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23-5 Transport in Plants
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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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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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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.
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 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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Water Pressure
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 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.
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 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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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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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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11. Movement from Source to Sink
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
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
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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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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