Adhesion is the attraction between molecules of different substances, while cohesion is the attraction between molecules of the same substance. These forces are crucial in biological systems due to the polarity of the water molecule. Water’s asymmetrical structure creates partial positive and negative charges, allowing it to form hydrogen bonds. This polarity enables water molecules to bond with each other (cohesion) and with other polar surfaces (adhesion). The interplay between these attractions is essential for transporting fluids, building complex tissues, and sustaining ecosystems.
Propelling Water Movement in Plant Life
Adhesion is involved in the upward movement of water within the vascular systems of terrestrial plants, overcoming gravity. This transport occurs primarily within the xylem, a network of specialized tubular cells extending from the roots to the leaves. Water molecules adhere strongly to the hydrophilic cellulose walls inside these narrow vessels.
This attraction is a component of capillary action, which helps pull the fluid upward over short distances. For taller plants, adhesion works with cohesion, forming the basis of the cohesion-tension theory of water transport. As water evaporates from the leaves through transpiration, a negative pressure pulls the entire column of water upward from the roots.
Adhesion prevents the water column from pulling away from the sides of the xylem vessels under tension. By anchoring the water to the vessel walls, adhesion maintains the continuity of the fluid column and counteracts the tendency of the water to break. Without this molecular attraction, the water column would fracture, making the sustained life of tall plants impossible. This mechanism delivers water and dissolved minerals to photosynthetic tissues.
Providing Structural Stability and Surface Interaction
Adhesion provides the binding forces necessary for the formation of complex, multicellular organisms and enables interaction with external surfaces. At the microscopic level, cell-to-cell adhesion organizes individual cells into functional tissues and organs. Specialized protein complexes, such as cadherins and integrins, act as molecular glue, facilitating the attachment of cells to each other and to the surrounding extracellular matrix.
Cellular Adhesion
Cadherins ensure cells of a similar type stick together to form structured tissues like epithelia. Integrins attach cells to the extracellular matrix, which provides mechanical stability for organs. This cellular adhesion is fundamental for development, tissue repair, and immune response.
Organismal Adhesion
On a larger scale, specialized organisms utilize adhesion for locomotion and stability. Geckos employ a dry adhesion mechanism using millions of microscopic hairs (setae) that branch into tiny spatulae. These structures increase the surface area, allowing weak van der Waals forces to generate enough cumulative force to support the animal’s weight on smooth vertical surfaces.
Many insects, like flies, use a “wet adhesion” strategy. They secrete a fluid from their foot pads to create a strong capillary attraction between the foot and the surface, allowing them to cling to surfaces against gravity.
Enabling Water Retention in Soil and Substrates
Adhesion governs how water is held within the soil and porous substrates, maintaining viable terrestrial ecosystems. As water infiltrates the ground, adhesive forces cause polar water molecules to cling to the surfaces of soil particles, especially clay. Clay particles, with their high surface area, exert a strong adhesive pull on the water molecules.
This attraction allows a film of water to be retained against the downward pull of gravity, creating soil moisture. The layer of water held directly against the particle surface is tightly bound by adhesion and is generally unavailable to plant roots. This adhesive layer then attracts subsequent layers of water molecules through cohesion, forming a thicker film that is accessible to plants.
The capacity of soil to retain plant-available water is related to the surface area of its constituent particles. Finer-textured soils like clay hold more total water due to their higher adhesive capacity. By adhering water to solid material, adhesion effectively stores the fluid in the root zone, ensuring a sustained supply for plant life after rainfall.