Question

Explain how plants without a heart, can pump water up sometimes hundreds of feet above ground. Base you explanation in the characteristics of water molecules and water potentials.

Answer 1

Transportation of water inside plants occurs by two features;

1. Cohesive and adhesive properties of water molecules to form an unbroken continously water column in xylem.

2. Transpiration pull or tension exerted on this water column.

Xylem vessels are tubular structures extending from roots to the top of plants. One end of xylem tube is connected with root hairs via pericycle endodermis and cortex and the other end is connected with the substomatal cavity in the leaves via mesophyll cells.

The water gets filled inside the xylem capillaries and due to cohesiom and adhesion properties of water, it forms a continous water column. This water column cannot be broken away because of cohesion and adhesion properties of water. This water column is just like a steel rope which is extended from substomatal cavities in leaves to the roots. If this rope is pulled from the top , the entire rope will move upward. In plants, this pull is generated by the process of transpiration called as transpiration pull.

water evaporates from the mesophyll cells to the intercelkular spaces as a result of active transpiration. Loss of water from mesophyll cells causes a decrease in water potential ie the cell sap of mesophyll becomes more concentrated and consquently their water potential is decreased. The water moves from higher water potential to lower water potential, thus water moves from cell to cell and finally movement of water within the leaf is transmitted to the water filled in the xylem elements. This exerts a pull and water, filled in xylem comes in a state of tension.

This pull is called transpiration pull. This tension generated at the top of water column is transmitted downwards from petiole, stem and finally reaches to the roots. This pull results in the upward movement of water.