Cells rely on the plasma membrane to maintain a stable internal environment while still exchanging matter and information. This depends on a simple physical principle: a water-repelling membrane core makes some substances cross easily and others not.

Plasma membranes as barriers

The plasma membrane forms the boundary between the cytosol and the extracellular environment.

This labeled plasma membrane diagram highlights the phospholipid bilayer plus embedded proteins, cholesterol, and carbohydrate chains. It helps reinforce that selective permeability is rooted in the lipid core, while membrane proteins provide specialized pathways for substances that cannot pass through the hydrophobic interior. Source

Its most important functional property is selective permeability, meaning the membrane is not equally permeable to all substances. This allows cells to:

• Keep essential solutes at useful concentrations

• Exclude harmful or disruptive substances

• Establish internal conditions (e.g., pH and ion composition) compatible with enzyme function

• Regulate what enters and leaves, rather than equilibrating with the surroundings

Selective permeability is fundamentally a consequence of the membrane’s chemical architecture, especially its hydrophobic interior.

The hydrophobic interior: why it matters

Amphipathic phospholipids create a nonpolar core

Membranes are built primarily from phospholipids, which are amphipathic (having both water-attracting and water-repelling regions):

• Hydrophilic heads interact with water on either side of the membrane

• Hydrophobic fatty acid tails cluster together away from water, forming a nonpolar interior

This diagram shows a phospholipid bilayer with hydrophilic head groups facing the aqueous environments and hydrophobic fatty acid tails oriented inward. The tail-to-tail arrangement creates the membrane’s nonpolar core, which underlies selective permeability by excluding most polar and charged solutes. Source

This tail-to-tail region behaves like a thin layer of oil. Because many biologically important molecules are polar or charged (and therefore interact strongly with water), moving them into the nonpolar core is energetically unfavourable. As a result, the hydrophobic interior acts as the main barrier to transport.

Permeability depends on polarity, charge, and size

Crossing the membrane requires a molecule to enter (at least briefly) the hydrophobic core. The more strongly a substance is hydrated in water (typical of polar and charged solutes), the less likely it is to partition into the membrane interior.

Key determinants of permeability:

This figure summarizes how chemical properties predict membrane permeability: overall nonpolar molecules cross most readily, while polar and especially charged species are effectively blocked by the hydrophobic core. The examples (e.g., $O_2$, $CO_2$, $H_2O$, glucose, and ions like $Na^+$) illustrate why polarity/charge is usually a stronger barrier than size alone. Source

• Nonpolarity: more nonpolar = more permeable

• Charge: any ion (positive or negative) is strongly excluded

• Size: smaller molecules generally cross more readily than larger ones, but polarity/charge is usually the dominant factor

What can and cannot cross the hydrophobic core

Substances that cross relatively easily

These molecules can dissolve in, and diffuse through, the hydrophobic interior:

• Small nonpolar molecules (e.g., many lipids and hydrophobic compounds)

• Some small uncharged polar molecules to a limited degree (their polarity still restricts passage)

Even when such molecules can cross, the membrane still functions as a selective barrier because different molecules cross at different rates.

Substances that are strongly restricted

The hydrophobic core largely blocks:

• Ions (e.g., $Na^+$, $Cl^-$) due to charge and hydration shells

• Large polar molecules (e.g., many sugars and amino acids) due to size and polarity

When cells need these substances to cross, they do not rely on the phospholipid core; instead, they use specific membrane components that provide alternative pathways while preserving the barrier function of the hydrophobic interior.

Biological significance of selective permeability

Maintaining distinct internal conditions

By separating internal and external environments, membranes support homeostasis at the cellular level. Selective permeability enables cells to:

• Accumulate nutrients without losing them rapidly

• Retain metabolites and signalling molecules

• Prevent uncontrolled movement of solutes that would disrupt water balance

Supporting gradients that drive cellular processes

Because the membrane restricts many solutes, cells can maintain concentration differences across the membrane. These differences are essential for numerous cellular functions, including controlled transport and signalling, and they depend on the membrane remaining a strong barrier to most polar and charged substances.