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Books say that water moves from places with a high water potential to places with a low water potential. The movement of water in plants are explained because water potential is lowest in leaves and highest in roots.

There is also a theory called cohesion-tension theory for explaining the movement of water in plants. This theory says that the water lost at leaves will pull up the chain of water molecules.

I want to know whether the cohesion-tension theory is a part of the water potential explanation or not. For example, if there is a tree with the trunk height of 40 m. The water at 20 m height in the xylem of this tree will move up to 21 m. This is because the water potential in xylem is lower than the water potential at 20 m (?). But I am having a difficult time explaining why it is so by using the cohesion-tension theory. For example, I want to know which water potential is lower at 21 m than at 20 m, and why? I tried to think from the matrix potential, but I was not successful.

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The xylem actually creates a long thin string of water from the roots to the leaves of a tree. This string remains continuous by two forces. One is the cohesion of water molecules and the other is adhesion of water with xylem walls. Now in leaves water is evaporated. That decreases the pressure of water there. Because the protoplasm of those cells on leaves now have higher density than the others. This causes inflow of water inside those leaves by osmosis. This process in turns creates a pressure which pulls all the water string to rise a little and also helps the root hairs to absorb more water.

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Let me arrange some information briefly.

Cohesion-tension theory : phenomena that pulls water molecules at leaves producing tension + cohesion along entire stream of water molecules.

Cohesion of water molecules mainly arises from high-strength hydrogen bond and the tension that presses the stream is generated from various mechanisms.

On the other hand water potential deals with every potential per unit volume that generates forces upon water molecules. Tension/cohesion pulling water molecules can be viewed upward-potential and gravitational potential can be viewed as downward-potential for instance. Simply water molecules go upward only if when net water potential(cohesion-tension + gravity + capillary action + osmosis pressure + etc.) is directed upward.

Check http://www.fmf.uni-lj.si/~podgornik/download/cohesion-tension-review.pdf

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Most plant physiologists now accept the "cohesion-tension theory" as an explanation for the ascent of sap. According to this theory, water moves up the trunk of a tree in narrow, elongated cells near the periphery of the trunk, referred to as the xylem, and does not require the expenditure of metabolic energy. The movement of water only depends upon three important physical-chemical properties of water.

The first important property of water is that it always moves from a region with a more positive water potential, to a region with a more negative potential. Water potential is a measure of the energy available in a solution of water. Thus, water moves out of the leaves and into the air because the water potential of the air is more negative; water moves out of the tree trunk and into the leaves because the water potential of the leaves is more negative; water moves out of the roots and into the trunk because the water potential of the trunk is more negative; and water moves out of the soil and into the roots because the water potential of the roots is more negative.

The second important physical-chemical property of water is that it is a cohesive molecule. In other words, water molecules tend to bind to one another through the formation of hydrogen bonds. The cohesiveness of water molecules gives the thin water columns in a tree trunk a very great tensile strength. This prevents breakage of the water column when great longitudinal stresses are placed upon it as it is pulled out of the leaves and into the air.

The third important property of water is that it adheres very tightly to the walls of xylem cells in the tree's transport pathway. Adhesion of water to these cell walls maintains the full hydration of the pathway for water transport. This prevents breakage of the water column, and allows water transport even when a tree is water-stressed in a dry environment.

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