Name: Translocation of Mineral Ions
Uploaded: 2019-07-19
Duration: 8 min 6 s
Description: Translocation of mineral ions occurs in the plants through special channels called phloem. Watch this video to learn how the minerals are absorbed and transl...

Question

Describe the transpirational pull and cohesion-tension theory of ascent of sap.

Accepted Answer

The loss of water in the form of water vapor to the atmosphere from the mesophyll cells through transpiration creates a negative pressure. This assists the process of upward transportation of xylem sap from the passage cells of the endodermis through xylem vessels in the leaves that is called ascent of sap. It is best explained by the Cohesion-tension theory. This theory is proposed by Dixon and Joly in 1894. It is supported by Curtis and Clark in 1951, Levitt in 1969.

The theory has two essential features:

The cohesion of water and adhesion between water and xylem tissues

Transpirational pull

i. Cohesion of water and adhesion between water and xylem tissues: The water molecules have a strong mutual attraction which is called cohesion. The water molecules also tend to stick to the wall of the xylem which is called adhesion. Cohesion force provides tensile strength to the water column. Both of these forces help to maintain the continuity of the water column from roots to leaves. The forces that act against the cohesive force of water and which try to break up the water column in the plant are the weight of the water column itself. But the magnitude of the cohesive force i.e. about 300 atmosphere is sufficient for maintaining the continuity of the water column in plants.

ii. Transpirational pull: The Transpirational pull is the pulling force that is responsible for lifting the water column. The negative tension is gradually transmitted downwards via xylem tissues of the leaf, stem, and finally to the roots. As a result, there is a continuous upward movement of the water column in the plant. Thus, the transpirational pull acts as pull from above on the whole water column of the plant which draws the water of xylem vessels of roots towards leaves in an upward direction.

Describe the transpirational pull and cohesion-tension theory of ascent of sap.

Important Questions on Transport in Plants

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