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Transport Mechanisms in Plants Explained

Apr 9, 2025

OCR (A) Biology A-Level: Topic 3.3 - Transport in Plants

Introduction

  • Plants require a transport system to distribute nutrients to all cells.
  • Xylem Tissue: Transports water and dissolved minerals upward via transpiration (passive process).
  • Phloem Tissue: Distributes sugars throughout the plant via translocation (active process).

Vascular Bundle

In Roots

  • Xylem and phloem are in the vascular bundle, aiding in transport and structural support.
  • Xylem Vessels: Arranged in an X shape within the vascular bundle.
    • Provides resistance to mechanical forces.
    • Surrounded by endodermis (supplies water) and the inner pericycle layer.

In Stems

  • Xylem: Located inside for support and flexibility.
  • Phloem: Found outside the vascular bundle.
  • Cambium Layer: Meristem cells that produce new xylem and phloem.

In Leaves

  • Forms the midrib and veins.
  • Dicotyledonous Leaves: Network of veins for transport and support.

Xylem and Phloem

Xylem Vessels

  • Transport water and minerals; provide structural support.
  • Made of dead tissue forming continuous columns.
  • Contains pits for lateral water movement.
  • Lignin: Provides flexibility through spiral patterns.
  • Water moves upwards only.

Phloem Vessels

  • Composed of living cells for translocation.
  • Sieve Tube Elements & Companion Cells:
    • Sieve tubes transport sugars (e.g., sucrose) as sap.
    • Companion cells produce ATP for active transport.
    • Plasmodesmata link cells for communication and substance flow.

Transpiration

  • Plants absorb water through roots, release as water vapor via stomata.
  • Transpiration Stream: Supplies water for photosynthesis, growth, elongation, minerals, and temperature control.
  • Involves osmosis and evaporation/diffusion of water vapor.
  • Potometer: Measures transpiration rate using water replacement.
  • Factors Affecting Transpiration:
    • Leaf number and size, stomata characteristics, cuticle, light, temperature, humidity, air movement, water availability.

Adaptations

  • Xerophytes: Adapted to dry conditions with smaller leaves, thick cuticles, and stomatal closure.
  • Hydrophytes: Adapted to water environments with open stomata, large surface area, and buoyancy features.

Movement of Water in Roots

  • Water enters through root hair cells into xylem due to water potential gradient.
  • Root Hair Cells: Increase surface area for absorption.
  • Minerals absorbed via active transport.
  • Pathways:
    • Symplast Pathway: Water moves through cytoplasm and plasmodesmata.
    • Apoplast Pathway: Water moves through cell walls.
  • Casparian Strip: Forces water into symplast pathway at endodermis.

Water Movement in Xylem

  • Water moves from xylem into mesophyll cells via water potential gradient.
  • Root Pressure: Drives water upward through active transport of minerals.
  • Cohesion-Tension Theory: Cohesion and surface tension aid water movement.
  • Capillary Action: Supports upward water movement.

Translocation

  • Energy-requiring process moving assimilates (e.g., sucrose) in the phloem.
  • Active Loading: Companion cells use ATP to create a diffusion gradient for sucrose entry.
  • Mass Flow: Hydrostatic pressure gradient drives flow from source to sink.
  • Sucrose enters sieve tubes, reducing water potential, causing osmosis.
  • Pressure Changes: Drive water and sucrose movement into sinks.