and wind speed on transpiration

10. Factors affecting the rate of transpiration
Rate of transpiration will be fast if
Rate of transpiration will be slow if
Humidity
dry
damp/ humid
Temperature
warm
cold/ cool
Light
bright
dim
Wind
windy
still

11. Quick check Are you able to:
1) Define the term transpiration and explain that transpiration is a consequence of gaseous exchange in plants.
2) Describe the factors that affect transpiration rate

12. Windier conditions increases transpiration rate because the boundary layer around the leaf is smaller. (T/F)
Transpiration rate is lower at night due to lower temperature. (T/F)
The water vapour concentration gradient is due to the difference in water vapour concentration between the atmosphere and the leaf. (T/F)