Lecture Notes: Mechanism of Water and Mineral Movement in Plants

Introduction

• Previous topics: Movement of water and minerals into soil and roots, and within leaves.
• Current focus: Movement of water and minerals up the xylem, essential for leaf hydration.

Key Concepts

Water as a Dipole

• Dipole Nature: Water molecules have partial charges (hydrogen: positive, oxygen: negative).
• Hydrogen Bonds: Water molecules bond through cohesion due to polarity.

Water as a Universal Solvent

• Interaction with Minerals: Water can dissolve minerals due to polarity (e.g., nitrates, phosphates).
• Polarity Importance: Critical for cohesion and interaction with xylem vessel walls.

Xylem Structure

• Nature of Xylem Cells: Composed of dead cells, forming continuous vessels.
• Pits: Unlignified sections in xylem vessels; permeable to water and minerals.
• Polarity of Walls: Cellulose walls have slight polarity facilitating water movement via adhesion.

Mechanism of Water and Mineral Movement

Adhesion and Cohesion

• Adhesion: Water sticks to xylem vessel walls (lignin and cellulose) due to polar interactions.
• Cohesion: Water molecules stick to each other, forming a continuous column.

Transpiration Pull

• Transpiration: Loss of water vapor from leaves creates a pull effect.
• Continuous Chain: Cohesion and adhesion form a chain of water and minerals.

Pressure Gradient and Mass Flow

• Pressure Gradient: Water loss in leaves reduces pressure at the top of the xylem relative to the bottom.
• Mass Flow: Water and minerals move from high to low pressure areas, enabling upward movement.
• Role of Root Absorption: Roots absorb more water and minerals as xylem content is pulled upwards.

Application in Exams

• Key Phrases: Mention adhesion, cohesion, pressure gradient, and mass flow.
• Explanation Strategy: Describe how water loss leads to lower pressure and initiates mass flow upwards.

Conclusion

• Understanding the interaction of water's polarity with its environment is crucial for explaining xylem movement.
• Emphasis on the roles of cohesion and adhesion in maintaining water columns and driving nutrient flow.