Friday, February 1, 2013

Cohesion-Adhesion-Tension Theory

Ever wonder how water moves through plants? The leading theory explaining this phenomenon describes plants as a conduit for water balance between the water-rich biosphere and comparatively water-sparse atmosphere. You can visualize this as a concentration gradient map; the soil has much more water than the atmosphere, so water wants to move across the soil-air interface. Plants provide the means for this transfer.

The cohesion-adhesion-tension theorized process occurs in six major steps. (1) Water loss from leaf cells due to lower humidity in the air (water potential in the mesophyll becomes more negative). (2) Water from xylem flows into the mesophyll (water tension in the xylem becomes more negative, and exerts tension on water in xylem cells). (3) Water molecules stick together (cohesion), water molecules also stick to the cell walls (adhesion), and tension is transmitted down the xylem to the roots. (4) Water flows from root cells into the xylem (water potential in the root cells becomes more negative). (5) Water flows from soil into the root cells (water potential in the soil becomes more negative). (6) Precipitation replenishes the water potential in the soil.

There is a fair amount of evidence supporting this theory. Water potentials can be measured in all parts of plants, from leaf surfaces and leaf xylems, to root cells and root xylems. The data support the idea of plants as conduits for water concentration balance.

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