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Mass Flow through Sieve Tube Elements

Sucrose loaded into the sieve tubes causes the sieve tubes to have a lower water potential.Water moves from xylem vessels into the sieve tubes down the water potential gradient by osmosis.This creates a high hydrostatic pressure which causes the mass flow of water containing dissolved sucrose through the sieve tube elements.Although mass flow is a passive process, the whole process is driven by the active transport of hydrogen ions so as a whole the process is active.

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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

active transport
The movement of molecules or ions across a membrane, often against their concentration gradient, using ATP and carrier proteins.
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.
hydrostatic pressure
The pressure exerted by a fluid, such as blood, that forces fluid out of the capillaries to form tissue fluid.

More in Mass Transport in Plants

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