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Q2:
What are porins? What role do they play in diffusion?
Solution :
Definition and Structural Identity of Porins
Porins are a specialized class of transmembrane proteins that function as large, barrel-shaped channels. Structurally, they are composed of a series of $\beta$-sheets that fold into a cylinder, creating an aqueous pore that traverses the lipid bilayer of membranes. [Per the Fluid Mosaic Model of biological membranes]. These proteins are primarily found in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts.
Mechanism of Diffusion Facilitation
Porins facilitate the process of facilitated diffusion, a form of passive transport that does not require metabolic energy (ATP). The role they play can be analyzed through the following logical sequence:
- Size-Selective Permeability: The internal diameter of the porin channel determines the size of the molecules that can traverse the membrane. They allow for the passive passage of relatively large molecules, such as small proteins, disaccharides, and various ions, which would otherwise be restricted by the hydrophobic core of the phospholipid bilayer.
- Aqueous Pathway Creation: By providing a hydrophilic environment within the hydrophobic membrane interior, porins create a "water bridge." This allows polar and charged solutes to bypass the insulating properties of the lipid tails. [Based on Fick's Law of Diffusion, where the channel increases the permeability coefficient ($P$) of the membrane for specific solutes].
- Selectivity Filters: While some porins are non-specific, many exhibit selectivity based on the charge and size of the solute, acting as a sieve that regulates the influx and efflux of metabolites between organelles and the cytosol.
Diagrammatic Representation of a Porin Channel
Comparative Analysis Table
| Attribute | Description |
|---|---|
| Energy Requirement | None (Passive Transport) |
| Transport Direction | Along the concentration gradient |
| Specificity | Depends on pore size and amino acid side chains |
| Biological Significance | Crucial for plastid and mitochondrial homeostasis |
Final Solution: Porins are integral membrane proteins forming large, barrel-shaped aqueous channels that facilitate the passive diffusion of large molecules, ions, and proteins across the outer membranes of mitochondria, chloroplasts, and bacteria without the expenditure of cellular energy.
More Questions from Class 11 Biology Transport in Plants EXERCISES
- Q1: What are the factors affecting the rate of 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?
- 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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