Cohesion in biology refers to the property of like molecules or entities sticking together. This attraction among similar components plays a widespread role in maintaining structure and enabling processes within living systems. It is a concept that underpins many biological phenomena, from the microscopic interactions of water molecules to the large-scale transport of fluids in organisms.
Understanding Cohesion in Biology
Cohesion involves the attractive forces between molecules of the same substance. In biological contexts, water molecules provide an example of this property. Water molecules are polar, meaning they have a slightly negative charge on the oxygen atom and slightly positive charges on the hydrogen atoms. This polarity allows individual water molecules to form weak electrostatic attractions, known as hydrogen bonds, with neighboring water molecules.
These hydrogen bonds form, break, and reform, creating a dynamic network where water molecules are attracted to one another. The collective strength of these hydrogen bonds gives water its high cohesive properties. This molecular “stickiness” is why water tends to bead up on surfaces and forms drops.
Cohesion Versus Adhesion
While cohesion describes the attraction between molecules of the same type, adhesion refers to the attraction between molecules of different substances. For example, water molecules exhibit adhesion when they cling to other polar or charged surfaces, such as the cellulose walls within plant vessels.
Cohesion and adhesion frequently work in tandem within biological systems. Capillary action, where water moves up narrow tubes against gravity, relies on both water’s cohesive forces holding the column together and its adhesive forces allowing it to cling to the tube walls. Their combined effects are often necessary for biological processes.
Cohesion in Action: Vital Roles in Biology
Cohesion plays a role in the transport of water within plants, particularly through the xylem tissue. The “cohesion-tension theory” explains how water moves from roots to leaves, often against gravity. As water evaporates from the leaves during transpiration, it creates a pulling force, or tension, which draws more water upward. The cohesive forces between water molecules ensure they remain connected, forming an unbroken column pulled up through the xylem vessels, allowing for efficient water and nutrient distribution throughout the plant.
Cohesion is also responsible for surface tension, a phenomenon where the surface of a liquid resists external forces. At the surface of water, molecules are attracted to their neighboring water molecules than to the air above them, creating a dense, film-like layer. This allows certain organisms, like water striders, to glide across the water’s surface without sinking. The spherical shape of water droplets also results from these cohesive forces, as molecules pull inward to minimize surface area.
Beyond plants and surface phenomena, cohesion contributes to the integrity of biological fluids and structures. In blood, for instance, cohesive forces among components help maintain the structural integrity of blood clots, which is important for stopping bleeding. Specialized proteins and cellular junctions also ensure cohesion between cells and their surrounding extracellular matrix. This cellular cohesion is important for the formation and stability of tissues and organs within complex organisms.