Question

Transpiration cohesion theory explains that, the upward pull of water is transmitted from top to bottom by cohesion of molecules caused by
a. Hydrophilic cell walls
b. Hydrogen bonds
c. Oxygen bonds
d. Surface tension

Answer 1

Transpiration is the biological process of a plant in which water moves through a plant and gets evaporated from aerial parts, such as leaves, stems, and flowers. Water is required by the plants for various different processes but only a small amount of water taken up by the roots is used for growth and metabolism and the rest amount is lost in the atmosphere.

Complete answer:
Transpiration is the process of evaporation of water from the plant body. Transpiration is a biological process of the plant to eliminate the excess water in the form of vapor. This process takes place in the aerial parts, such as leaves, stems, and flowers. A plant gets water to perform its different metabolism processes from the groundwater through its roots. A very small amount of this water is used by the plants, the rest of the water is lost by the plants through transpiration and guttation.

Water is taken at the roots by the process of osmosis and it travels through the xylem tissue of the plant. Adhesion and cohesion features act in tandem to pull water molecules from the roots and pass through xylem tissue. Finally, in the stomata of leaves, transpiration takes place.

The theory of cohesion was first proposed by two botanists Henry Dixon and Joly. Cohesion is the force of mutual attraction between water molecules. The water molecule is transported through the vessels. The wall of the vessels is composed of lignin and cellulose and they have a high affinity for water which is known as adhesion.

The transpiration causes in the leaves due to negative pressure or tension in xylem sap which is transmitted to the root. This is called transpiration pull and is responsible for the movement of the water column upward. The transpiration pull or tension has been found due to the inner attraction of water molecules to one another i.e. cohesion force. This cohesive force between the water molecules prevents the water column from breaking. Hydrogen bonds present in the water molecules which prevent the division of water and water is pulled upward through the xylem.

Hence, the correct answer is option (B).

Note: Transpiration is a process of the excretory system of a plant. In this process, the plant eliminates the lost extra water through the leaves. The water is taken to the roots from the roots due to the transpiration pull and this transpiration pull comes on effect due to the cohesive bond of the water molecule. The hydrogen bond of the water prevents the water from the division.