Plant Transport Systems

Vascular Tissue Overview

• Dual system transports substances & supports plant body
  • Xylem: moves $\text{H}_2\text{O}$ + minerals root → shoot; vessels & tracheids (dead), lignified walls give strength
  • Phloem: moves sugars (mainly $\text{C}<em>6\text{H}</em>{12}\text{O}_6$) source → sink; sieve-tube elements + companion cells (living)

Spatial Arrangement

• Roots: xylem & phloem central → resist stretching
• Stems: bundled near cortex → resist bending/compression
• Leaves: grouped as veins/vascular bundles

Water & Mineral Uptake (Roots)

• Enter via root hairs
  • Water: osmosis (down $\Psi_w$ gradient)
  • Minerals: active transport (ATP-dependent)
• Movement to xylem through two pathways
  • Symplast: cytoplasm & plasmodesmata
  • Apoplast: cell walls/intercellular spaces (blocked by Casparian strip before xylem)

Ascent of Sap – Driving Forces

1. Root Pressure
   • Active ion accumulation ↓$\Psi_w$; water enters osmotically, pushes sap upward (not dominant)
2. Capillary Action
   • Adhesion (water ↔ walls) + Cohesion (water ↔ water) pulls column up narrow xylem tubes
3. Transpiration Pull (primary)
   • Evaporation at stomata → negative pressure (tension) → continuous pull root → leaf (cohesion-tension mechanism)

Transpiration Stream Route

Soil → Root hairs → Cortex → Xylem (root) → Xylem (stem) → Xylem (leaf) → Mesophyll → Air spaces → Stomata → Atmosphere

Roles in Leaves

• Water: photosynthesis substrate & turgor maintenance
• Minerals: N, K, P etc. for metabolism

Phloem Translocation (Pressure-Flow Model)

• Source: mature photosynthetic leaves / storage tissues releasing sugar
• Sink: growing tissues, roots, fruits, storage organs

1. Sugar actively loaded into phloem at source ↓$\Psi_w$
2. Water osmoses in from adjacent xylem → high turgor (pressure)
3. Bulk flow drives sap along pressure gradient toward sinks
4. Sugar unloaded at sink (use/storage); water returns to xylem → pressure drops

Key Takeaways

• Xylem: unidirectional, passive (tension); Phloem: bidirectional, active loading/unloading
• Cohesion-Tension theory explains most upward water movement
• Pressure-Flow hypothesis explains long-distance sugar transport
• Efficient coordination of xylem & phloem maintains plant hydration, nutrition, and growth