Download Transpiration continued

(transpiration continued)
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Evaporation of water at the surface of the
mesophyll cells followed by the loss of water
vapor from plant leaves through the stomata
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
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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
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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
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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)
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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
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pond
Leaf
stem
•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
No leaf
-Reduced
transpiration
-Spines – deterrent
from animals eating
-Photosynthesis occurs
here
- Stem cells store water
root
Garden
* HW
Desert
- Extensive root
system to absorb
as much water as
possible
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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