Plant Physiology — Transport
'Water is the medium of life. In plants, the journey of water from root to leaf is a remarkable story of physics and biology.' — Plant Physiology
1. Chapter Overview
Plants are SESSILE organisms that must transport water, minerals, and food across their bodies WITHOUT a pumping heart. This chapter explains the MECHANISMS of transport: DIFFUSION (passive movement), OSMOSIS (water movement across membranes), PLASMOLYSIS, the ASCENT OF SAP through xylem (transpiration pull), and the TRANSLOCATION of food through phloem (pressure flow hypothesis). The chapter also covers ABSORPTION by roots.
2. Means of Transport
Diffusion
- Definition: Movement of molecules from HIGHER to LOWER concentration (down the gradient)
- No energy required — PASSIVE
- Rate depends on: Concentration gradient, temperature, molecular size, medium
- Examples: CO₂ entering leaf, O₂ leaving leaf
Facilitated Diffusion
- Diffusion AIDED by transport PROTEINS (carrier/channel)
- STILL passive (down gradient) — NO ATP required
- Channel proteins: Porins (water channels — aquaporins)
- Carrier proteins: Specific binding (uniport, symport, antiport)
Active Transport
- Movement AGAINST concentration gradient (low → high)
- REQUIRES ENERGY (ATP)
- Pumps: Na⁺/K⁺ pump, proton pumps (H⁺-ATPase)
- HIGHLY selective
Comparison
| Feature | Diffusion | Facilitated Diffusion | Active Transport |
|---|---|---|---|
| Energy | NONE | NONE | ATP required |
| Gradient | DOWN | DOWN | AGAINST |
| Carrier needed | NO | YES | YES |
| Saturation | NO | YES | YES |
| Selectivity | LOW | HIGH | VERY HIGH |
3. Osmosis
- Definition: Diffusion of WATER across a SELECTIVELY PERMEABLE membrane
- Water moves from HIGH water potential (dilute solution) to LOW water potential (concentrated solution)
Water Potential (Ψ)
- Ψ = Ψ_s + Ψ_p (solute potential + pressure potential)
- Pure water: Ψ = 0 (MAXIMUM)
- Solutions: Ψ is NEGATIVE (solute potential reduces it)
- Water moves from HIGHER Ψ (less negative) to LOWER Ψ (more negative)
Osmotic Pressure vs Osmotic Potential
- Osmotic pressure: PRESSURE needed to prevent osmosis
- Osmotic potential: = -Osmotic pressure (always NEGATIVE)
Plasmolysis
- Cell placed in HYPERTONIC solution → water LEAVES → protoplast SHRINKS
- Cell becomes FLACCID (plasmolysed)
- If placed in HYPOTONIC solution → water enters → deplasmolysis
- Demonstrates that the cell membrane is SELECTIVELY PERMEABLE
Imbibition
- Absorption of water by hydrophilic COLLOIDS (seeds, wood)
- Mechanism: Affinity of MATRIX for water
- Produces GREAT force (helps seed GERMINATION)
4. Absorption of Water by Roots
Pathways for Water Movement
| Pathway | Description | Speed |
|---|---|---|
| Apoplast | Through CELL WALLS and INTERCELLULAR spaces (NON-living) | FAST (no barriers) |
| Symplast | Through PLASMODESMATA and cytoplasm (LIVING) | SLOWER |
| Transmembrane | From cell to cell across MEMBRANES | SLOWEST |
Casparian Strip
- WATER-IMPERMEABLE band in the ENDODERMIS of roots
- FORCES water from the apoplast into the SYMPLAST pathway
- Important for SELECTIVE mineral uptake
Root Pressure
- POSITIVE pressure generated in xylem by root cells actively pumping ions
- Can cause GUTTATION (water droplets on leaf margins in mornings)
- NOT sufficient for tall trees — TRANSPIRATION PULL is the main force
5. Transpiration
Definition
- Loss of WATER VAPOUR from plant surfaces (mainly through STOMATA)
Types
| Type | Site | Amount |
|---|---|---|
| Stomatal | Leaves (through stomata) | 90-95% (MAJOR) |
| Lenticular | Bark (lenticels) | 0.1-1% |
| Cuticular | Leaf surface through cuticle | 5-10% |
Mechanism of Stomatal Opening
- Guard cells: Kidney-shaped cells surrounding stomatal pore
- Opening: Guard cells PUMP in K⁺ → water enters by osmosis → turgid → pore OPENS
