Lecture Notes: Transpiration in Plants

Introduction

• Transpiration: Loss of water vapor from plant surfaces, mainly leaves.
  • Important for plant structure and transport.
  • Relevant for biology exams.

Main Concepts

1. Process of Transpiration

• Evaporation: Water evaporates mostly from leaves.
• Stomata: Tiny pores on leaves where evaporation occurs.
  • Guard Cells: Control opening and closing of stomata via osmosis.
    • Turgid Guard Cells: Stomata open.
    • Flaccid Guard Cells: Stomata close.
  • Osmosis Driven By:
    • Internal CO2 concentration
    • High temperatures (closes stomata due to production of abscisic acid)

2. Factors Affecting Transpiration Rate

• Environmental Conditions:
  • Dry/low humidity increases transpiration.
  • Breezy weather increases transpiration.
  • Warm and sunny conditions increase transpiration.

Leaf Structure and Adaptations

• Waxy Cuticle Layer:
  • Thick layer on top, thinner below.
• Stomata Distribution:
  • More stomata on the underside of the leaf.
  • Reduces direct sunlight exposure.

Transpiration Stream

• Continuous Flow: Water moves from root to leaf.
• Importance: Essential for photosynthesis.
• Pathway:
  • Water absorbed at roots (osmosis).
  • Enters xylem, travels up to leaves.
  • Xylem: Dead tissue, reinforced with lignin.

Xylem and Phloem

• Xylem: Responsible for water transport.
  • Tracheids and Vessels: Form long, hollow tubes.
• Diagrams: Important to learn and label for exams.

Root Structure

• Root Hairs: Increase surface area for water absorption.
• Root Zones:
  • Zone of Protection: Root cap.
  • Zone of Cell Production: Meristem.
  • Zone of Cell Elongation.
  • Zone of Differentiation: Root hairs.

Cohesion-Tension Model

• Dixon and Joly: Developed by scientists from Trinity College, Dublin.
• Explains: Water transport against gravity.
• Important Elements:
  • Transpiration
  • Adhesion
  • Cohesion
  • Tension
• Root Pressure: Must be explained before the model in an exam answer.

Exam Preparation Tips

• Know leaf adaptations and diagrams.
• Understand stomatal functions and water transport.
• Be familiar with root structure and zones.
• Study the cohesion tension model comprehensively.
• Use additional materials such as videos, textbooks, and past papers.