Plant Transport Systems - OCR A Level Biology Module 3.3

Introduction

• Plant transport systems move water and sugar.
• Two main transport systems:
  • Water from the roots to the leaves.
  • Sugars from photosynthesis in the leaves to other cells.
• Plants do not need a transport system for oxygen; it moves by diffusion.

Water Transport in Plants

Water Movement into Roots

• Water uptake is powered by active transport of mineral ions into root hair cells.
• Water follows by osmosis down a water potential gradient.
• Root hair cells have a high surface area and many mitochondria for active transport.

Pathways for Water Movement

• Apoplast Pathway: Through cell walls; fastest route.
• Symplast Pathway: Through cytoplasm via plasmodesmata.
• Vacuolar Pathway: Through vacuoles, overlaps with symplast.

Casparian Strip

• Located in the endodermis; blocks the apoplast pathway.
• Forces water into symplast or vacuolar pathways.
• Ensures mineral ions do not leak out, maintaining water potential gradient.

Xylem

• Made of dead cells with lignin in cell walls for strength.
• Xylem vessels are continuous tubes, allowing cohesive and adhesive properties of water (capillary action).
• Lignin Patterning: Spiral, annular, or bordered pits for lateral movement.

Transpiration Stream

• Water moves up the xylem due to root pressure, cohesion, adhesion, and evaporation from leaves.
• Transpiration cools leaves, maintains turgor pressure, and supplies water for photosynthesis.

Leaf Structure and Water Movement

• Waxy Cuticle: Prevents water loss.
• Upper Epidermis: Transparent barrier.
• Palisade Mesophyll: Photosynthesis, packed with chloroplasts.
• Spongy Mesophyll: Air spaces for gas exchange.
• Water evaporates from mesophyll and diffuses out of stomata.

Measuring Transpiration - Potometer

• Mass Potometer: Measures mass loss.
• Bubble Potometer: Tracks bubble movement to measure water uptake.
• Factors affecting transpiration: light, air movement, humidity.

Adaptations to Water Availability

Xerophytes

• Adaptations for arid environments (e.g., cacti, marram grass).
• Marram Grass: Rolled leaves with stomata in pits, reduces water loss.

Hydrophytes

• Adaptations for water-rich environments (e.g., water lilies).
• Stomata on upper surface, air spaces for buoyancy and gas exchange.

Sugar Transport in Phloem

• Phloem: Living cells, sieve plates, companion cells.
• Mass Flow: Driven by pressure differences from source to sink.
• Loading Mechanism: Co-transport of H+ and sucrose into phloem.

Summary

• Key differences in structure and function between xylem and phloem.
• Importance of adaptations for different environments.
• Understanding of plant transport is essential for studying plant biology.