Sugar and Water Movement in Sugar and Water Movement in PlantsPlants

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• Water is moved up from the roots of the plants, up the stem and out the leaves by the Transpiration-Pull Theory (Cohesion-Tension Pull theory).

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• The theory is based on the three key properties of water:

1) Cohesion: the ability of water molecules to stick together

2) Adhesion: the ability of water molecules to stick to the sides of hollow tubes.

3) The high surface tension of water.

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• Water molecules are absorbed into the roots through the process of osmosis.

• Here water is forced through the endodermis into the xylem, but only if the xylem has room for it.

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• At the leaves, water is being used in photosynthesis or it is evaporated out of the leaf through the stomata (a process known as transpiration)

• Due to the cohesion of water molecules, as one is pulled out of the stomata or used in the leaf, another takes its place.

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• Due to adhesion, the water is able to stick to the sides of the xylem tubes and not slide back down towards the root.

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• This occurs throughout the entire length of xylem in the plant!

• Due to these properties, water can be pulled against the force of gravity upwards of hundreds of feet!

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• As water is being pulled up through the stem, more water is forced through the endodermis of the root to replace water lost at the leaves, so there is a continual flow of water through the plant.

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1. Glucose is created at the leaf (The Source) during photosynthesis. Glucose is changed into the disaccharide sucrose in order to be transported in the plant.

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2. This sucrose is actively transported (uses energy) into the phloem cells in the leaf.

3. There is a high concentration of sugar (sucrose) in the phloem at the source.

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4. Because there is so much sugar in the phloem, some water moves in from the xylem to try and balance out the concentration.

5. This creates a high amount of pressure in the phloem near the source.

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6. Meanwhile sugar is being actively transported into the root or any other storage area in the plant (The Sink).

7. Sinks have lots of sugar (usually joined together to form starch).

8. The phloem cells around a sink have low amounts of sugar and therefore water will leave them and cause them to have a low amount of pressure.

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Sugar MovementSugar Movement9. Due to the difference in pressure in the phloem cells at

the Source and the Sink, sugar will be forced down the phloem along this pressure gradient.

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• This process is known as the Pressure-Flow Theory, which still has yet to be proven.

• It was suggested by Ernst Munch, a German plant physiologist, in 1926.