Cohesion, the attraction between like molecules, plays a pervasive role in supporting life on Earth. This property influences many biological processes, especially those involving water. Water’s ability to stick together is foundational to its unique characteristics, enabling diverse life forms to thrive.
The Unique Nature of Water’s Cohesion
Water’s cohesive properties originate from its molecular structure. A water molecule is polar, with slight positive charges on its hydrogen atoms and a slight negative charge on its oxygen atom. These partial charges allow water molecules to form weak attractions, called hydrogen bonds, with neighbors. Multiple bonds create a strong, interconnected network.
Extensive hydrogen bonding results in strong cohesive forces. Adhesion, water’s attraction to different surfaces, often polar or charged, works alongside cohesion. Both properties contribute to phenomena like capillary action. Surface tension, which allows water’s surface to resist external forces, is a direct result of these strong cohesive forces at the water-air interface.
Cohesion’s Role in Plant Water Transport
Cohesion, working alongside adhesion, is important for water transport in plants. This process explains how water moves from roots to leaves against gravity. Water molecules form a continuous column within the plant’s xylem vessels due to strong cohesive forces. Adhesion helps this water column stick to the xylem walls.
Transpiration, the evaporation of water from leaves, creates tension at the leaf surface. This tension pulls the continuous water column upwards, like sipping through a straw. Water’s cohesive strength prevents the column from breaking, even in tall trees. This transport system delivers water and dissolved nutrients to plant cells, supporting photosynthesis and maintaining plant structure.
Surface Tension: A Cohesive Support for Life
Surface tension, a direct result of water’s cohesive forces, forms a taut, elastic “skin” on the water’s surface. This property allows organisms to interact with the water-air interface without sinking. For instance, water striders, lightweight insects, glide effortlessly across ponds. Their weight is insufficient to break the hydrogen bonds at the water’s surface.
Mosquito larvae also use surface tension, hanging from the water’s surface to access air for breathing. This provides a stable platform for small aquatic organisms. They can hunt, lay eggs, or move within their habitat.
Maintaining Biological Systems Internally
Beyond external interactions and plant transport, cohesion helps maintain the internal integrity of biological systems. In animals, water’s cohesive nature contributes to the continuous flow of fluids in circulatory systems, like blood. Cohesion ensures blood plasma moves efficiently through vessels, aiding in the transport of nutrients, hormones, and waste products.
In plants, cohesion helps maintain turgor pressure within cells. Water entering plant cells by osmosis creates internal pressure, pushing the cell membrane against the rigid cell wall. This turgor pressure provides structural support, preventing wilting and allowing plants to stand upright.