23-5 transport in plants -...

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Copyright Pearson Prentice Hall

23-5 Transport in Plants


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Water Transport

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.


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Water Transport

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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Water Transport

Cohesion is the attraction of molecules of the same

substance to each other.

Adhesion is the attraction between unlike molecules.


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Water Transport

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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Water Transport

Capillary action causes

water to move much

higher in a narrow tube

than in a wide tube.


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Water Transport

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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Water Transport

Transpiration

In tall plants, the major force in water transport

comes from the evaporation of water from leaves

during transpiration.


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Water Transport

When water is lost

through transpiration,

osmotic pressure

moves water out of the

vascular tissue of the

leaf.

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Water Transport

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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Water Transport

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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Water Transport

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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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.


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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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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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Nutrient Transport

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


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Nutrient Transport

Sugars produced during

photosynthesis are

pumped into the phloem

(source).

Sugar

molecules

Movement

of water

Movement

of sugar

Phloem Xylem

Source cell


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Nutrient Transport

As sugar concentrations

increase in the phloem,

water from the xylem

moves in by osmosis.

Movement

of water

Movement

of sugar

Sugar

molecules

Phloem Xylem

Source cell


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Nutrient Transport

Movement

of water

Movement

of sugar

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

Phloem Xylem

Source cell


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Nutrient Transport

…. toward a region that

uses these nutrients

(sink).

Xylem Phloem

Movement

of water

Movement

of sugar

Sink cell


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Nutrient Transport

If part of a plant actively

absorbs nutrients from

the phloem, osmosis

causes water to follow.

This decreases pressure

and causes a movement

of fluid in the phloem

toward the sink.

Movement

of water

Movement

of sugar

Xylem Phloem Sink cell

23-5 transport in plants -...

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