- Closing: K⁺ leaves → water leaves → guard cells flaccid → pore CLOSES
- Stomata OPEN in light, CLOSE in dark (generally)
Factors Affecting Transpiration
| Factor | Effect on Transpiration |
|---|---|
| Light | INCREASES (stomata open) |
| Temperature | INCREASES (↑ evaporation) |
| Humidity | DECREASES (↓ gradient) |
| Wind | INCREASES (removes vapour) initially; EXCESS wind may DECREASE |
| CO₂ | LOW CO₂ → stomata OPEN |
Transpiration Pull (Cohesion-Tension Theory)
- Water evaporates from leaf (transpiration)
- TENSION develops in xylem (negative pressure)
- Water molecules COHERE (hydrogen bonds) — continuous column
- Column is PULLED upward from roots
- Water enters roots from soil (mass flow)
6. Uptake and Transport of Minerals
- Minerals absorbed by ROOT HAIRS (active/passive)
- Transported through XYLEM along with water (transpiration stream)
- Essential elements: 17 required by plants (macronutrients + micronutrients)
7. Phloem Transport (Translocation)
- Source to Sink: Sugars (sucrose) travel from SOURCE (leaves) to SINK (roots, fruits, seeds)
- Pressure Flow Hypothesis (Münch, 1930):
- Sucrose LOADED into phloem at source (active transport)
- ↑ solute concentration → water enters phloem by OSMOSIS
- ↑ pressure → flows toward SINK (lower pressure)
- At sink: Sucrose UNLOADED → water leaves → pressure drops
8. Common Mistakes
- Osmosis is DIFFUSION of water, NOT of solute: Water moves, not the dissolved substance
- Plasmolysis occurs in HYPERTONIC solution, NOT hypotonic: Hypertonic = water leaves cell
- Water potential of pure water is ZERO, not maximum positive: All solutions have NEGATIVE water potential
- Phloem transport is BIDIRECTIONAL: Different sieve tubes simultaneously transport in different directions (source→sink)
- Ascent of sap is MAINLY by transpiration pull, NOT root pressure: Root pressure is significant only in small plants or at night
9. CBSE Exam Focus
- Diffusion vs Osmosis vs Active transport (3/5-mark)
- Water potential — concept and numerical problems (3-mark)
- Plasmolysis — experiment, significance (3-mark)
- Transpiration — types, mechanism, importance (5-mark)
- Transpiration pull — cohesion-tension theory (5-mark)
- Pressure flow hypothesis for phloem transport (5-mark)
10. Self-Test (5+ Q&A)
Q1: What is the difference between apoplast and symplast pathways of water movement? A: Apoplast: Through cell WALLS and intercellular spaces (non-living) — FAST. Symplast: Through PLASMODESMATA and CYTOPLASM (living) — SLOWER but controlled.
Q2: Explain the cohesion-tension theory of water transport. A: Transpiration creates TENSION in xylem. Water molecules COHERE (H-bonds), forming a CONTINUOUS column. The column is PULLED upward from roots to leaves like a ROPE.
Q3: What is guttation? What causes it? A: Exudation of WATER DROPLETS from leaf margins (hydathodes). Caused by ROOT PRESSURE at night when transpiration is LOW.
Q4: How do guard cells regulate stomatal opening? A: Guard cells take up K⁺ from surrounding cells → water enters by OSMOSIS → turgid → pore OPENS. Loss of K⁺ → water leaves → flaccid → pore CLOSES.
Q5: What is the role of the Casparian strip? A: Water-impermeable band in root ENDODERMIS that FORCES water from the APOPLAST into the SYMPLAST pathway. This acts as a CHECKPOINT — only SELECTED ions reach the xylem.
11. Conclusion
Plant transport is a MASTERFUL use of physical principles — cohesion, tension, osmosis, and pressure. The TRANSPIRATION PULL moves water and minerals from root to leaf through xylem. The PRESSURE FLOW HYPOTHESIS explains how food (sucrose) moves from SOURCE to SINK through phloem. The Casparian strip provides SELECTIVE control over mineral uptake. Understanding these mechanisms is ESSENTIAL for agriculture — irrigation, transpiration, and nutrient management all depend on these principles.
