How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Transport in Plants covers diffusion, osmosis, water potential, plasmolysis, imbibition, transpiration, and ascent of sap — focus on understanding the mechanisms and comparing processes.
Key Concepts (Quick Recap)
Means of Transport
| Type | Energy | Direction | Example |
|---|
| Diffusion | Passive (no ATP) | High concentration → Low concentration | O₂, CO₂ movement in cells |
| Facilitated diffusion | Passive (no ATP) | High concentration → Low concentration (via membrane proteins) | Glucose uptake via carriers |
| Active transport | Active (requires ATP) | Low concentration → High concentration | Ion uptake by root cells |
| Osmosis | Passive (no ATP) | Higher water potential → Lower water potential | Water uptake by root hairs |
Water Potential (Ψw)
- Definition: The potential energy of water in a system compared to pure water
- Unit: Megapascals (MPa) or Pascals (Pa)
- Pure water: Ψw = 0 MPa (highest possible value, by convention)
- Adding solutes: Decreases water potential (Ψw becomes negative)
- Pressure (turgor): Increases water potential (positive contribution)
FormulaΨw = Ψs + Ψp
- Ψs = Solute potential (matrix potential) — always negative
- Ψp = Pressure potential (turgor pressure) — usually positive
| Solution type | Solute concentration | Water potential | Water movement |
|---|
| Hypotonic | Lower outside (higher water potential) | Higher outside (less negative) | Water enters the cell → cell swells |
| Isotonic | Equal inside and outside | Equal | No net movement |
| Hypertonic | Higher outside (lower water potential) | Lower outside (more negative) | Water leaves the cell → cell shrinks |
Osmosis and Related Terms
| Term | Definition |
|---|
| Osmosis | Movement of water across a semi-permeable membrane from higher water potential to lower water potential |
| Osmotic pressure | The pressure required to prevent osmosis; proportional to solute concentration |
| Turgor pressure | The pressure exerted by the cell contents against the cell wall when water enters |
| Wall pressure | The equal and opposite pressure exerted by the cell wall against the turgor pressure |
| Plasmolysis | Shrinkage of protoplast away from cell wall in a hypertonic solution |
| Deplasmolysis | Recovery of a plasmolysed cell when placed in a hypotonic solution |
| Turgid | Cell is fully swollen with water (in hypotonic solution) |
| Flaccid | Cell has lost water and is soft (in hypertonic solution) |
| Incipient plasmolysis | Stage when the protoplast just begins to pull away from the cell wall |
Imbibition
- Definition: The absorption of water by hydrophilic colloidal substances (seeds, wood) causing them to swell
- Examples: Swelling of seeds when soaked, expansion of dry wood when wetted
- Conditions: (1) Presence of hydrophilic colloids, (2) Difference in water potential between absorbent and water
- Imbibition pressure: The pressure developed by swelling of imbibant; can be very high (enough to break seed coats)
Transpiration
- Definition: The loss of water vapour from the aerial parts of a plant (mainly through stomata)
- Types:
- Stomatal transpiration: Through stomata (80–90% of total)
- Cuticular transpiration: Through the cuticle (5–10%)
- Lenticular transpiration: Through lenticels (1–2%)
- Factors affecting transpiration:
- External: Light, temperature, humidity, wind, water availability
- Internal: Leaf area, leaf structure, stomatal density, cuticle thickness
Ascent of Sap (Water Rise in Xylem)
| Theory | Explanation | Proposed by |
|---|
| Root pressure theory | Hydrostatic pressure in roots pushes water up | Priestley |
| Cohesion-tension theory (most accepted) | Transpiration pull + cohesion of water molecules + adhesion to xylem walls | Dixon and Joly |
| Capillarity theory | Rise due to capillary action in narrow xylem vessels | — |
| Imbibition theory | Water rises by imbibition through xylem walls | — |
Cohesion-tension theory (Transpiration pull theory) — most widely accepted: 1. Water molecules have strong cohesive forces (hydrogen bonds) → form a continuous column 2. Transpiration at the leaves creates a pull (tension) 3. This pull is transmitted down the xylem column 4. Adhesion of water to xylem walls helps support the column 5. Water moves upward from roots to leaves
Root Pressure
- Positive hydrostatic pressure in the xylem of roots
- Due to active transport of ions into the root xylem
- Can push water up a few metres
- Evidence: Guttation (water droplets at leaf margins in early morning)
- Not sufficient to explain water rise in tall trees
I. Choose the Best Answer (MCQs)
Q1. The movement of water across a semi-permeable membrane is called (a) Diffusion (b) Osmosis (c) Active transport (d) Imbibition
Answer(b) Osmosis — Osmosis is the movement of water across a semi-permeable membrane from a region of higher water potential to a region of lower water potential. Diffusion is the movement of any molecule from high to low concentration (no membrane required).
