Cohesion, the attraction between like molecules, plays a fundamental role in all life on Earth. This property is especially significant for water, a substance that underpins nearly every biological process. Without water’s unique cohesive characteristics, the systems that sustain organisms would not be possible. Understanding cohesion provides insight into life’s governing physical principles.
What Cohesion Is
For water, this strong attraction arises from its molecular structure. A single water molecule (H₂O) has a bent shape, with the oxygen atom holding a slight negative charge and the two hydrogen atoms carrying slight positive charges. This uneven distribution of charge makes water a polar molecule.
These partial positive and negative charges allow adjacent water molecules to form weak electrical attractions called hydrogen bonds. The partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of a neighboring water molecule. Each water molecule can form up to four such hydrogen bonds with others, creating a network of interconnected molecules.
Hydrogen bonding is responsible for water’s strong cohesive forces. The continuous formation, breaking, and reforming of these hydrogen bonds give liquid water its fluid properties while maintaining its structural integrity.
Water Transport in Plants
Water’s cohesive properties facilitate its transport through plants, particularly in the cohesion-tension theory. Plants absorb water and dissolved nutrients from the soil through their roots. This water must then travel against gravity, sometimes hundreds of feet, to reach the leaves where photosynthesis occurs.
This upward movement relies on a continuous column of water within specialized plant tissues called xylem vessels. As water evaporates from the leaves through tiny pores called stomata—a process called transpiration—it creates a negative pressure, or tension, in the leaf. This tension acts like a suction, pulling the water column upwards.
The strong cohesive forces between water molecules ensure the water column remains unbroken as it is pulled, creating a continuous chain from roots to leaves. Adhesion, the attraction of water molecules to the hydrophilic (water-attracting) walls of the xylem vessels, also assists this movement by counteracting gravity and preventing the column from breaking. The combined forces of cohesion, adhesion, and transpirational pull allow water and dissolved minerals to be transported efficiently throughout the entire plant, supporting its growth and survival.
Surface Tension and Aquatic Life
The strong cohesive forces among water molecules also create a phenomenon known as surface tension at the water-air interface. Molecules within the bulk of the water are attracted in all directions by neighboring water molecules. However, molecules at the surface are only attracted inward and sideways, leading to a net inward pull that causes the surface to contract and behave like a stretched elastic film.
This “skin-like” property of water’s surface supports various forms of aquatic life. For example, small insects like water striders can walk across the surface of ponds without sinking. Their light weight and specialized, hydrophobic legs distribute their mass over a large area, preventing them from breaking the water’s surface tension.
Surface tension also enables the formation of spherical water droplets, as the cohesive forces pull the liquid into the shape with the smallest possible surface area for its volume. This property is also evident when a paper clip or needle can float on water, despite being denser than water. These instances demonstrate how cohesion creates unique environments and supports ecological interactions in aquatic systems.