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Q12:
Discuss the factors responsible for ascent of xylem sap in plants.
Solution :
1. Theoretical Framework: The Cohesion-Tension-Transpiration Theory
The ascent of xylem sap, the upward movement of water and minerals from roots to the highest canopy leaves, is primarily explained by the Cohesion-Tension-Transpiration Pull Model. This process is passive, requiring no metabolic energy expenditure by the plant [Per the thermodynamic principles of water potential gradients].
2. Core Contributing Factors
The movement of sap relies on the physical properties of water and the anatomical architecture of the xylem vessel elements and tracheids.
- Transpiration Pull: Water evaporates from the mesophyll cell walls into the intercellular spaces and eventually into the atmosphere through stomata. This creates a negative pressure (tension) in the xylem sap column [Reference: Dixon and Joly, 1894].
- Cohesion: The mutual attraction between water molecules due to hydrogen bonding creates a continuous, unbroken water column within the xylem conduits [Property of Polar Molecules].
- Adhesion: The attraction of water molecules to the polar surfaces of the xylem vessel walls (cellulose and lignin) prevents the column from slipping downward due to gravity [Capillary Action Principle].
- Surface Tension: Water molecules are more strongly attracted to each other in the liquid phase than to water in the gas phase, providing the necessary strength to withstand the tension generated by evaporation.
3. Mathematical Representation of Water Potential
The movement follows a water potential gradient ($\Psi_w$) from a higher potential (roots) to a lower potential (atmosphere). The total potential is governed by the formula:
$$\Psi_w = \Psi_s + \Psi_p$$
Where:
- $\Psi_s$ = Solute potential (usually negative)
- $\Psi_p$ = Pressure potential (positive in turgid cells, negative in xylem under tension)
4. Visual Representation of the Xylem Conduit
5. Summary of Physical Mechanisms
| Mechanism | Physical Basis | Role in Ascent |
|---|---|---|
| Cohesion | Hydrogen Bonding | Maintains column integrity. |
| Adhesion | Surface Adsorption | Resists gravitational pull. |
| Transpiration | Evaporative Cooling/Diffusion | Generates negative pressure (tension). |
Final Solution: The ascent of xylem sap is driven by the Transpiration Pull, which creates a negative pressure gradient, facilitated by the Cohesive and Adhesive properties of water, which maintain the continuity of the water column from root to leaf against the force of gravity.
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?
- Q10: What role does root pressure play in water movement in plants?
- Q11: Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to 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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