Interactive Physics Lab
Biology 101 Classroom
Cohesion (Water ❤️ Water)
Cohesion is the attraction between molecules of the same substance. For water, this occurs because water molecules are polar—having a positive hydrogen end and a negative oxygen end. They form dynamic hydrogen bonds with one another.
🧬 Biology 101 Key Term: Surface Tension. Cohesion at the surface of a body of water creates a thin "skin" because the top water molecules are pulled tightly downward and inward. This allows things like leaves and water insects to float safely on top!
Adhesion (Water ❤️ Other Things)
Adhesion is the attraction between molecules of different substances. Water molecules will enthusiastically stick to other polar or charged surfaces, such as glass beaker walls, cotton towels, and plant vascular walls.
🧪 Biology 101 Key Term: The Meniscus. When water is placed in a glass graduated cylinder, it climbs the sides of the glass due to adhesion. This creates a curved dip called a concave meniscus. Always read the volume at the absolute bottom of this curve!
Capillary Action in Nature
When you combine cohesion and adhesion, a powerful physical phenomenon occurs: Capillary Action. This is the movement of water upwards through a narrow tube or porous material, entirely against the pull of gravity.
- Adhesion pulls the water molecules upward along the polar walls of the tube.
- Cohesion drags the rest of the water molecules behind, keeping the fluid column intact.
- Diameter Effect: Look at the simulator! If you make the tube narrower, the fluid rises significantly higher because there is more wall-contact area relative to the weight of the water column.
How Giant Trees Drink (Transpiration)
Giant redwood trees do not have mechanical pumps to drink water. Instead, they rely on a process called Transpirational Pull, powered by adhesion and cohesion.
🍃 The Ascent of Sap: As water evaporates from microscopic pores in plant leaves (stomata), it pulls on the molecular chain of water below it. Thanks to cohesion, the chain of water doesn't break. Thanks to adhesion, the water clings to the walls of the tree's microscopic water-pipes (xylem), climbing all the way from the roots up to the canopy!