📌Definition Table

Term	Definition
Xylem	Vascular tissue that transports water and dissolved minerals upward from roots to leaves.
Phloem	Vascular tissue that transports organic compounds (mainly sucrose) bidirectionally.
Transpiration	Evaporation of water from leaf surfaces, driving water uptake through xylem.
Cohesion–tension theory	Model explaining how hydrogen bonding between water molecules and adhesion to xylem walls enables upward water transport.
Translocation	The movement of sugars and other organic compounds in phloem, from sources (e.g., leaves) to sinks (e.g., roots, fruits).
Companion cell	Phloem cell that actively loads sucrose into sieve tubes via ATP-driven transport.

📌Introduction

Transport in plants is essential for distributing water, minerals, and organic nutrients to all tissues. While diffusion and osmosis are sufficient for unicellular organisms, multicellular plants evolved vascular systems — xylem for water and minerals, and phloem for photosynthates. These systems ensure efficient long-distance transport, even in tall trees. The cohesion–tension model explains how transpiration drives upward water flow, while the pressure-flow hypothesis explains phloem transport. Understanding these processes links molecular interactions like hydrogen bonding to ecological phenomena such as global water cycling.

📌 Xylem Structure and Water Transport

• Xylem vessels are elongated, dead cells aligned end to end, forming continuous tubes.
• Thickened lignin walls prevent collapse under tension and provide structural support.
• Water moves upward by:
  • Cohesion: hydrogen bonding between water molecules keeps them connected.
  • Adhesion: water sticks to hydrophilic xylem walls, aiding capillarity.
  • Transpiration pull: evaporation from stomata creates negative pressure, pulling water upward.
• This passive process requires no metabolic energy from plants.

🧠 Examiner Tip: Always state that water transport in xylem is a passive process driven by transpiration and cohesion–tension, not by pumping or active forces.

📌 Phloem Structure and Sugar Transport

• Phloem consists of sieve tube elements (lacking nuclei and ribosomes) supported by companion cells.
• Companion cells actively load sucrose using ATP and proton pumps, creating high solute concentration.
• This lowers water potential, drawing water into sieve tubes by osmosis.
• Hydrostatic pressure builds at sources (e.g., leaves) and pushes phloem sap toward sinks (e.g., roots, fruits, storage organs).
• This mechanism is called the pressure-flow hypothesis.

🧬 IA Tips & Guidance: Experiments can include measuring transpiration with a potometer under varying humidity, temperature, or wind conditions. For phloem, aphid stylet experiments can demonstrate direction and composition of translocation.

📌 Regulation of Transport in Plants

• Stomata control transpiration by regulating gas exchange. Guard cells respond to light, CO₂, and water availability.
• Environmental factors (humidity, temperature, wind, light) influence transpiration rate.
• Seasonal changes affect transport — sugars flow to roots in winter and toward shoots/flowers in spring.

🌐 EE Focus: An EE could investigate how environmental conditions (light intensity, humidity, CO₂ levels) influence transpiration rates, or how different plant adaptations (xerophytes vs hydrophytes) alter vascular transport efficiency.

📌 Plant Adaptations for Transport

• Xerophytes: reduced leaf area, thick cuticles, sunken stomata to minimize water loss.
• Hydrophytes: large air spaces for buoyancy, stomata on upper surfaces, reduced vascular tissue.
• Halophytes: salt glands, succulence, and selective ion transport to survive in saline environments.
• These adaptations link plant transport physiology to survival in diverse habitats.

❤️ CAS Link: Students could build simple potometer models and present findings on plant water use efficiency to promote sustainable agriculture or gardening in their communities.

📌 Applications of Plant Transport

• Explains how water and minerals reach leaves for photosynthesis.
• Provides the basis for sugar movement into fruits, essential in agriculture.
• Connects cellular-level hydrogen bonding to ecosystem-level water cycling.

🔍 TOK Perspective: The cohesion–tension theory relies on indirect evidence, since we cannot directly “see” water columns in xylem. TOK reflection: How do models built on indirect evidence shape our confidence in scientific explanations?

📝 Paper 2: Be ready to label xylem and phloem structures, explain cohesion–tension and pressure-flow hypotheses, describe experiments (potometers, aphid stylets), and apply transport mechanisms to real-world plant adaptations.