A1.1.4 Adhesion of water to materials that are polar or charged and impacts for organisms Notes

Adhesion of Water to Polar or Charged Materials

1. Water is polar (covered in the A1.1.3), meaning it's attracted to other polar or charged materials.
2. This attraction allows water to "stick" to surfaces and move along them.

Capillary Action: Adhesion and Cohesion Working Together

1. It requires both adhesion and cohesion where:
   1. Adhesion pulls water up along the walls of narrow spaces.
   2. Cohesion (hydrogen bonds between water molecules) pulls the rest of the water column along.
2. This results in water moving upward through small tubes or pores without external energy input.

Capillary Action in Soil

1. In soil, water occupies the spaces between particles, moving through pores of varying sizes.
2. Porous soils, such as those rich in sand or silt, have numerous small spaces that allow water to rise through capillary action.
3. When the soil is dry, water is drawn upward from deeper layers or underground sources, rehydrating the upper layers.
4. This matters for organisms because it aids:
   1. Water distribution: Capillary action redistributes water throughout the soil profile, making it accessible to plant roots even when the surface is dry.
   2. Survival in arid conditions: In dry environments, capillary action draws water from deeper soil layers upward, helping plants access moisture.

Capillary Action in Plant Cell Walls

1. Water molecules adhere to cellulose in the cell wall through hydrogen bonding.
2. Cohesion between water molecules creates a continuous network of water throughout the cell wall (recall from A1.1.3).
3. As water evaporates from leaf surfaces during transpiration, adhesive forces pull more water from the xylem into the cell walls.

Why It Matters for Organisms

1. Maintains moist cell walls:
   1. Moist cell walls allow CO₂ to dissolve and diffuse into cells for photosynthesis.
   2. Without adhesion keeping cell walls hydrated, gas exchange would be impaired.
2. Supports water transport:
   1. Water adheres to the cellulose-lined walls of xylem vessels.
   2. Combined with cohesion, this allows water to be pulled upward through the plant from roots to leaves (part of the cohesion-tension mechanism covered in the previous article).
3. Structural support:
   1. Water in cell walls contributes to turgor pressure, helping plants maintain structure.

Summary: How Adhesion Benefits Organisms