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Q6:
How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
Solution :
1. Mechanism for Neutral Solutes
Neutral solutes (non-polar molecules) move across the plasma membrane primarily through the process of Simple Diffusion. Because the plasma membrane is composed of a lipid bilayer, it possesses a hydrophobic core [Per the Fluid Mosaic Model, proposed by Singer and Nicolson, 1972].
- Non-polar molecules (e.g., $O_2$, $CO_2$) are lipophilic and can readily dissolve in the hydrophobic lipid bilayer.
- These molecules move down their concentration gradient (from a region of higher concentration to a region of lower concentration).
- This is a passive process, meaning it does not require the expenditure of metabolic energy in the form of $ATP$ [Per the Second Law of Thermodynamics regarding entropy-driven spontaneous processes].
2. Transport of Polar Molecules
Polar molecules (e.g., $H_2O$, amino acids, glucose, ions like $Na^+$, $K^+$) cannot move across the membrane via simple diffusion. The hydrophobic interior of the lipid bilayer acts as an effective barrier to charged or highly polar substances because they cannot dissolve in the non-polar tails of the phospholipids [Based on the principle of "like dissolves like"].
3. Transport Mechanisms for Polar Molecules
To overcome the hydrophobic barrier, polar molecules utilize specialized membrane proteins to facilitate their movement:
| Mechanism | Description |
|---|---|
| Facilitated Diffusion | Uses carrier proteins or channel proteins to move molecules down the concentration gradient without energy consumption. |
| Active Transport | Uses specific membrane pumps (e.g., $Na^+/K^+$-ATPase pump) to move molecules against their concentration gradient, which requires the hydrolysis of $ATP$ [Per the chemiosmotic theory]. |
Visual Representation of Membrane Transport
Summary of Transport Logic
- Neutral/Non-polar substances: Utilize Simple Diffusion to traverse the lipid bilayer directly.
- Polar/Charged substances: Are restricted by the hydrophobic core.
- Resolution: Polar molecules require Facilitated Diffusion (passive) or Active Transport (requiring $ATP$) mediated by transmembrane proteins (channels, carriers, or pumps).
Final Solution: Neutral solutes move via simple diffusion across the lipid bilayer. Polar molecules cannot cross this way due to the hydrophobic nature of the lipid tails; they are instead transported via facilitated diffusion or active transport through specialized membrane-bound protein channels and carrier molecules.
More Questions from Class 11 Biology Cell – The Unit of Life EXERCISES
- Q1: Which of the following is not correct?
- Q2: New cells generate from
- Q3: Match the following Column I Column II (a) Cristae (i) Flat membranous sacs in stroma (b) Cisternae (ii) Infoldings in mitochondria (c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus
- Q4: Which of the following is correct:
- Q5: What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
- Q7: Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
- Q8: What are the characteristics of prokaryotic cells?
- Q9: Multicellular organisms have division of labour. Explain.
- Q10: Cell is the basic unit of life. Discuss in brief.
- Q11: What are nuclear pores? State their function.
- Q12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
- Q13(i): Describe the structure of the following with the help of labelled diagrams. (i) Nucleus
- Q13(ii): Describe the structure of the following with the help of labelled diagrams. (ii) Centrosome
- Q14: What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.
CBSE Solutions for Class 11 Biology Cell – The Unit of Life
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
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- 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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