Transport

Biology

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Copyright Pearson Prentice Hall

23-5 Transport in Plants

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23-5 Transport in Plants Water Pressure

Water Pressure
Xylem tissue forms a continuous set of tubes that
runs from the roots through stems and out into the
spongy mesophyll of leaves.
Active transport and root pressure cause water to
move from soil into plant roots.
Capillary action and transpiration also are needed
to transport water and minerals.

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How is water transported throughout a
plant?

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The combination of root pressure, capillary
action, and transpiration provides enough
force to move water through the xylem
tissue of even the tallest plant.

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Cohesion is the attraction of molecules of the same
substance to each other.
Adhesion is the attraction between unlike molecules.

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The tendency of water to
rise in a thin tube is called
capillary action.
Water is attracted to the
walls of the tube, and
water molecules are
attracted to one another.

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Capillary action causes
water to move much
higher in a narrow tube
than in a wide tube.

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Tracheids and vessel elements form hollow
connected tubes in a plant.
Capillary action in these structures causes water to
rise well above the level of the ground.

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Transpiration

In tall plants, the major force in water transport
comes from the evaporation of water from leaves
during transpiration.

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When water is lost
through
transpiration, osmotic
pressure moves water
out of the vascular
tissue of the leaf.

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The movement of water
out of the leaf “pulls”
water upward through the
vascular system all the
way from the roots.
This process is known as
transpirational pull.

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Controlling Transpiration

The water content of the leaf is kept relatively
constant.
When there is a lot of water, water pressure in the
guard cells is increased and the stomata open.
Excess water is then lost through the open
stomata by transpiration.

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When water is scarce, the opposite occurs.
Water pressure in the leaf decreases. The guard cells
respond by closing the stomata.
This reduces further water loss by limiting
transpiration.
When too much water is lost, wilting occurs. When a
leaf wilts, its stomata close and transpiration slows
down. This helps a plant conserve water.

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23-5 Transport in Plants Nutrient Transport

How are the products of photosynthesis
transported throughout a plant?

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Nutrient Transport
Many plants pump sugars into their fruits.
In cold climates, plants pump food into their roots
for winter storage.
This stored food must be moved back into the
trunk and branches of the plant before growth
begins again in the spring.

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Movement from Source to Sink

A process of phloem transport moves sugars
through a plant from a source to a sink.
A source is any cell in which sugars are produced
by photosynthesis.
A sink is any cell where the sugars are used or
stored.

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When nutrients are pumped into or
removed from the phloem system, the
change in concentration causes a
movement of fluid in that same direction.

As a result, phloem is able to move
nutrients in either direction to meet the
nutritional needs of the plant.

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One idea that explains how phloem transport takes
place is called the pressure-flow hypothesis.

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Phloem Xylem
Sugar
molecules
Sugars produced during
photosynthesis are
pumped into the phloem
(source).
Source cell

Movement
of water
Movement
of sugar
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Phloem Xylem
Sugar
molecules
As sugar concentrations
increase in the phloem,
water from the xylem
moves in by osmosis.
Source cell

Movement
of water
Movement
of sugar
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Phloem Xylem
Sugar
molecules
This movement causes
an increase in pressure at
that point, forcing
nutrient-rich fluid to move
Source cell
through the phloem from
nutrient-producing
regions ….

Movement
of water
Movement
of sugar
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…. toward a region that Movement
uses these nutrients of water
Movement
(sink). of sugar

Sink cell
Phloem Xylem Slide
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If part of a plant actively Movement
absorbs nutrients from of water
Movement
the phloem, osmosis of sugar
causes water to follow.
This decreases pressure
and causes a movement
of fluid in the phloem
toward the sink.

Sink cell
Phloem Xylem Slide
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23–5

Continue to: Click to Launch:
- or -

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23–5

In a plant stem, water moves from
a. leaves to roots through xylem.
b. roots to leaves through xylem.
c. leaves to roots through phloem.
d. roots to leaves through phloem.

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23–5

Which of the following is NOT involved in the
movement of water in xylem tissue?
a. cohesion
b. osmosis
c. capillary action
d. adhesion

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23–5

When nutrients are pumped into the phloem
system of a plant, the increased concentration
a. causes fluid to move into the system.
b. causes fluid to move out of the system.
c. has no effect on the movement of fluid.
d. causes fluid to move into the xylem vessels.

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23–5

In a plant, sugar is moved from source cells to
sink cells by a process of
a. phloem transport.
b. xylem transport.
c. osmosis.
d. diffusion.

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23–5

In very tall trees, which of the following is
primarily involved in moving water to the top of
the tree?
a. transpirational pull
b. capillary action
c. root pressure
d. osmosis

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Transport

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