Hydrogen Bonding Causes Cohesion
Definition
Cohesion
The attraction between water molecules due to hydrogen bonding.
- In water, cohesion results from hydrogen bonds between water molecules.
- Each water molecule can form hydrogen bonds with neighbouring water molecules.
- These bonds hold water molecules together.
Consequence 1: Transport of Water Under Tension in Xylem
Cohesion enables water transport in plants through xylem vessels.
- How It Works
- Water evaporates from leaves through stomata (transpiration).
- This creates tension (negative pressure) that pulls water upward through the xylem.
- Hydrogen bonds between water molecules allow them to be pulled as a continuous column from roots to leaves.
- This is called the cohesion-tension theory.
- Why Cohesion Matters
- Without cohesion, the water column would break under tension.
- Hydrogen bonds hold water molecules together, preventing the column from snapping.
- This allows water to move upward against gravity, even in tall trees (over 100 meters high).
Cohesion allows water transport without the plant needing to use energy to pump water upward.
Consequence 2: Surface Tension and Use of Water Surfaces as Habitats
Cohesion creates surface tension at the water-air interface.
- What is Surface Tension?
- Water molecules at the surface are pulled inward by hydrogen bonds with molecules below and beside them.
- This creates a "skin" or elastic-like layer at the water's surface.
- Surface tension makes the water surface behave like a stretched membrane.
- Water surfaces as habitats: Small organisms can walk on water due to surface tension.
- Pond skaters (water striders): Insects with hydrophobic legs that distribute their weight across the water surface without breaking through.
- Mosquito larvae: Hang from the underside of the water surface, using surface tension for support.
Surface tension, caused by cohesion through hydrogen bonding, allows organisms to live on or interact with water surfaces as habitats.
Self review- What is cohesion?
- What type of bond causes cohesion in water?
- How does cohesion enable water transport in xylem?
- Why doesn't the water column in xylem break under tension?
- What is surface tension?
- How is surface tension related to cohesion?
- Give an example of an organism that uses surface tension as a habitat.
- How do pond skaters walk on water?
