Syntheic Group Of Proteins

Transport in the Xylem

Cohesion-Tension Theory of water movement.

 Water in leaves develops large negative ΨP (tension)

 This tension pulls the water up through the xylem

 Cohesion is required to maintain the water column integrity

 Cohesion allows the pull from the leaves to extend all the way through the xylem to the roots and ultimately to the soil.

 This also depends on adhesion of water to the apoplast

 The pulling force arises from differences in ΨW.

 Neither capillary action nor air vacuums are sufficient to explain the large pull from the leaves.

 Instead, this results from fact that air has an extremely low ΨW, around - 95MPa.

Evidence for theory

 Evidence that transport is in the xylem: put radioactive potassium in soil or water. Ask where radioactivity goes. It is found in the xylem.

 Evidence of pulling:

* Cut stem and place cut portion of upper stem in water with dye. Dye moves up the stem in the xylem.

* Measure trunk diameter at the top and bottom of a tree over the course of a day. The top of the trunk shrinks before the bottom.

 Evidence that water is cohesive enough to withstand forces in the xylem: Place water in a capillary tube and spin in a centrifuge. Increase spin until the water in the column separates

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