Q2. Water potential of pure water is (a) 100 MPa (b) 0 MPa (c) -100 MPa (d) 1 MPa
Answer(b) 0 MPa — By convention, the water potential of pure water at standard temperature and pressure is taken as 0 MPa (the highest possible value). Adding solutes makes water potential negative.
Q3. A cell placed in a hypertonic solution will (a) Swell (b) Shrink (c) Remain the same (d) Burst
Answer(b) Shrink — In a hypertonic solution, water potential is lower outside the cell. Water moves out of the cell by osmosis, causing the protoplast to shrink (plasmolysis).
Q4. The most widely accepted theory for ascent of sap is (a) Root pressure theory (b) Capillarity theory (c) Cohesion-tension theory (d) Imbibition theory
Answer(c) Cohesion-tension theory — The cohesion-tension theory (transpiration pull theory), proposed by Dixon and Joly, is the most widely accepted explanation for the ascent of sap. It explains water rise in tall trees through transpiration pull, cohesion of water molecules, and adhesion to xylem walls.
Q5. Most transpiration occurs through (a) Cuticle (b) Lenticels (c) Stomata (d) Hydathodes
Answer(c) Stomata — Stomatal transpiration accounts for 80–90% of total water loss. Cuticular transpiration (5–10%) and lenticular transpiration (1–2%) are much smaller. Hydathodes are involved in guttation, not transpiration.
Q6. Guttation is caused by (a) Transpiration pull (b) Root pressure (c) Capillarity (d) Imbibition
Answer(b) Root pressure — Guttation is the loss of water in the form of droplets from leaf margins through hydathodes. It occurs due to root pressure pushing water up when transpiration is low (e.g., early morning, humid conditions).
Q7. Active transport differs from diffusion in that it (a) Does not require energy (b) Moves substances from low to high concentration (c) Moves substances from high to low concentration (d) Does not require carrier proteins
Answer(b) Moves substances from low to high concentration — Active transport moves substances against the concentration gradient (low to high) using ATP energy and carrier proteins. Diffusion moves substances along the gradient (high to low) without energy.
II. Fill in the Blanks
1. The movement of molecules from high concentration to low concentration is called diffusion. 2. The movement of water across a semi-permeable membrane is called osmosis. 3. Water potential of pure water is 0 MPa. 4. Adding solutes decreases (makes more negative) the water potential. 5. When a cell is placed in a hypotonic solution, water enters the cell and it becomes turgid. 6. When a cell is placed in a hypertonic solution, water leaves the cell and it becomes flaccid (plasmolysed). 7. The shrinkage of the protoplast away from the cell wall is called plasmolysis. 8. The pressure exerted by the cell contents against the cell wall is called turgor pressure. 9. The absorption of water by hydrophilic colloids causing them to swell is called imbibition. 10. The loss of water vapour from aerial plant parts is called transpiration. 11. Most transpiration occurs through stomata. 12. The most accepted theory for ascent of sap is the cohesion-tension theory (transpiration pull theory). 13. Guttation occurs through hydathodes. 14. Root pressure is responsible for pushing water up the xylem, but it is not sufficient for tall trees. 15. The cohesion-tension theory was proposed by Dixon and Joly.
III. True or False (with corrections for false statements)
Q1. Osmosis requires energy.
AnswerFalse — Correction: Osmosis is a PASSIVE process — it does not require energy (ATP). Water moves from higher water potential to lower water potential through a semi-permeable membrane without energy input. Active transport requires energy.
Q2. Water potential increases when solutes are added.
AnswerFalse — Correction: Adding solutes DECREASES water potential (makes it more negative). Pure water has the highest water potential (0 MPa). Solute potential (Ψs) is always negative.
Q3. Transpiration is a necessary evil.
AnswerTrue — Transpiration is called a "necessary evil" because it causes water loss (evil) but is also necessary for: (1) creating transpiration pull for ascent of sap, (2) cooling the plant, (3) maintaining turgor pressure, and (4) facilitating mineral transport.
Q4. Root pressure can push water to the top of tall trees.
AnswerFalse — Correction: Root pressure can push water up only a few metres. It is NOT sufficient to push water to the top of tall trees. The cohesion-tension theory (transpiration pull) is the main mechanism for water rise in tall trees.
