Chapter 8 — Transport in Plants
Key Concepts Quick Recap
| Term | Definition |
|---|
| Diffusion | Movement of molecules from high to low concentration (no energy) |
| Osmosis | Diffusion of water through a semipermeable membrane from dilute to concentrated solution |
| Plasmolysis | Loss of water by osmosis → cell membrane shrinks away from cell wall |
| Turgor pressure (TP) | Pressure exerted by cell contents on cell wall (positive) |
| Osmotic potential (ψs) | Reduction in water potential due to solutes; always negative |
| Water potential (ψ) | ψ = ψs + ψp; drives water movement |
| Apoplast | Movement of water through cell walls and intercellular spaces (no crossing membranes) |
| Symplast | Movement of water through protoplasts via plasmodesmata |
| Transpiration | Loss of water as vapour from aerial plant parts (mainly stomata) |
| Ascent of sap | Upward movement of water through xylem from roots to leaves |
| Translocation | Transport of organic solutes (sucrose) through phloem |
I. Choose the Best Answer (MCQs)
1. Osmosis is best defined as: (a) Movement of solute molecules from low to high concentration (b) Movement of water through a semipermeable membrane from dilute to concentrated solution (c) Active transport of ions (d) Movement of water against concentration gradient
Answer(b) Movement of water through a semipermeable membrane from dilute to concentrated solution Osmosis is a passive process — water moves down its own concentration gradient (from low solute concentration to high solute concentration) through a selectively permeable membrane. No energy is required.
2. A cell placed in a hypertonic solution will: (a) Swell (b) Remain unchanged (c) Plasmolyse (d) Burst
Answer(c) Plasmolyse In a hypertonic solution, external solute concentration > cell solute concentration → water moves out of the cell by osmosis → cell loses turgor → protoplast shrinks away from the cell wall → plasmolysis.
3. The most accepted theory for ascent of sap is: (a) Root pressure theory (b) Vital theory (c) Cohesion-tension theory (d) Atmospheric pressure theory
Answer(c) Cohesion-tension theory Proposed by Dixon and Joly (1894). Transpiration from leaves creates tension (negative pressure) in xylem. This tension pulls the water column upward. The water column is maintained by cohesion between water molecules (H-bonds) and adhesion to xylem walls.
4. Which factor does NOT affect the rate of transpiration? (a) Light (b) Humidity (c) Temperature (d) Soil pH
Answer(d) Soil pH Transpiration rate is affected by: light (opens stomata), humidity (high humidity reduces transpiration), temperature (higher temp = faster evaporation), wind (removes water vapour), water availability. Soil pH does not directly affect transpiration rate.
5. Phloem transport occurs by: (a) Cohesion-tension mechanism (b) Pressure flow (mass flow) hypothesis (c) Root pressure (d) Active transport only
Answer(b) Pressure flow (mass flow) hypothesis Munch's pressure flow hypothesis: sugar loaded into phloem at source (leaf) lowers water potential → water enters by osmosis → creates pressure → sugar solution flows through phloem to sink (roots, fruits) → sugar unloaded → water leaves → pressure released.
6. Guard cells in stomata are: (a) Dead cells (b) Living cells with chloroplasts (c) Sclerenchyma cells (d) Epidermal cells without chloroplasts
Answer(b) Living cells with chloroplasts Guard cells are kidney-shaped (dicots) or dumbbell-shaped (grasses/monocots) living epidermal cells with chloroplasts. They control stomatal opening/closing through changes in turgor pressure regulated by K⁺ ion movements.
7. Root pressure can be demonstrated by: (a) Plasmolysis experiment (b) Exudation from cut stem stump (c) Weighing leaves (d) Measuring CO₂ absorption
Answer(b) Exudation from cut stem stump When a stem is cut close to the soil, water continues to ooze from the cut surface — this is due to root pressure (positive pressure generated in roots by active ion absorption). Root pressure is greatest at night when transpiration is minimal.
II. Short Answer Questions
Q1. Explain osmosis with a diagram. Distinguish between endosmosis and exosmosis.
