3.6.1 Water Transport in the Xylem

Features of Xylem Vessels:

1. Transport and Support:

• Xylem vessels transport water and dissolved minerals from roots to leaves.
• They also provide structural support to the plant.

2. Continuous Tubes:

• Xylem vessels are made of dead cells with no cytoplasm or organelles.
• They have open ends, forming long continuous tubes for water flow.

3. Pits:

• Contain pits (small pores) that allow water to move laterally between vessels, ensuring flexibility in water transport.

4. Lignin:

• Thickened with lignin, a strong, waterproof substance.
• Lignin is deposited in spiral or ring patterns, which strengthens the xylem while allowing flexibility and preventing collapse under tension.

The Process of Water Transport:

1. Root Absorption:

• Water enters the plant through root hair cells by osmosis.
• Mineral ions are actively transported into the root, lowering the water potential and drawing in water.

2. Movement to the Xylem:

• Water moves through the root cortex to the xylem by three pathways:
• Apoplast Pathway: Through the cell walls.
• Symplast Pathway: Through the cytoplasm via plasmodesmata.
• Vacuolar Pathway: Through the vacuoles.
• At the endodermis, the Casparian strip blocks the apoplast pathway, forcing water through the symplast pathway, ensuring selective uptake of minerals.

3. Ascent of Water:

• Cohesion-Tension Theory explains how water moves upward in the xylem:
• Cohesion: Water molecules stick to each other due to hydrogen bonding.
• Adhesion: Water molecules stick to the walls of xylem vessels.
• Tension: Transpiration at the leaf surface creates negative pressure, pulling water up the xylem.

4. Transpiration:

• Water evaporates from the surfaces of mesophyll cells in leaves and diffuses out through stomata.
• This evaporation drives the upward movement of water in the xylem.

Key Terms:

• Transpiration: The loss of water vapour from the aerial parts of a plant.
• Cohesion-Tension Theory: Explains the movement of water through xylem due to cohesion, adhesion, and tension.
• Lignin: A strong, waterproof substance that reinforces xylem vessels.
• Casparian Strip: A waterproof barrier in the endodermis that forces water into the symplast pathway.

Adaptations of Xylem:

1. Hollow Tubes: No cell contents ensure an uninterrupted water column.
2. Thick Walls: Lignin strengthens vessels and prevents collapse.
3. Pits: Allow lateral water movement if one vessel is blocked.
4. Continuous Columns: Open ends enable water to flow seamlessly.