Adhesion, in a biological context, refers to the tendency of dissimilar particles or surfaces to cling to one another. This fundamental property allows various biological structures to connect, interact, and function within living systems and their environments. It involves a range of forces that enable cells, tissues, and organisms to attach to substrates or to each other.
The Science Behind Biological Adhesion
Biological adhesion stems from various intermolecular forces. Van der Waals forces, weak attractions between molecules, contribute to transient adhesive interactions. Hydrogen bonds, stronger than van der Waals forces, form when a hydrogen atom is attracted to another electronegative atom. Electrostatic interactions, attractions between oppositely charged regions, also contribute to specific binding events. Hydrophobic interactions drive nonpolar molecules to associate in water, minimizing contact with the environment.
These forces facilitate binding of structures like molecular receptors or adhesive proteins. Adhesion is distinct from cohesion, which describes attractive forces between similar molecules, such as water molecules. While cohesion maintains substance integrity, adhesion enables biological components to interact with their surroundings, forming stable connections.
Adhesion Across the Tree of Life
Adhesion manifests in diverse life forms, enabling organisms to thrive. Animals employ adhesive mechanisms for locomotion, defense, and feeding. Geckos use millions of microscopic hairs (setae) on their toe pads, interacting with surfaces via van der Waals forces, to cling to almost any surface. Barnacles, marine crustaceans, produce a strong, quick-setting cement to attach to rocks, ship hulls, and other submerged surfaces, enduring harsh ocean currents.
Plants depend on adhesion for growth and structural support. Climbing vines, like ivy, develop aerial roots or tendrils that secrete adhesive compounds or form suction cups to grip walls and trees, ascending for sunlight. Plant root systems adhere to soil particles, anchoring the plant and facilitating water and nutrient absorption. This also stabilizes soil, preventing erosion.
Fungi use hyphae to secrete enzymes and adhere to substrates for nutrient breakdown and absorption. Mycorrhizal fungi form symbiotic relationships with plant roots, adhering to the root surface to enhance plant nutrient uptake, receiving sugars in return. Bacteria often form communities by adhering to surfaces. They secrete extracellular polymeric substances (EPS) that create a sticky matrix, allowing attachment to host tissues, medical implants, or environmental surfaces, important for survival.
Adhesion’s Role in Internal Biological Systems
Within an organism, adhesion is fundamental to tissue and organ formation, maintenance, and function. Cell-cell adhesion, mediated by proteins like cadherins and integrins, enables cells to recognize and bind. This binding is essential for organizing cells into distinct tissues, like epithelial or muscle tissues, providing structural integrity to the organism.
Beyond cell-cell interactions, cells adhere to the extracellular matrix (ECM), a network of proteins and carbohydrates. This cell-matrix adhesion, often involving integrins, is important for tissue architecture and mechanical stability. It plays a role in processes like wound healing, where cells migrate and adhere to the ECM to close injuries, and in embryonic development, guiding cell movements and differentiation.
Adhesion is important in the immune system, where immune cells adhere to target cells or vessel walls. For instance, white blood cells adhere to blood vessel linings to exit the bloodstream and reach infection or inflammation sites. In the nervous system, adhesion molecules guide neuron growth and branching, ensuring proper connections form during development and maintaining neural networks.
Shaping Ecosystems Through Adhesion
Adhesion extends its influence beyond individual organisms, shaping ecosystems. Biofilms are communities of microorganisms encased in a self-produced adhesive matrix of polysaccharides, proteins, and DNA. These biofilms form on virtually any surface in aquatic or terrestrial environments, from rocks in streams to plants and animals. They play an important role in nutrient cycling, facilitating organic matter breakdown and element transformation.
Plant root adhesion contributes to ecosystem stability by binding soil particles. This prevents soil erosion, maintaining fertile topsoil for plant growth and supporting diverse terrestrial ecosystems. In marine environments, various organisms’ adhesive abilities contribute to complex habitat creation. For instance, corals adhere to substrates and secrete calcium carbonate, building reef structures that provide shelter and food for marine species, forming biodiverse ecosystems. Attachment of algae and invertebrates to underwater surfaces contributes to the structural complexity and productivity of coastal and intertidal zones.