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Plants move water and assimilates differently

By UK A Level Biology · independent study · myAudience

Xylem water travels toward evaporating leaves through a cohesive column under tension; mature vessels are not ATP-driven pumps. Phloem source loading can draw in water and raise hydrostatic pressure, while sink unloading supports lower pressure and mass flow. Energy used in loading is distinct from pushing sap along the entire tube. This is one source-to-sink route, not a rule that phloem always flows down. The potometer estimates 4 mm³ per minute water uptake, a proxy rather than exact transpiration loss.

Original myAudience scientific illustration; not a documentary photograph or measured specimen. · Original generated project artwork; no source textbook figures reused. Sources support scientific review, not copied-art licensing.

Plants move water and assimilates differently

Evaporation from leaves helps create tension in a continuous water column in xylem. Cohesion supports the column, while water entering roots replaces losses. This movement is not caused by every xylem cell pumping water with ATP; mature vessel elements are not living pump cells.

Phloem moves substances such as sucrose between sources and sinks. Loading and unloading can establish pressure differences that drive flow. A potometer measures water uptake, which is a useful proxy but not exactly the same measurement as transpiration loss.

Evaporation lowers leaf-water potential and supports tension transmitted through cohesive xylem water. Sugar loading into a phloem source can draw in water and increase hydrostatic pressure, while unloading at a sink helps maintain a pressure difference. Tracer evidence follows labelled material; ringing interrupts tissues and needs careful interpretation about which structures were affected. A transport model is strengthened by appropriate evidence, not by a single arrow or by calling a potometer reading identical to measured water loss.

Worked example: A potometer capillary has cross-sectional area 0.8 mm² and its bubble moves 25 mm in 5 minutes. Estimate uptake rate.

Volume displaced = 0.8 × 25 = 20 mm³.

Divide by 5 minutes.

Answer: 4 mm³ per minute.

Check your understanding: Is a developing fruit usually a source or sink for sucrose?

Answer: A sink.

Why: It receives assimilates for growth or storage rather than necessarily exporting them.

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