Role of Xylem and Phloem in Plant Transport

Role of Xylem Water in the Transport of Elaborated Sap

• Xylem water is crucial for the movement of elaborated sap (phloem sap) within plants.

Functions of Xylem

• Transport of Water and Mineral Salts:
    - Xylem transports water and mineral salts from the roots to the leaves.
• Photosynthesis Interaction:
    - In the leaves, sugars produced by photosynthesis are loaded into the phloem.
    - This loading of sugars lowers the water potential in the phloem vessels.
• Osmosis Discovery:
    - Due to the decreased water potential, water moves from the xylem into the phloem by the process of osmosis.

Hydrostatic Pressure in Phloem

• Increase in Pressure:
    - The transfer of water into the phloem increases hydrostatic pressure inside these vessels.
• Movement of Elaborated Sap:
    - This increased pressure pushes the elaborated sap towards other plant parts such as:
      - Roots
      - Fruits
      - Growing tissues

Importance of Interaction Between Xylem and Phloem

• The interaction between xylem and phloem is essential for:
    - Distribution of nutrients throughout the plant.
    - Proper functioning and growth of the plant.

Movement of Fluid in Xylem vs Phloem

• The movement of sugars in phloem depends on the movement of water in the phloem.
• Important differences exist in the mechanisms of fluid movement in xylem and phloem.

Driving Force for Fluid Movement

• Xylem:
    - Transpiration (evaporation) from leaves serves as the driving force.
    - This is combined with cohesion and tension of water in vessel elements and tracheids.
    - This process is passive and does not require energy.
• Phloem:
    - The driving force is the active transport of sucrose from source cells into phloem sieve tube elements.
    - This process requires energy.

Cells Facilitating Fluid Movement

• Xylem:
    - Composed of non-living vessel elements and tracheids.
• Phloem:
    - Composed of living sieve tube elements which are supported by companion cells.

Type of Pressure Potential

• Xylem:
    - Exhibits negative pressure potential due to pull from the top.
    - The pressure potential $ext{Ψp}$ decreases due to the combined effects of transpiration, cohesion, and tension of water.
• Phloem:
    - Exhibits positive pressure potential due to push from the source.
    - The pressure potential $ext{Ψp}$ increases due to the influx of water, which boosts turgor pressure at the source.