slide 1 of 30 copyright pearson prentice hall biology

Copyright Pearson Prentice Hall

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Biology


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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

How is water transported throughout a plant?



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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

How are the products of photosynthesis transported throughout a plant?



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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


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23–5


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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

END OF SECTION

slide 1 of 30 copyright pearson prentice hall biology

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movement of sugars

movement of water molecules

pull of water

faster water

high concentration of

capillary action causes

sink cell

source cell