Chapter 25, Problem 1LTB

Summary Introduction

To write:

The reason behind a column of water in a xylem tube that remains intact despite tremendous tension.

Introduction:

Water and minerals are taken from the soil, together called xylem sap, travel up the xylem, and the cells that help absorb water inside a plant. The tendency for similar molecules to bond such as molecules of water linked by hydrogen bonding, is said as cohesion. The tendency for unlike molecules to stick together, such as water is hydrogen-bonded to other polar molecules, is called adhesion.

Explanation of Solution

The pulling force on a column of xylem sap is generated by the water evaporated from leaves. Stomata, the pores on leaf surfaces, control the exchange of gas and water loss in plants. From stomata, when water evaporates, the forces of adhesion and cohesion on the water molecules that remain inside the leaves create tension or negative water pressure. Tension is the force that allows the plunger to pull a column of water into a syringe.

Water adhesion to xylem cell walls and cohesion to other molecules of water even under extreme tension maintain continuity of xylem sap. Tension in plants causes water molecules to be drawn out into the leaf from the xylem to take the place of water that has evaporated. This in turn, increases tension on the water molecules immediately below them in the xylem, causing water molecules to move toward the leaves, roots and the soil.

Conclusion

Tension in leaf cell walls is important because it helps to maintain the transpiration stream. When water is sucked through the xylem, it creates low pressure in the xylem. The continuity is maintained together by cohesion and adhesion during transpiration.