1. (transpiration continued)

2. transpiration
Evaporation of water at the surface of the mesophyll cells followed by the loss of water vapor from plant leaves through the stomata

3. Water vapor loss Cell surface: walls of mesophyll wet
Water evaporates from these wet walls
Air space:
Full of water vapor
Stomata:
- Water vapor diffuses from inside the leaf through the stomata into the air

4. Loss of water from leaves reduces water pressure at the top of the xylem
This is how water moves up the xylem
Transpiration reduces pressure at the top of xylem so that there is less pressure at the base.
Water continues to move up.
98% transpiration of water
2% for photosynthesis

5. Transpiration rate Temperature:
Faster when the temp. is high than low.
Due to kinetic energy of water molecules
Therefore diffusion happens faster
Increased rate of evaporation from cell walls to mesophyll cells inside
Humidity:
Faster when air is dry than when humid
Due to steeper diffusion gradient for water vapor between air space inside the leaf and the air when the concentration of water is low

6. Wind speed is fast – rate of transpiration increases
Due to wind taking away humid air just outside leaf
Helps maintain diffusion gradient for water vapor from the leaf and into air
Light intensity:
Sunshine = stomata open to let CO2 in
Water vapor also diffuses out when stomata open
When dark – stomata close (traps water vapor in leaf). Transpiration slower in dark

7. Mechanism of water uptake
Water potential = the measure of how much water there is and how easily water molecules can move around compared to other molecules around it.
Lots of water = high water potential
Water moves from a high water potential area to a region of low water potential (water potential gradient)

8. Water moves up – water potential in air is less than in plant roots (gradient between roots and air)
Less water vapor in air – lower the water potential
Air space in leaf has high water potential (due to presence of water vapor)
This is how water vapor diffuses into the air
This reduces water potential in the air space around mesophyll
Cell wall of mesophyll still has high water potential and therefore water moves from cell wall into air space.

9. As water evaporates from cell wall – water potential in cell wall is reduced
This is how water moves from vessels into the cell walls of the mesophyll
Transpiration produces tension which draws water up through xylem vessels (transpiration pull)
Properties of water:
Cohesion – water molecules stick to each other
Adhesion- water molecules stick to other surfaces

10. Adaptations of the leaf /stem / root
pond
Leaves on top so photosynthesis can occur / CO2 from air
Stomata on upper side of leaf
Contains air space so that oxygen can diffuse from the leaves to the roots
Garden* HW
Desert
No leaf
Reduced transpiration
Spines – deterrent from animals eating
Photosynthesis occurs here
Stem cells store water
- Extensive root system to absorb as much water as possible

11. wilting
Plant losses water from leaves (transpiration) faster than it absorbs it form the root
Total water content in plant is less and less
Water in vacuole and cytoplasm decreases
Cell begins to shrink
When cells loose some water – flaccid
Leaves become soft and floppy - wilting