What kind of substances cannot permeate membranes by passive diffusion :

RRB Staff Nurse Ahmedabad-2015

Applied Physiology

Medium

Appeared in: RRB Staff Nurse Ahmedabad-2015

Explanation

Hydrophilic ('water-loving') substances are polar or charged molecules that do not readily mix with lipids.
The core of the cell membrane is a lipid bilayer, which is hydrophobic ('water-fearing').
Due to this repulsion, hydrophilic substances cannot simply diffuse through the lipid core.
They require specialized transport mechanisms, such as protein channels or carrier proteins, to cross the membrane, which is a process different from simple passive diffusion.

Option A: Lipid-soluble substances are, by definition, able to dissolve in the lipid bilayer of the cell membrane, allowing them to pass through easily via passive diffusion.
Option B: Non-ionised (uncharged) substances are typically more lipid-soluble than their ionised (charged) counterparts. This allows them to diffuse across the hydrophobic membrane core more readily.
Option C: Hydrophobic ('water-fearing') is a term synonymous with lipid-soluble. These substances are chemically compatible with the lipid bilayer and can diffuse across it without assistance.

Visual 1: Diagram - A diagram of the cell membrane's phospholipid bilayer. It should show small, nonpolar molecules (like O₂, CO₂) passing directly through the bilayer, while showing ions and large polar molecules (like glucose) being blocked or requiring a transport protein.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Cell Membrane Permeability and Passive Transport as background academic context rather than a clinical decision trigger.
This concept is fundamental to pharmacology. The ability of a drug to be absorbed and distributed throughout the body depends on its ability to cross cell membranes.
For a drug to cross the blood-brain barrier (a highly selective membrane), it must generally be highly lipid-soluble. Hydrophilic drugs cannot easily enter the brain.
The pH of different body compartments can affect whether a drug is in its ionised or non-ionised form, which in turn affects its absorption and excretion.

How to Approach the Question

First, identify the key terms in the question: 'cannot' and 'passive diffusion'. The word 'cannot' indicates you are looking for the exception.
Recall the basic structure of a cell membrane: it is a phospholipid bilayer with a hydrophobic (lipid-based) interior.
Apply the principle of 'like dissolves like'. Substances that are chemically similar to the membrane's core will pass through easily.
Evaluate each option: 'Lipid-Soluble', 'Non-ionised', and 'Hydrophobic' all describe substances that are compatible with the lipid core and can pass through.
Identify the outlier: 'Hydrophilic' describes water-loving substances, which are repelled by the lipid core and therefore cannot pass through by simple passive diffusion.

Concept Tested & Keywords

Concept Tested: Cell Membrane Permeability and Passive Transport
Stem keywords: permeate membranes, passive diffusion
Lead-in keywords: cannot
Negative lead-in flag: The question uses the negative keyword 'cannot', asking to identify the substance that is unable to cross the membrane via passive diffusion.

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