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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.
Capillary action—the result of water molecules’ ability to stick to one another and to the walls of a tube—contributes to the movement of water up the cells of xylem tissue. As shown here, capillary action causes water to move much higher in a narrow tube than in a wide tube. Applying Concepts Which force—adhesion or cohesion—causes the water to stick to the walls of the glass tube?
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Water Transport
Capillary action causes water to move much higher in a narrow tube than in a wide tube.
Capillary action—the result of water molecules’ ability to stick to one another and to the walls of a tube—contributes to the movement of water up the cells of xylem tissue. As shown here, capillary action causes water to move much higher in a narrow tube than in a wide tube. Applying Concepts Which force—adhesion or cohesion—causes the water to stick to the walls of the glass 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.
Root pressure, capillary action, and transpiration contribute to the movement of water within a plant. Transpiration is the movement of water molecules out of leaves. The faster water evaporates from a plant, shown in A, the stronger the pull of water upward from the roots, shown in B.
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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.
Root pressure, capillary action, and transpiration contribute to the movement of water within a plant. Transpiration is the movement of water molecules out of leaves. The faster water evaporates from a plant, shown in A, the stronger the pull of water upward from the roots, shown in B.
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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
Phloem
Xylem
Sugar molecules
Sugars produced during photosynthesis are pumped into the phloem (source).
Source cell
The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis. Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration.
Movement of water
Movement of sugar
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Nutrient Transport
Phloem
Xylem
Sugar molecules
As sugar concentrations increase in the phloem, water from the xylem moves in by osmosis.
Source cell
The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis. Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration.
Movement of water
Movement of sugar
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Nutrient Transport
Phloem
Xylem
Sugar molecules
This movement causes an increase in pressure at that point, forcing nutrient-rich fluid to move through the phloem from nutrient-producing regions ….
Source cell
The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis. Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration.
Movement of water
Movement of sugar
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Nutrient Transport
…. toward a region that uses these nutrients (sink).
Movement of water
Movement of sugar
The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis. Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration.
Sink cell
Phloem
Xylem
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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
The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis. Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration.
Sink cell
Phloem
Xylem
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23–5
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23–5
In a plant stem, water moves from
leaves to roots through xylem.
roots to leaves through xylem.
leaves to roots through phloem.
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?
cohesion
osmosis
capillary action
adhesion
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23–5
When nutrients are pumped into the phloem system of a plant, the increased concentration
causes fluid to move into the system.
causes fluid to move out of the system.
has no effect on the movement of fluid.
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
phloem transport.
xylem transport.
osmosis.
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?
transpirational pull
capillary action
root pressure
osmosis
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