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Q7:
What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Solution :
Theoretical Foundation: Water Potential Dynamics
The movement of water in plant systems is governed by the water potential ($\Psi_w$), which is defined as the sum of solute potential ($\Psi_s$) and pressure potential ($\Psi_p$). The relationship is expressed by the equation:
$\Psi_w = \Psi_s + \Psi_p$
[Per the thermodynamic definition of water potential]
Step 1: Analyzing the Effect of Positive Pressure
In a standard, unconfined system, the pressure potential ($\Psi_p$) of pure water at atmospheric pressure is $0$ MPa. When a mechanical pressure greater than the atmospheric pressure is applied to a system (pure water or a solution), the pressure potential ($\Psi_p$) becomes positive.
By increasing the physical force applied to the water, we increase the internal hydrostatic pressure. This increment in pressure directly increases the total water potential ($\Psi_w$) of the system.
Step 2: Mathematical Derivation of Potential Increase
Given the equation $\Psi_w = \Psi_s + \Psi_p$, if $\Psi_s$ remains constant (in the case of pure water, $\Psi_s = 0$), then:
$\Delta\Psi_w = \Delta\Psi_p$
[By the Additive Property of Potential Energy]
As the external force increases, $\Psi_p$ transitions from $0$ to a positive value. Consequently, the water potential ($\Psi_w$) increases. This makes the water potential of the solution higher than it was under atmospheric conditions, effectively increasing the "chemical potential" or the free energy of the water molecules.
Step 3: Biological Implications and Movement
Water always moves from a region of higher water potential to a region of lower water potential [Per the Second Law of Thermodynamics and the Principle of Osmosis].
- Increased Diffusion Pressure: The application of pressure increases the ability of water to move or do work.
- Reversal of Osmosis: If this pressure is applied to a solution separated from pure water by a semi-permeable membrane, and the pressure applied exceeds the osmotic pressure, the flow of water can be reversed (the basis of Reverse Osmosis).
Visual Representation of Water Potential Change
Summary Comparison Table
| Condition | Pressure Potential ($\Psi_p$) | Water Potential ($\Psi_w$) |
|---|---|---|
| Atmospheric Pressure | $0$ | $\Psi_s$ |
| Pressure > Atmospheric | Positive ($>0$) | Increased ($\Psi_s + \Psi_p$) |
Final Solution: When a pressure greater than atmospheric pressure is applied to pure water or a solution, its pressure potential ($\Psi_p$) increases, which in turn increases the total water potential ($\Psi_w$). This elevation in water potential increases the free energy of the water molecules, enhancing their capacity to diffuse or move into regions of lower water potential.
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?
- 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?
- 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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