Free Science Lessons: Transpiration and Cohesion-Tension Theory

Overview

Objective: Understand transpiration in plants and the cohesion-tension theory of water movement.

Photosynthesis
- Takes place in the leaves.
- Requires water (reactant) and carbon dioxide (CO2).
- Produces oxygen (O2).
Leaf Structure
- Main site: Palisade mesophyll in the top half.
- Water enters leaves via xylem vessels in the vascular bundle.
- Waxy Cuticle: Waterproof layer reducing evaporation.
- Stomata: Tiny pores on lower leaf surface; open during photosynthesis.
  - Allow CO2 to diffuse in and O2 to diffuse out.

Evaporation and Diffusion
- Leaf cell surfaces covered with a thin water layer.
- Water evaporates, creating high water vapor concentration internally.
- Water vapor diffuses out when stomata are open.
Transpiration
- Evaporation and diffusion combined are called transpiration.
- Causes a decrease in water potential in leaf cells, moving water via osmosis from adjacent cells.
- Water moves from xylem to adjacent cells due to reduced water potential.

Process
- Water is pulled out of xylem vessels, creating tension.
- Water movement from roots through stem and out of leaves.
Cohesion and Adhesion
- Water molecules form hydrogen bonds (Cohesion).
- Water bonds to xylem vessel walls (Adhesion).
Capillary Action
- Water moves against gravity in thin tubes due to cohesion and adhesion.

Explanation
- Combined effects of transpiration pull, cohesion, and adhesion draw water up the plant.
- Still widely accepted after over 100 years.
Supporting Evidence
1. Cutting a plant stem results in air being sucked into the xylem, indicating tension.
  - Air disrupts cohesion; stops water movement.
2. Tree trunk diameter reduces when transpiration is maximal.
  - Suggests negative pressure/tension in xylem.