Q5. Plasmolysis occurs when a cell is placed in a hypotonic solution.
AnswerFalse — Correction: Plasmolysis occurs when a cell is placed in a HYPERTONIC solution. Water moves OUT of the cell, causing the protoplast to shrink away from the cell wall. In a HYPOTONIC solution, water enters and the cell becomes turgid.
IV. Match the Following
| Column A | Column B | Answer |
|---|
| 1. Diffusion | (a) Water movement across membrane | (d) Molecules from high to low concentration |
| 2. Osmosis | (b) Movement against concentration gradient | (a) Water movement across semi-permeable membrane |
| 3. Active transport | (c) Shrinkage of protoplast | (b) Movement against concentration gradient (requires ATP) |
| 4. Plasmolysis | (d) Molecules from high to low concentration | (c) Shrinkage of protoplast in hypertonic solution |
| 5. Imbibition | (e) Loss of water as vapour | (f) Absorption of water by hydrophilic colloids |
| 6. Transpiration | (f) Absorption of water by hydrophilic colloids | (e) Loss of water as vapour from aerial parts |
V. Answer Briefly (2-mark questions)
Q1. Differentiate between diffusion and osmosis.
Answer
| Feature | Diffusion | Osmosis | |---|---|---| | Definition | Movement of any molecules from high to low concentration | Movement of water across a semi-permeable membrane | | Substance | Any substance (gases, liquids, solids) | Only water (solvent) | | Membrane | No membrane required | Semi-permeable membrane required | | Energy | Passive (no ATP) | Passive (no ATP) | | Example | Perfume spreading in a room | Water entering root hair cells |
Q2. What is water potential? What is the water potential of pure water?
Answer
Water potential (Ψw) is the potential energy of water in a system compared to pure water at standard conditions. It determines the direction of water movement — water always moves from higher water potential to lower water potential. - Water potential of pure water = 0 MPa (highest, by convention) - Adding solutes decreases water potential (negative values) - Pressure increases water potential (positive contribution) - Formula: Ψw = Ψs + Ψp
Q3. What is plasmolysis? What happens to a cell in a hypertonic solution?
Answer
Plasmolysis is the shrinkage of the protoplast away from the cell wall when a cell is placed in a hypertonic solution. In a hypertonic solution: 1. Water potential is lower outside the cell than inside 2. Water moves OUT of the cell by osmosis 3. The protoplast shrinks and pulls away from the cell wall 4. The cell becomes flaccid If the plasmolysed cell is placed in a hypotonic solution, it can regain its shape (deplasmolysis).
Q4. What is imbibition? Give two examples.
Answer
Imbibition is the absorption of water by hydrophilic colloidal substances (proteins, cellulose, starch) causing them to swell. It occurs due to the difference in water potential between the dry material and water. Examples: 1. Swelling of dry seeds when soaked in water 2. Expansion of wooden doors and windows during the rainy season 3. Germination of seeds (imbibition of water is the first step)
VI. Answer in Detail (5-mark questions)
Q1. Explain the cohesion-tension theory of ascent of sap.
Answer
The cohesion-tension theory (also called transpiration pull theory) was proposed by Dixon and Joly (1894). It is the most widely accepted theory for the ascent of sap (water rise) in tall trees. Main principles: 1. Cohesion (Tensile strength of water): - Water molecules have strong cohesive forces due to hydrogen bonding - These forces hold water molecules together in a continuous column in the xylem - The tensile strength of the water column is very high (can withstand great pull) 2. Adhesion (Attraction to walls): - Water molecules have adhesive forces with the lignified walls of xylem vessels - This adhesion helps the water column stay attached to the xylem walls and prevents it from falling back 3. Transpiration pull (Tension): - Transpiration (water loss from leaves) creates a negative pressure (tension/suction force) at the leaf surface - This pull is transmitted through the continuous water column in the xylem all the way down to the roots - Water is pulled up from the roots to the leaves How it works — step by step: 1. Water evaporates from the mesophyll cells of the leaf (transpiration) 2. This creates a water potential gradient between mesophyll cells and xylem 3. Water is pulled from the xylem into the mesophyll cells 4. This creates a negative pressure (tension) in the xylem 5. The cohesive forces between water molecules transmit this pull down the entire water column 6. Water is pulled up from the roots to the leaves Evidence supporting the theory: - Water column in xylem is under tension (measurable) - Tracheids and vessels are well-adapted for water conduction - The theory explains water rise in the tallest trees - Cutting a tree trunk causes air to enter the xylem (breaking the water column), stopping water flow
Q2. Describe the factors affecting transpiration.
