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Q10:
What role does root pressure play in water movement in plants?
Solution :
1. Physiological Definition of Root Pressure
Root pressure is defined as the positive pressure that develops in the roots of plants due to the active accumulation of mineral ions from the soil into the vascular cylinder (stele). [Per the Chemiosmotic Theory of Ion Transport]. Because mineral ions are actively transported into the xylem, the water potential ($\Psi_w$) of the xylem vessels decreases, causing water from the soil to move into the root xylem via osmosis.
2. Mechanism of Generation
The development of root pressure follows a strictly sequential biophysical pathway:
- Active Ion Uptake: Root hair cells use ATP to pump ions against the electrochemical gradient into the symplast and eventually into the xylem elements.
- Osmotic Gradient Formation: The increased solute concentration in the xylem creates a gradient where $\Psi_{xylem} < \Psi_{soil}$.
- Hydrostatic Pressure Development: Water enters the xylem to balance the potential, exerting outward pressure against the rigid cell walls of the xylem vessels.
3. Physiological Significance and Limitations
Root pressure serves specific functions but is insufficient as the primary driver for tall trees.
| Function | Description |
|---|---|
| Re-establishment of columns | It helps push up water columns that may have broken under tension [Cohesion-Tension Theory]. |
| Guttation | It is responsible for the exudation of water droplets from leaf margins at night or early morning. |
4. Critical Constraints
While root pressure provides a measurable push, its efficacy is restricted by the following factors:
- Magnitude: Root pressure is a positive pressure, whereas the primary mechanism for water ascent (transpiration pull) is a negative pressure (tension).
- Height Limitation: Root pressure is inadequate to overcome the force of gravity in tall gymnosperms and angiosperms, where the required hydraulic lift exceeds the magnitude of developed osmotic pressure.
- Seasonal Dependency: It is most observable when transpiration is low (e.g., humid conditions, night-time), meaning it cannot sustain the high rates of water transport required during peak daylight hours.
Final Solution: Root pressure acts as a supplemental mechanism for water transport. It is essentially a positive hydrostatic pressure resulting from the active accumulation of mineral ions in the xylem. While it is significant for re-establishing broken water columns and facilitating guttation in small herbaceous plants, it is not the primary force for water movement in tall plants, as it lacks the necessary magnitude to counteract gravity under high transpiration conditions.
More Questions from Class 11 Biology Transport in Plants EXERCISES
- Q1: What are the factors affecting the rate of diffusion?
- Q2: What are porins? What role do they play in diffusion?
- Q3: Describe the role played by protein pumps during active transport in plants.
- Q4: Explain why pure water has the maximum water potential.
- Q5(a): Differentiate between the following: (a) Diffusion and Osmosis
- Q5(b): Differentiate between the following: (b) Transpiration and Evaporation
- Q5(c): Differentiate between the following: (c) Osmotic Pressure and Osmotic Potential
- Q5(d): Differentiate between the following: (d) Imbibition and Diffusion
- Q5(e): Differentiate between the following: (e) Apoplast and Symplast pathways of movement of water in plants.
- Q5(f): Differentiate between the following: (f) Guttation and Transpiration.
- Q6: Briefly describe water potential. What are the factors affecting it?
- Q7: What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
- Q8(a): (a) With the help of well-labelled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.
- Q8(b): (b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.
- Q9: How is the mycorrhizal association helpful in absorption of water and minerals in plants?
- Q11: Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
- Q12: Discuss the factors responsible for ascent of xylem sap in plants.
- Q13: What essential role does the root endodermis play during mineral absorption in plants?
- Q14: Explain why xylem transport is unidirectional and phloem transport bi-directional.
- Q15: Explain pressure flow hypothesis of translocation of sugars in plants.
- Q16: What causes the opening and closing of guard cells of stomata during transpiration?
CBSE Solutions for Class 11 Biology Transport in Plants
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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