Transcript 23.5_Transport_in_Plants

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Review What two forces are responsible for 90%
of the upward flow of water through a plant
Review What is the hypothesis that explains the
movement of fluid through phloem of a plant
Infer Explain how movement of sugars in phloem
contributes to homeostasis in a plant.
CH 23 PLANT STRUCTURE AND FUNCTION
23.5 Transport in Plants
Transpiration
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As water evaporates through open stomata, the
cell walls within the leaf begin to dry out
Dry cell walls draw water from cells deeper inside
the leaf’s vascular tissue so that water is pulled up
through xylem
The hotter and windier the day, the greater
amount of water pulled.
Cell Walls Pull Water Upward

Cohesion
 Water
molecules are attracted
to one another
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Adhesion
 Water
molecules form hydrogen
bonds with other substances.
Capillary Action
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Tendency of water to rise in a
thin tube
Water is attracted to walls tube
and water molecules are
attracted to one another
Thinner the tube, the higher the
water will rise.
Putting it all together
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Tubes of xylem are lined with cellulose cell walls
 Water
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adheres very strongly
Transpiration pull from leaves
Concentration gradient forces water in roots to a
much smaller amount.
Nutrient Transport
Pressure-flow
hypothesis
1. Membranes of sieve
tube cells use active
transport to move
sugars from their
cytoplasm into the
sieve tube itself.
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2. Water then follows by
osmosis, creating
pressure in the tube at
the source of the
sugars.
3. Sugars are actively
pumped out of the tube
and into the
surrounding tissue
where it is needed.
Water then leaves the
tube via osmosis.

During growing season
 Sugars
are directed into ripening fruits or into roots for
storage
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As growing season ends
 Plant
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drops its fruits and stores nutrients in the roots
As spring approaches
 Phloem
cells in roots pump sugars back into phloem
sap
 Pressure-flow system raises these sugars into stems
and leaves to support rapid growth.