2.1 Plant Physiology: Absorption by Roots

1. Introduction to Water Absorption

• Crucial for plant survival and growth.
• Primarily occurs through roots, specialized for this function.

1.1. Characteristics of Roots for Water Absorption

• Vast Surface Area: Extensive branching and millions of root hairs.
• Semi-Permeable Membrane: Cell membranes of root hairs allow selective water passage.
• High Solute Concentration: Cell sap has higher solute concentration than soil water, creating an osmotic gradient.

1.2. Structure of a Root Hair

• Microscopic, tubular extension of an epidermal cell.
• Optimized for absorption: Large surface area, thin cell wall, semi-permeable cell membrane, large central vacuole with high solute concentration.

2. The Process of Water Absorption

• Involves a combination of physical phenomena:
  • Imbibition: Initial absorption of water by solid components (e.g., cell wall).
  • Diffusion: Net movement of water molecules from higher to lower concentration.
  • Osmosis: Primary mechanism; movement of water across a semi-permeable membrane from higher to lower water potential.

3. Water Potential and its Components

• Water Potential: Measure of the potential energy of water, governing its movement.
• Components:
  • Osmotic Pressure: Pressure required to prevent water flow across a membrane (higher solute = more negative osmotic potential).
  • Root Pressure: Positive pressure in xylem due to active mineral transport into roots.
  • Turgor Pressure: Pressure exerted by cell contents against the cell wall when hydrated.

4. Turgidity, Flaccidity, and Plasmolysis

• Turgidity: Cell is fully hydrated, membrane pressed against cell wall (normal state).
• Flaccidity: Cell loses water, membrane pulls away from cell wall (wilting).
• Plasmolysis: Severe water loss, cell membrane completely pulls away from cell wall (in hypertonic solution).
• Deplasmolysis: Plasmolyzed cell regains water and becomes turgid (in hypotonic solution).

5. Absorption of Water and Minerals

• Water: Primarily by osmosis.
• Mineral Ions:
  • Passive Transport: Down concentration gradient, no energy.
  • Active Transport: Against concentration gradient, requires ATP and carrier proteins.

6. Ascent of Sap

• Upward movement of water and dissolved minerals from roots to leaves through the xylem.
• Forces Responsible:
  • Cohesion: Water molecules stick to each other (hydrogen bonds).
  • Adhesion: Water molecules stick to xylem vessel walls.
  • Transpirational Pull: Primary driving force; evaporation of water from leaves creates tension, pulling water up.

6.2. Experiments to Show Conduction of Water Through Xylem

• Experiment: Place a leafy shoot in colored water.
• Observation: Veins in leaves turn red, and only xylem tissue in stem cross-section is stained.
• Conclusion: Confirms xylem as the pathway for water conduction.