Answer
Osmosis is the diffusion of water molecules through a differentially permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).
Dilute solution | Membrane | Concentrated solution
Higher ψ | | Lower ψ
Water molecules move →
| Feature | Endosmosis | Exosmosis |
|---|
| Direction | Water enters cell | Water exits cell |
| Condition | Cell placed in hypotonic solution | Cell placed in hypertonic solution |
| Result | Cell swells (turgid) | Cell shrinks (plasmolysis) |
| Example | Dry raisin in water | Fresh cell in salt solution |
Q2. What is transpiration? Describe its types and significance.
Answer
Transpiration is the loss of water in the form of water vapour from aerial parts of the plant.
Types:
- Stomatal transpiration: Through stomata (90–95% of total). Most significant.
- Cuticular transpiration: Through cuticle of epidermis (3–8%). Higher in young leaves.
- Lenticular transpiration: Through lenticels (woody stems) (< 1%).
Significance: 1. Cooling effect: Evaporation absorbs latent heat → cools leaf surface. 2. Ascent of sap: Transpiration pull is the main driving force for water transport. 3. Mineral absorption: Water flow carries dissolved minerals upward. 4. Turgidity: Turgor pressure maintained for cell shape.
Transpiration paradox: Stomata must be open for photosynthesis (CO₂ entry) but this also causes water loss — the plant must balance these two needs.
Q3. Explain the cohesion-tension theory of ascent of sap.
Answer
Proposed by: H.H. Dixon and J. Joly (1894). Also called transpiration pull theory.
Mechanism: 1. Transpiration from leaf cells creates a water deficit → water potential drops in mesophyll cells. 2. Water moves from xylem (higher ψ) into mesophyll cells (lower ψ). 3. This creates tension (negative pressure) at the top of the xylem water column. 4. The water column is pulled upward because of:
5. Water is absorbed from soil by roots to replace what is lost → water moves up continuously.
- Cohesion: Strong H-bonds between water molecules maintain a continuous column.
- Adhesion: Water molecules adhere to the hydrophilic xylem walls.
Evidence supporting the theory:
- When xylem is air-blocked (embolism), water transport stops.
- Water rises in proportion to transpiration rate.
- Trunk diameter decreases during the day (tension compresses trunk) and increases at night.
Q4. Describe the pressure flow hypothesis of phloem transport.
Answer
Proposed by: E. Munch (1930).
Concept: Sugars move from source to sink by bulk flow due to pressure difference.
Steps: 1. At source (leaf):
- Photosynthesis produces sucrose.
- Sucrose loaded into phloem (sieve tubes) by active transport.
- Water potential in sieve tube drops → water enters from xylem by osmosis.
- Turgor pressure increases at source end.
2. At sink (root/fruit/growing region):
- Sucrose unloaded from sieve tubes (active/passive).
- Water leaves phloem → water potential rises.
- Turgor pressure decreases at sink end.
3. Pressure gradient:
- High pressure at source → low pressure at sink.
- Sucrose solution flows from source to sink (mass flow).
Evidence: Aphid stylet experiments — when aphid punctures phloem, sap exudes under pressure.
Common Mistakes
- Confusing osmosis (water movement) and diffusion (solute movement).
- Saying xylem transports food — xylem transports water; phloem transports food.
- Forgetting that water potential of pure water = 0, and addition of solute makes it negative.
- Confusing apoplast (cell wall, outside membrane) and symplast (through cytoplasm).
Exam Checklist
- Osmosis direction: from high ψ to low ψ (high water potential to low water potential).
- Plasmolysis: hypertonic solution → water exits → protoplast shrinks from wall.
- Transpiration: stomatal (90%), cuticular, lenticular.
- Cohesion-tension theory: Dixon and Joly; transpiration pull + cohesion + adhesion.
- Pressure flow: Munch; source → sink; sucrose loaded at source, unloaded at sink.
- Guard cell: kidney-shaped (dicot), dumbbell-shaped (monocot); chloroplasts present.
