Question

Transpiration driven ascent of sap depends mainly on the \(\quad\) physical properties of water. (a) Cohesion (b) Adhesion (c) Surface tension (d) All of these

Short answer

(d) All of these

Step-by-step solution

Define Cohesion

Cohesion refers to the attraction between molecules of the same kind. For example, water molecules are attracted to each other, which is a key property that facilitates the movement of water in plants.

Define Adhesion

Adhesion is the attraction between molecules of different kinds. In the context of the ascent of sap, this would be the attraction between water molecules and the walls of the plant's xylem tubes.

Define Surface Tension

Surface tension is the force that causes liquid surfaces to shrink into the minimum surface area possible, it is due to the cohesive forces between liquid molecules.

Determine which properties are involved

All these properties of water: cohesion, adhesion and surface tension are involved in the upward movement of water in xylem. Cohesion helps water molecules stick together and form a continuous stream, adhesion helps the upward movement of water by opposing gravity, and surface tension helps keep the water together and resist forces that would act to increase its surface area.

Key concepts

Cohesion in Plants

Understanding the role of cohesion in plants is essential for grasping how they transport water from roots to leaves. Cohesion refers to the powerful attraction between water molecules. This attraction arises because water is a polar molecule; it has a positive end and a negative end, which allows each molecule to form hydrogen bonds with its neighbors. In the narrow tubes of a plant's xylem, cohesion works to pull water molecules upward as a continuous column. This phenomenon is so strong that even when water is pulled into the highest leaves of tall trees, it doesn't break the chain of molecules.

Cohesion is not just a fascinating scientific fact; it's a vital property for plant survival. This integrity of the water column is critical during the transpiration process, where water evaporates from the leaf's surface. As each molecule exits, it tugs on the next one due to cohesive forces, essentially pulling water from the roots much like a chain being pulled upwards. This process is known as the transpiration-cohesion tension mechanism.

Adhesion in Plants

While cohesion keeps the water molecules bound together, adhesion is another piece of the puzzle in the ascent of sap in plants. Adhesion involves the attraction between water molecules and other substances, like the inner surfaces of xylem vessels. This adhesion causes water to clings to the walls of the xylem tubes, assisting in counteracting the downward pull of gravity.

In practical terms, think of adhesion as the 'grip' water has on the xylem walls that helps the water climb against gravity. Together, with the cohesive properties of water, adhesion is a key player in the ascent of sap. It's this balancing act between adhesion and cohesion that maintains a stable flow of water despite the varying sizes and shapes of xylem vessels across different plant species. This interaction also helps maintain water inside the plant's transport system during periods when transpiration rates are low.

Surface Tension in Water Transport

Surface tension may seem like an abstract concept, but it plays a tangible role in the everyday functioning of plants. Surface tension is what allows insects to skim across a pond's surface and droplets to form and cling to surfaces. In the context of plants, this same principle prevents water inside the xylem tubes from being easily pulled apart and thus supports the continuous stream necessary for water transport.

At a microscopic level, it's the cohesive forces among water molecules at the top of the water column that create surface tension, which is most noticeable where water meets air. These forces minimize the surface area of water, creating a sort of 'skin' that is harder to break through. In a plant, when water evaporates from a leaf (transpiration), surface tension at the air-water interface within the xylem helps resist the pull of gravity on the remaining water column beneath it, thereby aiding in its upward movement.

Xylem Water Transport

The xylem is the plant's conduit for water transport, functioning much like biological plumbing. This system of hollow, tube-like structures made of specialized cells is designed for efficient upward water movement from roots to leaves. But how does water defy gravity through the xylem? This is where the cohesion-tension theory comes in, integrating the properties of cohesion, adhesion, and surface tension.

As water is transpired from the leaves, it creates a negative pressure that draws more water upward from the roots. The combination of water's adhesive properties with the xylem walls and the cohesive forces between water molecules allows for this continuous stream. Furthermore, the narrow diameter and structural reinforcement of xylem vessels are adapted to withstand the tension generated by transpiring water. This remarkable system illustrates nature's intricate design, ensuring that water, essential for photosynthesis and nutrient distribution, reaches every part of the plant efficiently.