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.