Water and Mineral Transport (via Xylem)

Uptake:

• Water and minerals enter the plant through root hairs via osmosis (for water) and active transport (for minerals).

Transport: Movement occurs through xylem vessels by:

• Root Pressure: Pressure exerted by water entering the roots pushes water upward.
• Capillarity: The ability of water to move up narrow xylem vessels due to cohesion and adhesion forces.

Transpiration Pull: Water is drawn upward due to evaporation from leaf surfaces, creating a suction force.

Pathways:

• Apoplast Pathway: Through the cell walls.
• Symplast Pathway: Through the cytoplasm and plasmodesmata.

Food Transport (via Phloem)

Process:

1. Food produced during photosynthesis (mainly sucrose) is loaded into the phloem at the source (e.g., leaves).
2. It is transported to sinks (e.g., roots, fruits) where it is needed or stored.

Mechanism:

• Follows the pressure-flow hypothesis:
• Sugars are actively loaded into phloem, causing water to enter by osmosis, creating pressure.
• The pressure pushes the sugar solution to areas of low pressure (sinks).

1. Water Transport: Essential for photosynthesis, nutrient transport, and maintaining cell turgidity.
2. Mineral Transport: Supplies nutrients needed for growth, enzyme function, and structural development.
3. Food Transport: Distributes energy sources to growing regions, storage organs, and non-photosynthetic tissues.
4. Support: Xylem's lignified walls contribute to the mechanical strength of the plant.

Vascular bundle in a stem showing the position of cambium

Xylem tissue

Xylem is a key component of the vascular system in plants, responsible for the transport of water and dissolved minerals from the roots to other parts of the plant. It also provides mechanical support due to its rigid structure.

Structure of the xylem

Structure of xylem tissue

Xylem tissue consists of two primary elements:

Xylem Vessels

Structure:

1. Composed of long, hollow, cylindrical cells arranged end-to-end to form continuous tubes.
2. Mature xylem vessels are dead cells with no cytoplasm, allowing unimpeded flow of water.
3. The walls are thickened with cellulose and lignin.

Function:

1. Facilitate efficient water and mineral transport.
2. Provide mechanical support to the plant.

Xylem vessels

Tracheids

Structure:

1. Elongated cells with tapered ends, forming a network for water conduction.
2. Mature tracheids are also dead and hollow.
3. Walls are reinforced with lignin and have pits to allow lateral water movement.

Function:

1. Transport water in plants lacking vessels (e.g., gymnosperms).
2. Contribute to the structural strength of the plant.

Tracheids

Role of cellulose and lignin

Cellulose: Provides the basic structure and elasticity of the cell walls.

Lignin:

1. Deposited in the walls of xylem vessels and tracheids.
2. Strengthens and rigidifies the walls, preventing collapse under the pressure of water transport.
3. Contributes to the plant's ability to stand upright and withstand external forces.

Functions of xylem

1. Water and Mineral Transport: Conducts water and dissolved minerals from the roots to the leaves for photosynthesis.
2. Support: The lignified walls of xylem vessels and tracheids provide rigidity, supporting the plant's structure.
3. Mechanical Strength: Helps plants maintain their shape and resist bending forces caused by wind or gravity.

Xylem vessels

Direction of Transport: Movement in xylem is always upward, transporting water and mineral salts from roots to leaves.

Structure:

1. Hollow, dead cells without cytoplasm or nuclei, allowing efficient water transport.
2. Cells are joined end to end with no end walls, forming continuous tube-like structures.
3. Walls are reinforced with lignin, providing structural support.

Tracheids:

1. Elongated cells with tapering ends.
2. End walls have perforations (pits), making them less efficient than xylem vessels for conduction.
3. Present in some plants as an alternative or supplementary water transport system.

Structure: Made up of sieve-tube elements and companion cells.

Sieve-tube elements:

1. Joined end to end, with end walls forming sieve plates containing pores.
2. Have cytoplasm but lack a nucleus.

Companion cells:

1. Contain mitochondria to provide energy for transport.
2. Connected to sieve-tube elements by small pores called plasmodesmata.

Function:

1. Transports food (mainly sucrose) produced during photosynthesis to other parts of the plant.
2. Movement in phloem is bidirectional (both upward and downward).

Phloem tissue

Distribution of vascular bundles

Monocotyledonous Plants

• Roots: Xylem and phloem alternate in a ring-like arrangement.
• Stem: Vascular bundles are scattered randomly.

Cross-section of monocot root

Dicotyledonous Plants

• Roots: Xylem is centrally located and star-shaped, with phloem between the xylem arms.
• Stem: Vascular bundles are arranged in a ring around a central pith.

Cross-section of a dicot root

Monocotyledonous stem

• The vascular bundles (composed of xylem and phloem) are scattered randomly throughout the stem.
• This arrangement allows monocots to have more flexibility, which is why plants like grasses can bend without breaking.

Cross-section of monocot stem

Dicotyledonous stem

• The vascular bundles are arranged in a ring around the central pith (the central region of the stem).
• This arrangement gives dicots more structural support, which is why many dicots (like trees and shrubs) can grow taller and sturdier than monocots.

Cross-section of dicot stem

Absorption and movement of water and mineral salts

Root hairs

Structure:

1. Extensions of root epidermal cells.
2. Long and slender, increasing surface area for absorption.
3. Thin walls for shorter absorption distance.

Functions:

1. Absorb water by osmosis due to hypertonic root sap.
2. Absorb mineral salts by active transport.