Cohesion is the physical property of a substance where its molecules are attracted to one another. For water, this attraction results in a continuous liquid mass, a fundamental requirement for all biological life on Earth. This internal connectivity gives water unique properties that allow for essential biological processes, from the microscopic scale of a single cell to global ecosystems.
The Mechanism of Water Transport in Plants
Cohesion is the driving force behind the Cohesion-Tension Theory, which explains how vascular plants transport water against the pull of gravity, sometimes to heights exceeding 100 meters in tall trees. This mechanism relies entirely on water molecules remaining linked together in an unbroken chain within the plant’s xylem vessels. The process begins with transpiration, the evaporation of water vapor from the leaves’ stomata into the atmosphere.
As water evaporates from the leaf’s surface, it creates a negative pressure, or tension, on the remaining water in the xylem. This tension pulls the entire column of water upward, from the roots through the stem, similar to sucking on a straw. Cohesion ensures that as one water molecule moves up to replace the one lost to evaporation, it drags the next molecule behind it, preventing the column from breaking.
The narrow diameter of the xylem vessels helps maintain the integrity of this water column. This continuous, unbroken stream of water delivers water and dissolved mineral nutrients from the soil to every cell in the plant. Without the strong attractive forces between water molecules, this bulk flow would be impossible, and tall terrestrial life could not exist.
Forming Surface Tension for Aquatic Organisms
The strong cohesive forces between water molecules create a phenomenon known as surface tension at the interface where water meets air. At this boundary, water molecules are attracted more strongly to their neighboring water molecules than to the air molecules above them. This unequal attraction results in the water surface behaving like a thin, flexible, elastic membrane under tension.
This physical boundary is robust enough to support the weight of small organisms, providing a specialized habitat. Insects like water striders, for example, can distribute their weight across the water’s surface without breaking the hydrogen bonds that form this “skin.” Their lightweight bodies and hydrophobic (water-repelling) legs allow them to exploit this cohesive property for locomotion.
Surface tension also supports smaller life forms, collectively known as neuston, which live at the air-water interface. Cohesion allows aquatic plants to float and provides a stable platform for various larvae and microbes. This physical property defines a distinct ecological niche necessary for the survival of many small aquatic species.
The Role of Hydrogen Bonds in Cohesion
The inherent cohesiveness of water is a direct consequence of its molecular structure and the resulting formation of hydrogen bonds. A single water molecule is polar, meaning it has an uneven distribution of electrical charge. The oxygen atom tends to pull the shared electrons closer to itself, giving it a slight negative charge, while the two hydrogen atoms acquire slight positive charges.
This polarity allows a weak electrical attraction to form between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of a neighboring molecule. This specific type of weak intermolecular attraction is called a hydrogen bond. Though individually weak, these bonds are numerous; each water molecule can form up to four hydrogen bonds with its neighbors.
The continuous, rapid forming and breaking of this extensive network of hydrogen bonds throughout the liquid produces the collective force known as cohesion. This network is the fundamental chemical basis for water’s unique properties, essential for supporting biological processes.