Q.38 Arrange the following in the decreasing order of their permeability coefficients across a
lipid bilayer membrane.
(i) Urea
(ii) Glucose
(iii) H2O
(iv) Na+
(v) Tryptophan
(A) (i), (iii), (v), (ii), (iv) (B) (iii), (v), (ii), (iv), (i)
(C) (iii), (i), (v), (ii), (iv) (D) (i), (iii), (iv), (v), (ii)

Lipid Bilayer Permeability: Decreasing Order of Coefficients for H2O, Urea, Glucose, Na+, Tryptophan

Lipid bilayers allow passive diffusion based on molecule size, charge, and hydrophobicity. Small uncharged molecules cross fastest, while ions and large polars are slowest. The correct arrangement in decreasing permeability is H2O > urea > tryptophan > glucose > Na+.

Correct Answer

The right choice is (C) (iii), (i), (v), (ii), (iv): H₂O, urea, tryptophan, glucose, Na⁺. Water permeates quickest due to its tiny size. Ions like Na⁺ have near-zero permeability without channels.

Permeability Principles

Lipid bilayers are hydrophobic, favoring nonpolar solutes. Permeability coefficient (P) follows: P ∝ lipophilicity / (size × hydration). Uncharged small molecules like H₂O slip through easily. Charged Na⁺ barely crosses.

Substance Breakdown

• H₂O (iii): Highest P (~10⁻³ cm/s). Tiny and uncharged; oscillates through bilayer via transient defects.

• Urea (i): Next (~10⁻⁶ cm/s). Small, neutral, somewhat polar but diffuses faster than sugars.

• Tryptophan (v): Moderate (~10⁻⁷ cm/s). Large but highly hydrophobic indole ring aids solubility in lipids.

• Glucose (ii): Low (~10⁻¹⁰ cm/s). Polar hydroxyls and size hinder crossing; needs transporters.

• Na⁺ (iv): Lowest (<10⁻¹² cm/s). Hydrated ion; charge blocks hydrophobic core.

Option Analysis

Exam Tips

For GATE/NEET biology, recall: O₂ > H₂O > urea > glycerol > glucose >> ions. Tryptophan edges glucose via aromatic hydrophobicity. Practice with logP values: tryptophan (~0.8) > glucose (~-3).