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Q4:
Explain why pure water has the maximum water potential.
Solution :
Theoretical Foundation: Defining Water Potential ($\Psi_w$)
Water potential is defined as the chemical potential of water, representing the free energy per unit volume of water molecules. It serves as a measure of the tendency of water to move from one region to another. By thermodynamic convention, pure water is the reference state for this potential.
Step 1: The Thermodynamic Standard
In accordance with the standard definition established by plant physiologists, the water potential of pure water at standard temperatures (which is not under any pressure) is assigned a value of zero. This is expressed mathematically as:
$\Psi_w (\text{pure water}) = 0 \text{ MPa}$
[Per the thermodynamic convention in Biology and Physics]
Step 2: Analysis of Kinetic Energy and Solute Concentration
Water molecules possess kinetic energy. In a system containing only pure water, the molecules are in rapid, constant, and random motion. When solutes are added to this system, the following occurs:
- Solute Interaction: Solute molecules (such as salts or sugars) bond with water molecules via hydrogen bonding and hydration shells.
- Kinetic Reduction: These interactions restrict the freedom of movement of water molecules, thereby decreasing their kinetic energy.
- Entropy Implications: Since water potential is directly proportional to the kinetic energy of water molecules, the restriction of movement caused by solutes leads to a decrease in water potential.
Step 3: Comparative Analysis of Potentials
Because the addition of solutes always results in a reduction of free energy, any solution containing solutes will have a water potential lower than that of pure water.
| State of System | Water Potential ($\Psi_w$) Value | Justification |
|---|---|---|
| Pure Water | $0$ (Maximum) | Absence of solute-water binding energy. |
| Solution | $< 0$ (Negative) | Solutes lower the chemical potential of water. |
Step 4: Vector Representation of Water Movement
Conclusion
Pure water possesses the maximum water potential because it is devoid of any solutes that would otherwise restrict the kinetic energy of its molecules. As solutes are added, the water potential decreases (becomes more negative), meaning that water molecules are less "free" to move. Therefore, pure water acts as the baseline of maximum energy state ($0 \text{ MPa}$), from which all other aqueous systems are measured as negative values.
Final Solution: Pure water has the maximum water potential of zero because it lacks solutes; the addition of any solute decreases the kinetic energy of water molecules, resulting in a negative water potential value.
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.
- 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?
- 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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