Water possesses a remarkably high surface tension, a property often overlooked compared to its role as the universal solvent for life. This phenomenon arises from the strong cohesive forces between water molecules, primarily due to hydrogen bonding. Molecules at the water-air interface are pulled inward by their neighbors, creating an effect similar to a stretched, elastic membrane. Water’s surface tension is exceptionally high compared to other common liquids, directly impacting biological systems on both global and microscopic scales.
Supporting Surface-Dwelling Organisms
The high surface tension of water creates a boundary layer strong enough to support the weight of certain small organisms. This effect is readily observable in insects like water striders, which appear to walk or glide effortlessly across the surface of a pond. These organisms do not float due to buoyancy, but rather they are supported by the tension of the water’s surface “skin”. Their legs are specially adapted to utilize this tension, often being long and slender to distribute their minimal weight over a larger area.
The insect’s legs create small depressions, or dimples, in the water’s surface without actually penetrating the liquid-air boundary. These depressions increase the contact area and generate an upward force that counteracts the insect’s weight. Furthermore, the legs of water striders are covered in thousands of microscopic, non-wetting hairs. These hydrophobic structures trap air, which significantly enhances the water resistance of the legs and prevents the surface film from breaking.
The combined effect of weight distribution and the non-wetting surface of the legs allows the insect to remain suspended. If the surface tension were lowered, such as by adding a detergent, the supporting force would disappear, and the insect would immediately sink. This delicate balance between the organism’s specialized structure and the water’s cohesive forces demonstrates a direct biological reliance on this physical property.
Facilitating Internal Water Movement
Surface tension works in concert with related forces to drive the movement of fluids within living systems, particularly in plants. The movement of water and dissolved nutrients from the soil up to the highest leaves occurs against the constant pull of gravity. This transport relies on capillary action, a process where liquid flows in narrow spaces without external assistance. The cohesive forces between water molecules are foundational to this upward movement.
Water molecules adhere to the walls of the narrow xylem vessels within the plant stem, which is a process known as adhesion. Simultaneously, the strong hydrogen bonds cause water molecules to stick to each other, which is the property called cohesion. As water evaporates from the leaves through transpiration, the cohesive forces create a continuous, unbroken column of water extending all the way down to the roots. This is described by the cohesion-tension theory, which posits that the water column is pulled upward by the tension created by evaporation at the leaf surface.
The surface tension acting at the meniscus of the water within the leaf’s cell walls generates the negative pressure, or tension, that pulls the entire water column along. This tension is the primary driving force for water transport in tall trees, overcoming the force of gravity. Adhesion to the xylem walls prevents the water column from breaking, while cohesion ensures that the molecules follow the pull of the evaporating water.
Maintaining Respiratory System Function
The high surface tension of water, so beneficial to plants, would be detrimental to gas exchange in the lungs of air-breathing animals. The delicate, moist air sacs in the lungs, called alveoli, have an extremely small diameter and are lined with a thin layer of fluid. If the surface tension of this fluid remained high, the cohesive forces would cause the alveoli to collapse or stick together during exhalation. This collapse, known as atelectasis, would dramatically increase the energy required to re-inflate the lungs with every breath.
To counteract this powerful collapsing force, the body produces a lipoprotein mixture known as pulmonary surfactant. This substance is secreted by Type II alveolar cells and acts as a surface-active agent, reducing the surface tension at the air-water interface within the alveoli. A primary component of the surfactant is dipalmitoylphosphatidylcholine (DPPC), a lipid molecule that positions itself at the interface to disrupt the strong cohesive pull of the water molecules.
Pulmonary surfactant dynamically modulates surface tension based on the size of the alveolus. When the alveoli are smaller at the end of exhalation, the surfactant molecules become more concentrated, reducing the surface tension to near-zero levels, thus preventing collapse. If this mechanism fails, such as in cases of respiratory distress syndrome in premature infants, the inability to maintain open alveoli leads to severe breathing difficulty. The regulatory action of surfactant ensures that the work of breathing is manageable and maintains a stable surface area for efficient gas exchange.