23-5 Transport in Plants
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23-5 Transport in Plants
Water Pressure
Water Transport
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.
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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23-5 Transport in Plants
Water Pressure
Capillary Action
Water molecules are attracted to each other by a force
called cohesion. Cohesion is the attraction of
molecules of the same substance to each other.
Water molecules have a tendency to form hydrogen
bonds with each other, and with other substances.
Adhesion is the attraction between unlike molecules.
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23-5 Transport in Plants
Water Pressure
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.
Capillary action causes water to move much higher in
a narrow tube than in a wide tube.
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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23-5 Transport in Plants
Water Pressure
Transpiration
In tall plants, the major force in water transport
comes from the evaporation of water from leaves
during transpiration.
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23-5 Transport in Plants
Water Pressure
When water is lost through
transpiration, osmotic
pressure moves water out of
the vascular tissue of the leaf.
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23-5 Transport in Plants
Water Pressure
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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23-5 Transport in Plants
Water Pressure
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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23-5 Transport in Plants
Water Pressure
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
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.
Phloem carries out seasonal movement of sugars
within a plant.
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23-5 Transport in Plants
Nutrient Transport
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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23-5 Transport in Plants
Nutrient Transport
Phloem Xylem
One idea that explains how
phloem transport takes
place is called the
pressure-flow
hypothesis.
Sugars produced during
photosynthesis are
pumped into the phloem
(source).
Sugar
molecules
Source cell
Movement
of water
Movement
of sugar
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23-5 Transport in Plants
Nutrient Transport
Phloem Xylem
As sugar concentrations
increase in the phloem,
water from the xylem
moves in by osmosis.
Sugar
molecules
Source cell
Movement
of water
Movement
of sugar
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23-5 Transport in Plants
Nutrient Transport
Phloem Xylem
This movement causes
an increase in pressure at
that point, forcing
nutrient-rich fluid to move
through the phloem from
nutrient-producing
regions ….
Sugar
molecules
Source cell
Movement
of water
Movement
of sugar
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23-5 Transport in Plants
Nutrient Transport
…. toward a region that
uses these nutrients
(sink).
Movement
of water
Movement
of sugar
Sink cell
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Phloem Xylem
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23-5 Transport in Plants
Nutrient Transport
If part of a plant actively
absorbs nutrients from
the phloem, osmosis
causes water to follow.
Movement
of water
Movement
of sugar
This decreases pressure
and causes a movement
of fluid in the phloem
toward the sink.
Sink cell
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Phloem Xylem
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23-5 Transport in Plants
Nutrient Transport
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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