LESSON #6: TRANSPIRATION
Objectives:
Explain how transpiration occurs.
TRANSPIRATION
WHAT IS IT?
Transpiration is the loss of water from a plant
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It’s caused by the evaporation of water from
inside the leaves.
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The evaporation causes a slight shortage of water
in the leaf, which causes it to draw more water
up from the plant, which causes water to be
drawn up from the roots via cohesion.
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It has two beneficial effects
a) It transports minerals from the soil and
b) It cools the plant.
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FACTORS WHICH AFFECT TRANSPIRATION
The rate of transpiration is affected by
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2.
3.
4.
The amount of light
Temperature
Amount of air movement
Humidity of the surrounding air
The biggest rate of transpiration occurs in hot,
dry, windy, sunny conditions (ie perfect to dry
clothes)
The slowest rate occurs in cool, cloudy, damp,
miserable weather with no wind
WRITE A SHORT EXPLANATION WITH YOUR
NEIGHBOUR:
HOW DOES THE LEAF STRUCTURE HELP
PREVENT EXCESSIVE WATER LOSS?
HOW DO PLANTS TRANSPORT WATER AND FOOD?
Through the Xylem and Phloem
Using capillary action:
Adhesion: Surface of xylem (+ve) and water (-ve)
molecules
Cohesion: water to water
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http://www.youtube.com/watch?v=mc9gUm1mMz
c&feature=related
http://projects.cbe.ab.ca/resources/cts/agr1030/res
ources/multimedia/transpiration.swf
Transpiration virtual lab:
http://www.mhhe.com/biosci/genbio/virtual_labs/B
L_10/BL_10.html
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TRANSPIRATION VIRTUAL LAB:
http://www.mhhe.com/biosci/genbio/virtual_labs/BL
_10/BL_10.html
1.
2.
3.
Which factor causes the highest rate of
transpiration? Why?
Which plant has the highest rate of
transpiration? Why?
Which plant has the lowest rate of
transpiration? Why?
CAPILLARY ACTION
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Capillary action in plants is a good example of adhesion
and cohesion.
The inner surface of the xylem contains positive and
negative charges to which water forms hydrogen bonds.
This is called adhesion.
As water creeps up the sides of the xylem (adhesion) the
water molecules in the middle connect to other water
molecules because of cohesion.
The water moves up as the water molecules at the top of
the xylem enter the leaves and evaporates.
When a water molecule leaves the leaf, the molecule
behind it moves up causing a general movement of the
water up the tree.