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Phloem Unloading

Sucrose is actively transported out of the sieve tubes into the sink cells.This raises the water potential in the sieve tubes 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: OCR Biology A (H020), AS-level, 3.1.3 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
A measure of the tendency of water molecules to move out of a solution; pure water has a water potential of zero and adding solute lowers it.

More in Mass Transport in Plants

All 18 lessons in Mass Transport in Plants · All OCR AS-level Biology topics