Answer
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It is affected by both external (environmental) and internal (plant) factors: External (Environmental) Factors: 1. Light: - Light increases transpiration by opening stomata (light triggers guard cell turgor) - Light also increases temperature, which increases evaporation - Transpiration is highest during the day and lowest at night 2. Temperature: - Higher temperature increases the rate of evaporation of water from leaf surfaces - Increases the water vapour-holding capacity of air, creating a steeper gradient - Every 10°C rise approximately doubles transpiration rate 3. Humidity: - High humidity DECREASES transpiration (air already saturated with water vapour; less gradient) - Low humidity INCREASES transpiration (steeper water vapour gradient between leaf and air) 4. Wind: - Wind removes water vapour from around the leaf surface, maintaining a steep gradient - Increases transpiration rate - Strong winds can close stomata (protective response) 5. Water availability: - If soil water is insufficient, transpiration decreases - Water stress causes stomata to close, reducing water loss Internal (Plant) Factors: 1. Leaf area: - Larger leaf area = more stomata = more transpiration - Plants in dry habitats have small leaves (xerophytes) to reduce transpiration 2. Stomatal density: - More stomata per unit area = more transpiration - Stomata are usually more numerous on the lower surface (less exposed to direct sunlight) 3. Cuticle thickness: - Thick cuticle reduces cuticular transpiration - Xerophytes have a thick, waxy cuticle 4. Leaf structure: - Sunken stomata (in xerophytes) reduce transpiration - Hairy leaves trap moist air, reducing transpiration - Rolled leaves reduce the surface area exposed to air Significance of Transpiration: 1. Creates transpiration pull for ascent of sap 2. Cools the plant (evaporative cooling) 3. Maintains turgor pressure 4. Helps in mineral transport 5. But it is also called a "necessary evil" because it causes water loss
Q3. Compare diffusion, osmosis, and active transport.
Answer
| Feature | Diffusion | Osmosis | Active Transport | |---|---|---|---| | Definition | Movement of molecules from high to low concentration | Movement of water across a semi-permeable membrane | Movement against concentration gradient using energy | | Substance | Any substance | Only water (solvent) | Ions, molecules | | Membrane | Not required | Semi-permeable membrane required | Cell membrane with carrier proteins | | Energy | No ATP required (passive) | No ATP required (passive) | ATP required (active) | | Direction | High → Low concentration | High Ψw → Low Ψw | Low → High concentration | | Carrier proteins | Not required | Not required | Required | | Example | O₂ and CO₂ exchange in lungs | Water entering root hair cells | Na⁺/K⁺ pump, ion uptake by roots | | Selectivity | Non-selective | Selective (only water) | Highly selective (specific carriers) |
Common Mistakes to Avoid
- Confusing diffusion and osmosis: Diffusion = any molecule, no membrane needed. Osmosis = only water, requires a semi-permeable membrane.
- Confusing hypertonic and hypotonic: Hypertonic = more solutes outside = water leaves cell = plasmolysis. Hypotonic = fewer solutes outside = water enters cell = turgid.
- Saying water potential increases with solutes: Adding solutes DECREASES water potential (makes it more negative). Pure water = 0 MPa (highest).
- Confusing root pressure and transpiration pull: Root pressure pushes water up (limited to a few metres, active process). Transpiration pull pulls water up (can rise to tree tops, passive process).
- Confusing guttation and transpiration: Guttation = liquid water droplets from hydathodes (early morning, root pressure). Transpiration = water vapour from stomata (daytime, transpiration pull).
- Saying osmosis requires energy: Osmosis is PASSIVE — no ATP required. Active transport requires ATP.
- Confusing turgor pressure and wall pressure: Turgor pressure = pressure of cell contents ON the wall (outward). Wall pressure = pressure of the wall ON the cell contents (inward). They are equal and opposite.
Exam Checklist
- [ ] Can you differentiate diffusion, osmosis, and active transport?
- [ ] Can you define water potential and state the water potential of pure water?
- [ ] Can you explain what happens to a cell in hypotonic, isotonic, and hypertonic solutions?
- [ ] Can you define plasmolysis and deplasmolysis?
- [ ] Can you explain the cohesion-tension theory of ascent of sap?
- [ ] Can you list the factors affecting transpiration (external and internal)?
- [ ] Can you differentiate guttation and transpiration?
- [ ] Can you explain the significance of transpiration (and why it's a "necessary evil")?
- [ ] Can you define imbibition and give examples?
- [ ] Can you explain root pressure and its role in water transport?
