Cohesion in biology is the phenomenon where molecules of a single substance are attracted to one another. This internal attraction is particularly noticeable and significant in water, a substance that forms the basis of all known life. The tendency of water molecules to stick together gives rise to several unique properties that influence biological systems at both the microscopic and macroscopic level. This molecular self-attraction is foundational, shaping everything from the structure of a water droplet to the ability of a giant redwood tree to transport fluid against gravity.
The Role of Water and Hydrogen Bonds
The mechanism behind water’s powerful cohesive force originates from the structure of the water molecule itself. A water molecule consists of two hydrogen atoms and one oxygen atom held together by covalent bonds. However, the oxygen atom exerts a stronger pull on the shared electrons, a concept known as electronegativity. This unequal sharing of electrons creates a polar molecule, where the oxygen end has a partial negative charge and the hydrogen ends have partial positive charges.
These partial charges cause one water molecule to be weakly but consistently attracted to its neighbors. The partially positive hydrogen atom of one molecule is drawn toward the partially negative oxygen atom of an adjacent molecule, forming a specific type of intermolecular force called a hydrogen bond. While a single hydrogen bond is weak and transient, water molecules form vast networks of these bonds, connecting each molecule to up to four others at any given moment. This collective strength of countless hydrogen bonds provides the high internal attraction that defines water’s cohesive property.
How Cohesion Differs from Adhesion
Cohesion is specifically the attraction between like molecules, such as water clinging to other water molecules. Adhesion, on the other hand, is the attractive force that occurs between water molecules and molecules of a different substance. Both forces result from the polarity of water, but they describe two distinct types of interaction.
An example of adhesion is seen when water wets a surface, such as the way a drop spreads out on a clean glass plate. The water molecules are more strongly attracted to the charged silicon dioxide molecules in the glass than they are to each other. Conversely, on a waxy or nonpolar surface, the cohesive forces within the water are stronger than the adhesive forces, causing the water to pull into a spherical bead. This constant competition between the internal cohesive pull and the external adhesive pull dictates how water interacts with every biological surface it encounters.
Why Cohesion is Vital for Life
Cohesion is a property that makes possible the long-distance transport of water in plants. Trees and other vascular plants rely on the cohesion-tension theory to move water from the roots to the highest leaves, often against the force of gravity. As water evaporates from the leaves during transpiration, it creates a negative pressure, or tension, at the top of the plant.
Because the water molecules are strongly cohesive, the pull exerted by the evaporating molecules is transmitted down the entire column of water in the plant’s xylem vessels. This continuous, unbroken chain of water molecules acts like a rope, with the loss of one molecule at the top pulling the next one up from below. The combination of water’s cohesive strength and its adhesion to the narrow walls of the xylem allows the water column to resist breaking under the tension created by the transpiration pull.
Cohesion is also directly responsible for the phenomenon of surface tension, which is the measure of how difficult it is to break the surface of a liquid. Water molecules at the surface are pulled inward and sideways by their neighbors, but they lack the upward pull from molecules above them. This imbalance creates a strong, elastic film on the water-air interface. This film is strong enough to support small organisms, such as water striders, which can glide across the surface of a pond without sinking. This cohesive force at the surface influences the shape of water droplets.