Transport213-page9 - of the trunk, and most xylem is...

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Transport in Plants - 9 Moving through Xylem — The Cohesion–Tension Theory Although early researchers attempted to explain water movement by some mysterious "pumping" action within cells, no pumps have been found in plants. In 1893, Eduard Strasberger demonstrated that leaves were critical to water movement. He took trees of 20-meter height, cut them at the base of the trunk and placed them in a toxic copper sulfate solution. The solution progressively moved up through the severed trunk until it reached the leaves and the leaves died. Loss of roots did not affect movement of the solution. Additional research further demonstrated that positive root pressure could not explain water movement. Although root pressure is about 0.1 – 0.2MPa (or 1 – 2 atmospheres) ( and is a positive force in water movement during some circumstances discussed later ), a positive pressure would have to be sustained throughout the xylem
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Unformatted text preview: of the trunk, and most xylem is actually under a tension, a phenomenon that helps to explain how water appears to move upward throughout the plant. Ultimately it was been demonstrated that the loss of water through transpiration, discussed earlier, plays a significant role in water movement throughout the xylem. • Water lost by transpiration creates a negative water potential in cells that exerts a “pull” on the H 2 O in cell walls that is connected (by cohesion) to H 2 O in xylem resulting in a strong tension in the xylem. • As water evaporates out of the stomata, the film of water that coats mesophyll cells diminishes. Primary cell walls have adhesive properties; the remaining water is attracted to the walls (cellulose is very hydrophilic – think of the paper towel commercials), resulting in even less water and the water potential of the mesophyll cells decreases....
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This note was uploaded on 01/08/2012 for the course BIO 213 taught by Professor Makina during the Fall '09 term at SUNY Stony Brook.

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