Mass Flow Theory

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Mass Flow Theory

1. Sucrose produced by photosynthesis is actively pumped into the phloem vessels by the companion cells.

2. This decrease the water potential in the leaf phloem, so water diffuse from the neighbouring xylem vessel by osmosis.

3. This is increase the hydrostatic pressure in the phloem, so water and dissolved solutes are forced downwards to relieve the pressure. This is the mass flow : the flow of water together with its dissolved solutes due to a force.

4. In the roots, the solutes are removed from the phloem by active transport into the cells of the root.

5. In the same time, ion are being pumped into the xylem from the soil by active transport, reducing the water potential in the xylem vessel.

6. The xylem now has a lower water potential than the phloem, so water diffuse by osmosis from the phloem to the xylem.

7. water and its dissolved oins are pulled up the xylem by tension from the leaves.

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