(transpiration continued)

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

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

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

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

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

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)

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.

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

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

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