Cohesion describes the attraction between like molecules, a fundamental property influencing natural phenomena. In living organisms, water’s cohesive nature is particularly significant, impacting biological processes from the cellular level to entire ecosystems. This property allows life to thrive and adapt across diverse environments.
Water’s Cohesive Nature
Water molecules, composed of two hydrogen atoms and one oxygen atom, exhibit polarity. The oxygen atom carries a slight negative charge, while the hydrogen atoms possess slight positive charges. This uneven distribution of charge enables individual water molecules to form weak electrical attractions with neighboring water molecules, known as hydrogen bonds. These bonds are continuously forming and breaking, yet their collective strength creates a strong cohesive force, causing water molecules to stick together.
This inherent attraction between water molecules gives rise to several unique properties, including high surface tension. Surface tension causes the water’s surface to behave like a stretched, elastic film, resisting external forces. Water’s strong cohesive forces allow it to form droplets and maintain its shape, as seen when water beads on a non-absorbent surface.
Cohesion in Plant Water Transport
Cohesion plays a primary role in the transport of water and dissolved minerals from the roots to the leaves of plants, a process described by the cohesion-tension theory. Within the plant’s xylem vessels, water molecules form a continuous, unbroken column due to their strong cohesive forces. This continuous column is essential for efficient water transport against gravity.
As water evaporates from the leaves through a process called transpiration, it creates a negative pressure, or “pull,” at the top of this water column. This transpirational pull is transmitted down the xylem because of the strong cohesive forces between water molecules, effectively pulling successive molecules upward like a chain. Adhesion, the attraction between water molecules and the hydrophilic walls of the xylem vessels, also contributes by preventing the water column from breaking and helping to counteract gravity. This combination of cohesive and adhesive forces allows even the tallest trees to draw water hundreds of feet from the soil to their uppermost leaves.
Cohesion’s Influence in Animals and Cells
Cohesion is also fundamental to the stability and function of aqueous environments within animal bodies and at the cellular level. This attraction contributes to maintaining the structural integrity of cells, helping them resist changes in shape and volume.
Cohesion influences the properties of bodily fluids, such as blood, enabling efficient transport throughout the circulatory system. Blood plasma, which is predominantly water, flows smoothly through narrow capillaries without easily fragmenting, largely due to the cohesive forces holding the water molecules together. This ensures that nutrients, oxygen, hormones, and waste products are effectively circulated to and from tissues.
Surface tension, a direct consequence of cohesion, also allows certain organisms to interact uniquely with water surfaces. For example, lightweight insects like water striders can walk across the surface of a pond because their weight is insufficient to break the water’s high surface tension.