Quick Summary
Transport in plants involves two systems: xylem transports water and minerals upward; phloem translocates organic solutes (sugars) bidirectionally. Understanding osmosis, water potential, and the cohesion-tension theory of ascent of sap is essential. The pressure flow hypothesis for phloem transport is the other key concept.
Diffusion and Osmosis
Diffusion: Movement of molecules from high to low concentration (no membrane needed).
Osmosis: Diffusion of water through a semipermeable membrane from high water potential to low water potential.
Endosmosis: Water enters a cell (cell in hypotonic solution → turgor) Exosmosis: Water leaves a cell (cell in hypertonic solution → plasmolysis)
Water Potential (Ψw)
Ψw = Ψs + Ψp
| Component | Symbol | Value | Meaning |
|---|---|---|---|
| Solute potential | Ψs | Always negative | Solute lowers water potential |
| Pressure potential | Ψp | Positive (turgor) or negative (tension) | Pressure raises or lowers water potential |
| Pure water | Ψw | 0 | Reference |
Water moves from high Ψw (less negative) → low Ψw (more negative)
Plasmolysis and Turgor
Turgor pressure: Pressure exerted by water-filled vacuole on cell wall (makes cell firm)
Plasmolysis: Cell loses water in hypertonic solution → vacuole shrinks → plasma membrane pulls away from cell wall
Flaccid cell: Ψp = 0 (no turgor) Turgid cell: High Ψp (water fully absorbed) Incipient plasmolysis: Point when Ψp = 0 and plasmolysis just begins
Absorption of Water
Root hair cells absorb water by osmosis.
Pathways of water movement across root:
| Pathway | Route | Features |
|---|---|---|
| Apoplast | Through cell walls and intercellular spaces | Faster; no membrane crossing until Casparian strip |
| Symplast | Through cytoplasm and plasmodesmata | Slower; crosses membranes |
| Transmembrane | Through tonoplast and cell membrane | Crosses vacuoles |
Casparian strip: Waterproof lignin band in endodermal cell walls — blocks apoplast pathway, forces water through symplast before entering xylem.
Ascent of Sap
Cohesion-Tension (Transpiration Pull) Theory (Dixon & Joly):
1. Transpiration at leaf surface creates negative pressure (tension) 2. Cohesion of water molecules (H-bonds) — continuous water column 3. Adhesion of water to xylem wall — prevents column from breaking 4. Column is pulled upward from roots to leaves
Root pressure: Active water absorption by roots creates positive pressure — limited role in ascent.
Transpiration
Definition: Loss of water vapour from plant surfaces (mainly through stomata)
Types:
- Stomatal: 80–90% of total water loss
- Cuticular: Through cuticle (2–5%)
- Lenticular: Through lenticels (<1%)
Factors affecting transpiration:
| Factor | Effect on transpiration |
|---|---|
| High temperature | Increases (more evaporation) |
| Low humidity | Increases (greater gradient) |
| Wind | Increases (removes water vapour) |
| High CO₂ | Decreases (stomata close) |
| Water stress | Decreases (stomata close via ABA) |
Wilting: Excessive water loss → loss of turgor → stomata close → photosynthesis stops.
Phloem Transport — Pressure Flow Hypothesis (Münch)
Source: Leaf (photosynthesis produces sugar) Sink: Roots, fruits, storage organs
Steps: 1. Sugar loaded into sieve tubes at source → Ψs decreases → water enters by osmosis → high turgor 2. At sink: sugar unloaded → water leaves → low turgor 3. Pressure gradient (source → sink) drives mass flow of phloem sap
Evidence for: Movement bidirectional, matches sugar distribution. Evidence against: Not all predictions fully confirmed in gymnosperms.
Important Questions
One Mark
- Define plasmolysis.
- What is the Casparian strip and where is it located?
- State the cohesion-tension theory in one sentence.
- Which hormone causes stomatal closure during water stress?
Short Answer (3 marks)
- Explain the apoplast and symplast pathways of water movement in roots.
- What is water potential? Write the equation and explain each component.
- Describe the factors that affect the rate of transpiration.
Long Answer (5 marks)
- Explain the cohesion-tension theory of ascent of sap. Name the scientist who proposed it and state its evidence.
- Describe the pressure flow hypothesis of phloem translocation. What is the source and sink?
Common Mistakes
- Osmosis: water moves from high water potential to low water potential (NOT from low concentration to high — though that's equivalent for solute, water potential is the correct term).
- Casparian strip is in the endodermis, not epidermis.
- Transpiration pull is the most important force for ascent of sap — root pressure is minor.
- Phloem sap moves both upward and downward depending on the source/sink — unlike xylem (upward only).
Board Exam Tips
- Cohesion-tension theory (3 components: transpiration pull, cohesion, adhesion) is a standard 5-mark question.
- Pressure flow hypothesis with source-sink diagram is another 5-mark staple.
- Water potential equation and components appear as a 3-mark question.
- NEET: Osmosis, water potential, plasmolysis, and transpiration contribute 2–3 questions per paper.