(transpiration continued)

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

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