1. Transpiration

2. Definition
Transpiration is the evaporation of water from the aerial parts of plants.
Of all the water a plant absorbs, over 95% is transpired to the air as water vapor.

3. What is this process called? Stomatal Transpiration
What is most likely leaving through the stomata of the leaf picture here?
Water (H2O)
What is this process called?
Stomatal Transpiration 3

4. Stomatal transpiration
Cuticle
Prevents water loss
Mesophyll
Site of photosynthesis
Stomata
Guard cells
Openings allow gases and water to move in and out of leaf
Open and close the stomata

5. Importance of transpiration
Guard Cells
What process involves using CO2 and H2O releasing O2 as a waste product?
Photosynthesis
What is the plant using this process to make?
Carbohydrates-glucose
If the plant needs water for photosynthesis, why is water coming out of the stoma?
Guard Cells
What goes out? O2
H2O
CO2
What goes in?
Stoma Closed
Stoma Open
Stoma

6. Function of Stomata
These stomata (leaf openings) naturally allow water to evaporate out.
Why would the plant close stomata with guard cells?
Prevent excess water loss through transpiration. (conserve water)
So what is the point of having stomata?
Allows gas exchange for photosynthesis
Guard Cells
Stoma Open
Stoma Closed
Guard cells open by inflating with extra water. They do this by pumping K+ ions into the cell, which causes water to rush in via osmosis to diffuse the high ion concentration. 6

7. How do the guard cells react to the availability of water?
Function of Guard Cells
How do the guard cells react to the availability of water?
Dry – guard cells CLOSE
lots of H2O – guard cells OPEN 7

8. cells that open and close the stoma
Guard cells:
cells that open and close the stoma
Stomata: openings in leaf’s surface; when open:
GAS EXCHANGE: Allows CO2 in & O2 out of leaf
TRANSPIRATION:
Guard Cells
Stomata 8

10. 5. Characteristics of guard cells

11. The driving force of transpiration is the “vapor pressure gradient
The driving force of transpiration is the “vapor pressure gradient.” This is the difference in vapor pressure between the internal spaces in the leaf and the atmosphere around the leaf
Diffusional resistance comprises stomatal resistance and boundary layer resistance

12. Environmental factors that affect the rate of transpiration
Light
Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata , Light also speeds up transpiration by warming the leaf .

13. Temperature
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises.
Humidity When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

14. Wind When a breeze is present, the humid air is carried away and replaced by drier air.
Soil water A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration. If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

15. Cells turgid/Stoma open
Cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Changes in guard cell shape and stomatal opening and closing
(surface view)