Q.No 33. Choose the correct order of molecules according to their ability to diffuse across a lipid bilayer.
CO₂ diffuses fastest across lipid bilayers due to its small size and nonpolar nature, followed by H₂O, glucose, and RNA as the slowest.
The correct option is (A) CO₂ > H₂O > glucose > RNA.
Factors Governing Diffusion
Diffusion across a lipid bilayer depends on molecular size, polarity, and lipid solubility. Small, nonpolar molecules like CO₂ partition easily into the hydrophobic core and diffuse rapidly. Polar molecules face barriers due to the bilayer’s hydrophobic interior, with rates decreasing as size and polarity increase.
Option Analysis
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(A) CO₂ > H₂O > glucose > RNA: Correct. CO₂ (MW 44, nonpolar) has permeability ~0.35-1 cm/s. H₂O (MW 18, polar but small) permeates at ~10⁻³ cm/s via simple diffusion. Glucose (MW 180, polar with OH groups) diffuses very slowly (~10⁻¹⁰ cm/s), requiring facilitated transport. RNA (large, highly polar/charged) does not diffuse passively.
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(B) RNA > glucose > CO₂ > H₂O: Incorrect. RNA’s massive size (>10,000 Da) and polarity prevent diffusion; CO₂ and H₂O are far faster.
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(C) H₂O > CO₂ > RNA > glucose: Incorrect. CO₂ exceeds H₂O in rate due to higher lipid solubility; RNA/glucose are slowest.
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(D) H₂O > CO₂ > RNA > glucose: Incorrect. Matches (C); RNA cannot precede glucose.
Introduction to Diffusion Across Lipid Bilayers
Diffusion order molecules lipid bilayer CO2 H2O glucose RNA is a key concept in cell membrane transport for CSIR NET Life Sciences. Small nonpolar molecules like CO₂ diffuse fastest through the phospholipid bilayer’s hydrophobic core. This passive process follows Fick’s law, driven by concentration gradients without energy.
Key Factors Influencing Diffusion Rates
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Molecular Size: Smaller molecules (e.g., CO₂ MW 44) cross faster than large ones like RNA.
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Polarity and Solubility: Nonpolar CO₂ solubilizes easily; polar glucose and charged RNA struggle.
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Permeability Coefficients: CO₂ ~1 cm/s, H₂O ~0.001 cm/s, glucose <<10⁻⁹ cm/s.
Detailed Comparison Table
| Molecule | Size (MW) | Polarity | Permeability (cm/s) | Diffusion Ability |
|---|---|---|---|---|
| CO₂ | 44 | Nonpolar | ~0.35-1 | Highest |
| H₂O | 18 | Polar | ~10⁻³ | High |
| Glucose | 180 | Polar | ~10⁻¹⁰ | Low (needs facilitation) |
| RNA | >10k | Highly polar/charged | Negligible | None |
Applications in Cell Biology
In cells, CO₂ and O₂ diffuse freely for respiration; glucose uses transporters like GLUT. RNA transport involves nuclear pores, not bilayer diffusion. Understanding this aids CSIR NET prep on membrane permeability.
