Explain the cohesion tension theory.

The cohesion-tension theory, also known as the cohesion-adhesion theory or the transpiration pull theory, is a widely accepted explanation for the upward movement of water in plants, particularly in vascular plants like trees. This theory explains how water is transported from the roots to the leaves against gravity, primarily driven by the process of transpiration.

The key components of the cohesion-tension theory are cohesion, adhesion, and transpiration:

Cohesion: Water molecules have a strong tendency to stick to each other due to hydrogen bonding. This cohesion creates a continuous column of water within the plant's xylem, which is the specialized tissue responsible for transporting water and minerals from the roots to the rest of the plant. The cohesion between water molecules allows them to form a continuous chain, similar to how a chain of beads sticks together.

Adhesion: Adhesion refers to the attraction between water molecules and the walls of the xylem vessels. The walls of the xylem contain specialized cells with lignin-reinforced secondary walls, which have a hydrophilic (water-attracting) nature. This adhesion helps counteract the force of gravity and prevents the water column from collapsing.

Transpiration: Transpiration is the process by which water vapor escapes from the stomata (tiny pores) on the leaves' surfaces. As water vapor exits the leaf, it creates a negative pressure or tension within the leaf's cells and the interconnected xylem vessels. This tension pulls on the water molecules within the xylem, creating a continuous flow from the roots to the leaves.

The combined effects of cohesion, adhesion, and transpiration work together to generate a transpiration pull, which results in a continuous flow of water from the roots through the stem and into the leaves. This pull is able to lift water to great heights, including the tops of tall trees.

It's important to note that the cohesion-tension theory does not account for the active pumping of water by the plant. Instead, it relies on passive processes driven by physical properties of water and the interactions between water molecules and the plant's vascular tissues. This theory has been supported by various experimental observations and has become a fundamental concept in plant physiology and biology.