Water Potential in Plants: Osmosis, Pathways & Root Uptake Explained

Plant Physiology: Water Potential and Transport of Water in Plants

8.4 Water Potential (Ψw)

• Definition: Water potential measures the total kinetic energy of water molecules in a system. Higher concentration of water molecules → greater kinetic energy → higher Ψw.
  • Symbol: Ψw (Greek letter psi).
• In Plant Cells: Determined by two main factors:
  • Solute Potential (Ψs): Negative value; reflects effect of dissolved solutes (more solutes → more negative Ψs).
  • Pressure Potential (Ψp): Positive value; due to physical pressure on water (e.g., in turgid cells).
• Key Properties:
  • Pure water at atmospheric pressure: Ψw = 0 (maximum).
  • All solutions: Ψw < 0 (negative).
  • Water moves from higher Ψw to lower Ψw.
• Osmosis: Movement of water across a partially permeable membrane from higher Ψw to lower Ψw.
• Formula: Total Ψw = Ψs + Ψp.
• Applications: Measures water movement between systems (e.g., soil → root, leaf → air). Steeper gradient → faster flow.

Solute Potential (Ψs)

• Measure of how solutes lower Ψw.
• Always negative; more solute molecules → lower (more negative) Ψs.

Pressure Potential (Ψp)

• Due to pressure exerted by water against cell walls.
• Increases if pressure > atmospheric (e.g., osmosis into cells builds turgor pressure, making cells turgid).

8.5 Transport of Water in Plants

Uptake of Water by Roots

• Root Structure: Extensive branching + root hairs (extensions of epidermal cells) provide large surface area for absorption.
• Entry Process: Water and minerals enter root epidermal cells via active/passive transport.
• Path Inside Root: From epidermis → cortex → central xylem.
• Three Pathways to Xylem:
1. Apoplast Pathway:
   • Through continuous system of adjacent cell walls.
   • Discontinuous at endodermis due to Casparian strip (waxy band).
2. Symplast Pathway:
   • Through interconnected protoplasts (living cytoplasm) of cells.
   • Connected via plasmodesmata (cytoplasmic strands through cell wall pores).
   • Less important except for minerals at endodermis.
3. Vacuolar Pathway:
• Through cell membranes, cytoplasm, tonoplast (vacuole membrane), and vacuoles.
• Cell-to-cell via vacuoles; bypasses apoplast/symplast.
• Movement is negligible.

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