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

Overview of Plant Transport

  • Discussion of standard content related to transport in plants (B3.2)

  • Not all plants possess vascular tissue, but those that do utilize specialized tubes like xylem.

Xylem and Water Transport

Role of Xylem

  • Xylem is responsible for transporting water from roots to shoots, exclusively in an upward direction.

  • Relies on two primary forces: cohesion and adhesion.

Mechanism of Water Movement

  • Water evaporates from stomata (tiny openings on leaves).

  • Cohesion allows water molecules to attract each other, forming a continuous column.

  • Adhesion helps water molecules stick to the xylem walls, assisting in overcoming gravity through transpirational pull.

Structural Adaptations of Xylem

  • Composed of dead, hollow cells to maintain a continuous water column through cohesion.

  • Lignin (a polysaccharide) provides structural support and prevents collapse of xylem under pressure.

  • Pits in xylem allow for water passage between xylem and phloem.

Plant Tissue Distribution

Transverse Section of Plant Stem

  • Cross-section of the stem provides a view of vascular tissues: xylem and phloem.

  • Vascular bundles: analogous to human vascular system (arteries, veins, and capillaries).

  • Key Positioning:

    • Xylem: located towards the center of the vascular bundle.

    • Phloem: situated towards the outer part of the bundle.

Important Stem Features

  • Pith: innermost region, may serve a structural role.

  • Cortex: supports vascular bundles and is involved in photosynthesis.

  • Epidermis: protective layer that acts as waterproofing, keeping contents contained.

Comparison of Stem and Root Structure

Stem Structure

  • Similar components (epidermis, xylem, phloem, cortex, pith) present in stems but arranged in a specific manner.

Root Structure

  • Epidermis present on the outside with root hairs to increase water absorption surface area.

  • Xylem centrally located (often in X-shape) with phloem adjacent, reinforcing their functional relationship with slight structural variations.

Summary of Plant Transport Mechanisms

  • Key elements for water transport include transpirational pull, cohesion, adhesion, and the structural integrity provided by xylem adaptations.

  • Understanding the arrangement and function of plant tissues is crucial for studying plant physiology.