What kind of substances cannot penetrate membranes by passive diffusion?

RRB Staff Nurse Mumbai-2015

Applied Physiology

Easy

Appeared in: RRB Staff Nurse Mumbai-2015

Explanation

Hydrophilic substances.
The cell membrane consists of a lipid bilayer, creating a nonpolar, hydrophobic core.
Hydrophilic ('water-loving') molecules are polar and are repelled by this hydrophobic core, preventing them from passing through by simple passive diffusion.
These substances, like glucose and ions, require specific protein channels or carriers to cross the membrane.

Option A: Lipid-soluble substances are chemically similar to the membrane's lipid core, allowing them to dissolve in it and diffuse across easily.
Option B: Non-ionised (uncharged) substances are not repelled by the lipid bilayer as strongly as charged ions, allowing them to pass through more readily.
Option C: Hydrophobic ('water-fearing') substances are nonpolar and readily mix with the hydrophobic interior of the cell membrane, facilitating their diffusion across.

Visual 1: Diagram - A diagram of the cell membrane's phospholipid bilayer. It should illustrate the hydrophilic heads facing outwards and the hydrophobic tails facing inwards, with arrows showing that hydrophobic molecules can pass through while hydrophilic molecules are blocked.

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 diffusion as background academic context rather than a clinical decision trigger.
This principle is fundamental to pharmacology. For a drug to be absorbed and distributed throughout the body, it often needs to cross multiple cell membranes. Therefore, many drugs are designed to be lipid-soluble and non-ionised at physiological pH to enhance absorption.
The Blood-Brain Barrier (BBB) is a highly selective membrane that protects the brain. Only very lipid-soluble substances can cross it via passive diffusion, which is why many drugs cannot affect the central nervous system.
What if? If a patient takes a drug that is highly ionized in the stomach's acidic environment, its absorption in the stomach will be poor. However, when it reaches the more alkaline environment of the small intestine, it may become less ionized and more lipid-soluble, allowing for better absorption there.

How to Approach the Question

First, identify the keyword 'cannot'. This indicates you are looking for the exception—the substance that is blocked.
Recall the basic structure of a cell membrane: it's a phospholipid bilayer with a fatty, hydrophobic interior.
Apply the principle of 'like dissolves like'. Substances that are similar to the fatty interior (lipid-soluble, hydrophobic) will pass through easily.
Evaluate each option: 'Lipid-Soluble', 'Non-ionised', and 'Hydrophobic' all describe properties that facilitate passage across the lipid membrane.
Conclude that 'Hydrophilic' substances, being 'water-loving', are dissimilar to the 'fat-loving' membrane core and therefore cannot pass through by simple diffusion.

Concept Tested & Keywords

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