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Phloem Unloading

Sucrose is actively transported out of the sieve tubes into the sink cells.This creates a high water potential at the sink.The water potential gradient between the phloem and the xylem causes water to move out of the phloem into the xylem by osmosis.Sucrose is used up in the sink cells or converted into starch. This lowers the amount of sucrose in the cells and maintains the potential gradient between the phloem and the cells.

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Specification: AQA Biology (7401), AS-level, 3.3.4.2 Mass transport in plants. Topic: Mass Transport in Plants.

Key terms in this lesson

osmosis
The net movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.
water potential
The pressure created by water molecules, a measure of the tendency of water to move out of a solution; pure water has a water potential of zero and solutes make it more negative.
xylem
Plant vascular tissue, made of dead, lignified tubular cells, that transports water and mineral ions from the roots to the leaves.
phloem
Plant vascular tissue, made of sieve tube elements and companion cells, that transports organic solutes such as sucrose around the plant.

More in Mass Transport in Plants

All 15 lessons in Mass Transport in Plants · All AQA AS-level Biology topics