B3.2 HL Transport in Plants [IB Biology HL]

Transport in Plants

1. Transpiration

  • Definition: The process of water vapor evaporating from the stomata in leaves.

  • Movement Mechanism: Water moves upward through the plant via cohesion and adhesion, forming a continuous column.

  • Significance:

    • Water Supply: Essential for delivering water throughout the plant.

    • Potential Issues:

      • Water loss can be detrimental, especially in dry conditions.

      • Requires a balance between water loss and uptake.

2. Factors Affecting Transpiration

  • Humidity:

    • Transpiration slows or ceases in high humidity as it reduces the water potential gradient.

  • Stomata Function:

    • Openings on leaves regulate gas exchange and transpiration.

    • Close at night or during unfavorable conditions (e.g., leaf loss, winter).

3. When Transpiration Pull Stops

  • Root Pressure Mechanism:

    • Plants generate pressure at their roots to move water when transpiration ceases.

    • Active transport of mineral ions from soil into roots increases solute concentration.

    • Water follows minerals, resulting in higher pressure, pushing water up through the xylem.

4. Xylem vs. Phloem

  • Xylem:

    • Main role is upward transport of water and minerals.

    • Operates mainly through transpiration pull and root pressure.

  • Phloem:

    • Transport of carbohydrates (e.g., sugars) from sources (where they are produced, like leaves) to sinks (where they are stored or utilized, like roots or flowers).

5. Phloem Transport Mechanism

  • Phloem Loading:

    • Active process of pumping carbon compounds into phloem, creating high solute concentration.

    • Water moves into phloem via osmosis from xylem, creating hydrostatic pressure.

  • Pressure Gradient:

    • High pressure at source (where sugars are loaded) drives water and nutrients to areas of lower pressure (sinks).

    • Water re-enters xylem at sink to maintain balance.

6. Companion Cells and Sieve Tubes

  • Companion Cells:

    • Aid in active transport processes in phloem, supporting the function of phloem.

  • Sieve Tubes:

    • Composed of sieve tube elements with large holes (sieve plates) in their walls, allowing carbon compounds to move through.

    • Phloem cells disintegrate (losing their nucleus and organelles), requiring companion cells for metabolic functions.

7. Summary of Plant Transport Features

  • Form and Function:

    • Xylem: Specialized for upward water transport.

    • Phloem: Adapted for bi-directional transport of nutrients and carbohydrates.

  • Overall, successful plant functioning relies on effective water and nutrient transport mechanisms.