Cohesion, the attraction of water molecules to one another, is a fundamental property of water derived from its molecular structure. This characteristic is one of the most significant physical forces enabling life to exist and flourish across the planet. The ability of water to stick to itself and maintain a continuous body supports biological functions ranging from the microscopic movement of fluids to the mechanics of the tallest trees.
The Molecular Basis of Cohesion
Cohesion in water originates from the uneven distribution of electrical charge within the water molecule, a concept known as polarity. A single water molecule consists of one oxygen atom bonded to two hydrogen atoms, but the oxygen atom attracts the shared electrons more strongly. This differential pull creates a partial negative charge near the oxygen atom and partial positive charges near the two hydrogen atoms.
These partial opposing charges allow one water molecule to form a weak electrical attraction, called a hydrogen bond, with a neighboring water molecule. The partially positive hydrogen of one molecule is attracted to the partially negative oxygen of another. A vast network of these attractions forms throughout liquid water, even though an individual hydrogen bond is significantly weaker than the covalent bonds holding the molecule together.
This extensive network of hydrogen bonds provides water with its powerful cohesive force. The collective strength of these weak attractions causes water to be highly cohesive compared to other non-metallic liquids. This strong self-attraction allows water to resist forces that would otherwise pull it apart, which is required for its role in many biological systems.
Creating the Surface Film: Biological Significance of Surface Tension
The strong self-attraction of water molecules produces surface tension at the interface between liquid water and air. Molecules within the body of the water are pulled in all directions, resulting in a net zero force. However, molecules at the surface are only attracted inward and sideways, creating a net inward pull that minimizes the surface area.
This inward force causes the surface of the water to behave like a stretched elastic film. The biological significance of this surface film is demonstrated by aquatic insects, such as water striders. These insects glide across the surface without sinking because their light weight and specialized legs distribute their mass across the water’s highly tensioned surface.
Surface tension is also responsible for the formation of water droplets, a shape that minimizes the surface area to volume ratio. Droplet formation is crucial for distributing water in the environment and creating small, localized microbial habitats. For a substance to enter or leave the body of water, it must overcome this intermolecular force, which acts as a protective barrier in small-scale environments.
Water Movement Through Vascular Systems
The cohesive nature of water is foundational to the Cohesion-Tension Theory, which explains how water is transported from the roots to the highest leaves of plants. Cohesion allows water molecules to form a continuous, unbroken chain within the narrow tubes of the plant’s xylem tissue. This continuous water column is held together by the collective force of hydrogen bonds between the water molecules.
The upward movement is powered by transpiration, the process where water vapor evaporates from the leaves through small pores called stomata. As water leaves the leaf surface, it creates a negative pressure, or tension, that pulls the entire water column upward from the roots, similar to sucking on a straw. This tension can be substantial.
The cohesive strength of water is high enough to resist breaking under this extreme tension, allowing the water column to be pulled against the force of gravity. Adhesion, the attraction of water to the walls of the xylem vessels, also contributes by preventing the column from collapsing inward and maintaining the continuity of the fluid. Without this strong cohesive force, complex, tall plants would be incapable of moving the necessary water and dissolved minerals to their upper parts.