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7.08 Water: Roots → leaves


Pathways of Water Movement from Roots to Leaves

Water Absorption by Root Hairs

  • Root Hairs Function: Increase the surface area for water and mineral ion uptake from the soil.
  • Water Potential Gradient: Water moves from an area of higher water potential in the soil to lower water potential in root cells.

Movement Through the Root Cortex

  • Apoplast Pathway:
    • Description: Water travels through the cell walls and intercellular spaces without entering the cytoplasm.
    • Advantages: Faster movement as it bypasses cell membranes.
    • Limitation: Blocked by the Casparian Strip.
  • Symplast Pathway:
    • Description: Water moves from cell to cell through the cytoplasm via plasmodesmata.
    • Advantages: Allows selective uptake of minerals and regulation of water movement.

Role of the Endodermis and Casparian Strip

  • Endodermis:
    • A single layer of cells surrounding the vascular tissue.
  • Casparian Strip:
    • Composition: Made of suberin, a waterproof substance.
    • Function: Blocks the apoplast pathway, forcing water to enter the symplast pathway.
    • Passage Cells: Specialized cells in mature roots that facilitate water movement into xylem while controlling mineral entry.

Entry into Xylem

  • Process: Water moves from the endodermis into xylem vessels through pits.
  • Xylem Vessels: Continuous tubes formed by end-to-end joining of xylem vessel elements, enabling upward water transport.

Structure of Xylem and Adaptations for Water Transport

Xylem Composition

  • Xylem Vessel Elements:
    • Origin: Start as living cells but become dead at maturity.
    • Cell Walls: Lignified, providing strength and waterproofing.
  • Lignin:
    • Properties: Hard, strong, and waterproof.
    • Role: Reinforces xylem structure, preventing collapse under the negative pressure (tension) created during water transport.

Formation of Xylem Vessels

  • Cell Arrangement: Xylem cells join end-to-end, with end walls dissolving to form continuous tubes.
  • Pits:
    • Function: Unlignified areas in cell walls allowing lateral water movement between vessels and neighboring cells.
    • Importance: Maintain water flow continuity and enable bypassing of air bubbles.

Structural Adaptations Summary

  • Dead, Hollow Cells: Facilitate unobstructed water flow without protoplasmic resistance.
  • Lignified Walls: Provide structural support and prevent vessel collapse.
  • Narrow Diameter: Reduces the risk of air bubbles (air locks) disrupting water continuity.
  • Pits: Enable lateral water movement and bypass air bubbles, ensuring efficient transport.

Mechanism of Water Movement in Xylem: The Cohesion-Tension Theory

Key Components of the Theory

  • Transpiration:
    • Definition: Loss of water vapor from stomata in leaves.
    • Effect: Creates a negative pressure (tension) in the leaf’s air spaces and xylem.

  • Cohesion:
    • Description: Hydrogen bonds cause water molecules to stick together.
    • Role: Maintains a continuous water column within xylem vessels.
  • Adhesion:
    • Description: Water molecules adhere to the hydrophilic walls of xylem vessels.
    • Role: Prevents the water column from breaking under tension.
  • Tension:
    • Mechanism: Evaporation of water from leaves pulls water upward through the xylem.
    • Result: Generates negative pressure that drives water movement against gravity.

Step-by-Step Process

  1. Transpiration Occurs:
    • Water vapor exits the leaf via stomata, creating a water potential gradient.
  2. Tension in the Xylem:
    • Evaporation generates a pulling force (negative pressure) on the water in the xylem.
  3. Cohesion of Water Molecules:
    • Hydrogen bonding keeps water molecules joined, forming a continuous column.
  4. Adhesion to Xylem Walls:
    • Water molecules adhere to xylem vessel walls, supporting the column and preventing collapse.
  5. Water Uptake from Roots:
    • Negative pressure in the xylem pulls water from root hairs into the xylem vessels.
  6. Mass Flow:
    • All water molecules move together efficiently, transporting water and minerals from roots to leaves.

Conditions Affecting the Cohesion-Tension Theory

  • Environmental Factors:
    • High Temperature: Increases transpiration rates.
    • Low Humidity: Enhances water vapor loss from leaves.
    • Wind: Removes saturated air around stomata, promoting transpiration.
  • Xylem Structure:
    • Narrow Xylem Vessels: Enhance cohesion and adhesion, maintaining the water column.

Importance of the Cohesion-Tension Theory

  • Supports Photosynthesis and Cooling: Provides water for photosynthesis and facilitates plant cooling through transpiration.
  • Explains Upward Water Movement: Accounts for how water travels against gravity in tall plants.
  • Ensures Continuous Supply: Maintains a steady flow of water and nutrients from roots to leaves.

Practice Questions

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