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Q11:
Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Solution :
Step 1: The Transpiration Pull Model (Cohesion-Tension Theory)
The movement of water through the xylem is explained by the Cohesion-Tension-Transpiration Pull Model, proposed by Dixon and Joly (1894). Water movement from roots to the highest canopy is a passive process driven by the potential energy gradient created by transpiration.
- Cohesion: Mutual attraction between water molecules due to hydrogen bonding. [Per the properties of polar molecules].
- Adhesion: Attraction of water molecules to the polar surfaces of the xylem tracheary elements (tracheids and vessels).
- Surface Tension: Water molecules are more attracted to each other in the liquid phase than to water in the gas phase.
As water evaporates from the mesophyll cell surfaces into the intercellular spaces and then through the stomata [Transpiration], it creates a negative pressure (tension) in the xylem sap. This tension pulls the continuous water column upward, resisting gravity.
Step 2: Factors Influencing Transpiration
Transpiration rates are governed by both environmental and physiological variables:
| Factor | Effect on Transpiration | Mechanism |
|---|---|---|
| Temperature | Directly Proportional | Increases kinetic energy of water vapor; decreases relative humidity. |
| Light | Directly Proportional | Stimulates stomatal opening [via proton pumping in guard cells]. |
| Humidity | Inversely Proportional | Higher ambient water vapor reduces the diffusion gradient ($\Delta C$). |
| Wind Speed | Directly Proportional | Removes the boundary layer of moist air, maintaining a steep gradient. |
Step 3: Biological Significance of Transpiration
Transpiration is not merely an "unavoidable evil" but a functional necessity for plant survival:
- Mineral Transport: It facilitates the upward movement of minerals and ions from the soil to various plant parts [Per the Mass Flow Hypothesis].
- Thermoregulation: It exerts a cooling effect on leaf surfaces, often by as much as $10-15^\circ C$ via evaporative cooling.
- Turgor Maintenance: By maintaining a steady flux of water, it helps keep cells turgid, which is essential for structural support and the expansion of leaves and stems.
- Photosynthesis Supply: It ensures that the photosynthetic machinery (chloroplasts) receives a constant supply of water required for the photolysis of water ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$).
Final Solution: The Transpiration Pull Model posits that negative pressure generated by water evaporation from leaves pulls a continuous, cohesive column of water through the xylem. Influenced by temperature, light, humidity, and wind, this process is essential for nutrient translocation, plant cooling, and maintenance of cell turgor pressure.
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?
- 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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