Water is an indispensable substance, forming the foundation of all life on Earth. Among its unique characteristics, water’s “stickiness,” known as cohesion, stands out as a fundamental attribute that underpins countless biological phenomena, making life possible in diverse environments.
The Sticky Truth: What Cohesion Is
Cohesion describes the strong attraction between molecules of the same type. Water molecules are polar, with slight positive charges on their hydrogen atoms and a slight negative charge on their oxygen atom. This polarity allows them to form weak electrical bonds, called hydrogen bonds, with neighboring water molecules. Each water molecule can form up to four hydrogen bonds, creating a network that holds water together.
Adhesion is a related property, describing the attraction between water molecules and different substances. Water’s polarity allows it to interact with other charged or polar surfaces. Together, cohesion and adhesion create surface tension, the resistance of a liquid’s surface to rupture. At the water’s surface, molecules are more strongly attracted to neighboring water molecules than to the air, forming a thin, elastic “skin”.
Life’s Lifeline: Water Transport in Plants
One primary example of cohesion’s impact is water transport in plants. Plants absorb water and nutrients from the soil, moving them against gravity to great heights. This upward movement relies on cohesion and adhesion through capillary action. Capillary action allows water to move up narrow tubes like xylem vessels, as adhesive forces pull water molecules up the walls while cohesive forces link them in a continuous column.
As water evaporates from leaves through stomata, a process called transpiration occurs. This evaporation creates a negative pressure, or “pull,” at the top of the xylem’s water column. Because water molecules are strongly cohesive, this pull transmits down the column, drawing more water upward from the roots. Without these cohesive forces maintaining an unbroken chain, plants could not efficiently transport water and nutrients.
Beyond the Green: Cohesion’s Wider Impact
Cohesion’s influence extends beyond plant water transport, shaping various aspects of life and ecosystems. Surface tension, a direct result of cohesion, allows small organisms to interact uniquely with aquatic environments. For instance, water striders can walk on water, exploiting the “skin” created by cohesive forces at the surface. Their specialized legs distribute weight, preventing them from breaking through the water’s surface tension.
Water’s cohesive properties also contribute to maintaining the structural integrity of biological cells and tissues. As a primary component of most living organisms, the strong hydrogen bonding between water molecules helps give cells their shape and provides a stable internal environment for biological processes.
This cohesive nature also indirectly contributes to temperature regulation. Water has a high specific heat capacity, meaning it can absorb or release a large amount of heat with only a small temperature change. This property, linked to the energy required to break and form hydrogen bonds, allows water to buffer temperature fluctuations and maintain stable conditions for life.