A-Level Biology: Transpiration and Cohesion Tension Theory

Transpiration

• Definition: Loss of water vapor from the stomata, occurring through evaporation.
• Stomata: Pores primarily on the lower side of leaves, formed by guard cells.
• Conditions Affecting Transpiration:
  • Light Intensity: Positive correlation; higher light intensity opens more stomata, increasing surface area for evaporation.
  • Temperature: Positive correlation; higher temperature increases kinetic energy of water molecules, thus increasing evaporation rate.
  • Humidity: Negative correlation; more humid air lowers water potential gradient, slowing evaporation.
  • Wind: Positive correlation; wind removes surrounding humid air, maintaining water potential gradient, facilitating faster evaporation.

Cohesion Tension Theory

• Purpose: Explains how water moves up plants against gravity.
• Key Components:
  • Cohesion:
    • Water is dipolar with hydrogen bonds between molecules, allowing them to stick together.
    • Results in a continuous water column in the xylem.
  • Adhesion (Capillarity):
    • Water molecules stick to xylem walls, aiding upward movement.
    • Smaller xylem diameter increases the capillarity effect.
  • Root Pressure:
    • Water entering roots by osmosis increases pressure, pushing water upwards.

Full Process

1. Transpiration Pull:
   • Water evaporates from stomata, reducing water volume and pressure in that area.
   • Creates a negative pressure that pulls water up from below.
2. Continuous Water Column:
   • Due to cohesion, water moves as a continuous column up the xylem.
   • Adhesion to xylem walls prevents water from falling back.
3. Tension in Xylem:
   • Evaporation creates an upward pull, drawing xylem walls inward.
   • Narrower xylem enhances capillarity, further facilitating water movement.

Summary

• Transpiration: Water vapor loss through evaporation at the leaves.
• Cohesion: Connects water molecules; forms a continuous column.
• Adhesion: Water sticks to xylem; enhances upward movement.
• Tension: Evaporation-induced pull narrows xylem, boosting capillarity.

Further Learning

• Mass Transport in Plants: Explore transport of sucrose and other substances.
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