Cohesion is the attraction between molecules of the same substance, meaning water molecules sticking to other water molecules. This fundamental molecular tendency is foundational to the existence and function of nearly all biological systems on Earth. The collective action of these attractive forces gives water unique characteristics that allow it to support life, from the smallest cells to the tallest trees. Without this inherent stickiness, many complex processes necessary for life would be impossible.
The Unique Molecular Bond Driving Cohesion
Water molecules cohere due to their inherent polarity, a structural feature resulting from the uneven sharing of electrons between oxygen and hydrogen atoms. Oxygen is significantly more electronegative, pulling electrons closer to itself and gaining a slight negative charge. This leaves the two hydrogen atoms with slight positive charges, creating a molecule with distinct electrical poles.
This charge separation allows a weak electrostatic attraction, known as a hydrogen bond, to form between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of a neighboring molecule. While individually weak, the cumulative effect of these numerous and constantly forming hydrogen bonds creates a strong cohesive force throughout liquid water. This force is responsible for water’s tendency to form droplets and resist being pulled apart.
Facilitating Water Transport in Ecosystems
The cohesive property of water is applied on a vast scale in terrestrial ecosystems, particularly in the process of water transport through vascular plants. Water must be moved from the roots up to the leaves, sometimes defying gravity over distances exceeding 100 meters in tall trees. This upward movement is primarily explained by the cohesion-tension mechanism, which relies heavily on water’s self-attraction.
As water evaporates from the leaves during transpiration, it creates a negative pressure, or tension, within the continuous column of water extending down to the roots. Cohesion ensures the water molecules remain linked together, allowing the tension generated at the leaf surface to pull the entire column of water upward like a rope. This pulling force is possible because the hydrogen bonds are strong enough to prevent the water column from breaking under the stress.
The process is also aided by adhesion, which is the attraction between water molecules and the cell walls of the narrow xylem vessels. Adhesion helps counteract gravity and prevents the water column from slipping downward, a combined effect often referred to as capillary action. Together, cohesion and adhesion maintain the uninterrupted flow of water and dissolved minerals, supporting the metabolic needs of the entire plant structure.
Maintaining Surface Tension for Aquatic Life
Cohesion also manifests as surface tension, a phenomenon occurring at the air-water interface due to the imbalance of forces on the surface molecules. Water molecules deep within the liquid are pulled in all directions by their neighbors, but those at the surface are only pulled inward and sideways. This strong inward pull creates a dense, film-like layer that resists external force and allows water to form beads and droplets.
Ecologically, surface tension supports unique biological niches, enabling small organisms to move across the water’s surface without sinking. For instance, the water strider insect exploits this cohesive force, distributing its weight across the strong surface film with its long, non-wetting legs. This cohesive skin also plays a role in gas exchange and light reflection at the air-water boundary, influencing the microenvironment for many aquatic species.