Biology · Cells, Membranes & Organisation

Membrane structure, fluidity and selective permeability

Oxygen can cross a cell's lipid membrane by a route that a sodium ion does not readily use. Both are small, so a simple sieve with holes of one size is a poor model. Their chemical properties and the membrane's components matter.

The earlier biology chapters introduce living systems, evidence, water, ions and biological molecules. Here, polar means a molecule has unevenly distributed electrical charge, and an ion is an electrically charged atom or group of atoms. Concentration means amount of a substance per volume. No microscope or home experiment is required; the examples use explicitly constructed observations.

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By the end, you can

Use a membrane model to explain why different substances cross by different routes.

Relate molecular regions to the boundary

In the phospholipid bilayer model used here, each lipid has a polar head region and nonpolar hydrocarbon tails. Head regions face water on either side; tails occupy the membrane's interior. Two layers can therefore separate aqueous spaces without having a layer of water between their tails.

Hydrophilic describes favourable interactions with water; hydrophobic describes a region that mixes poorly with water. These words name chemical behaviour, not attraction and repulsion performed by conscious molecules. The lipid arrangement helps explain why the membrane's core presents a different environment from the surrounding solutions.

The diagram is a cross-section, not a photograph. A real membrane extends in two dimensions, bends, and contains many different lipids and proteins. Archaeal membranes can use a different lipid arrangement, so this particular bilayer should not be presented as the only possible membrane structure.

Different routes cross the same chemical boundary

A bilayer model, not a universal membrane formula. Proteins have different functions; not all form channels.

Read the diagram in words

A schematic bilayer has polar heads facing water on both sides and nonpolar tails between them. Oxygen follows a route through the lipid core; an ion is shown using a selective protein route. A separate receptor has a signalling connection without an open pore. The model is not to scale and does not portray all membrane chemistry.

Different routes cross the same chemical boundary
Structure	Location in the model
Outer polar head regions	outer aqueous interface
Inner polar head regions	cytosolic aqueous interface
Nonpolar tail regions	between the polar surfaces
Selective ion channel	spans both lipid layers
Surface receptor	spans the membrane with an outside binding site

Different routes cross the same chemical boundary
Route	Substance or signal	Associated structure	Relationship
Oxygen through the lipid core	Oxygen	Nonpolar tail regions	Material transport
Ion through a selective channel	Ion	Selective ion channel	Material transport
Outside binding connects to an internal response	Signal remains outside	Surface receptor	Signal reception

Diagram key

Outside: aqueous solution
Inside: aqueous cytosol
Polar head region
Nonpolar tails
Oxygen: lipid route
Ion: selective protein route
Channel
Receptor
Signal reception

A crossing route depends on chemistry

Small nonpolar molecules such as oxygen can pass through the lipid core relatively readily. Charged ions face a strong barrier there. Larger polar molecules such as sugars also often require membrane transport proteins for useful rates of crossing. Selective permeability means different substances cross at different rates and through different available routes, rather than a perfect division into permanently allowed and forbidden substances.

A channel provides a selective passage; a carrier binds a substance and changes conformation during transport. A receptor participates in signalling, and an attachment protein can connect the membrane to surrounding structures. These are distinct functions. A protein drawn across the membrane is not automatically an open channel.

Water can cross the lipid bilayer, and aquaporin channels greatly increase water permeability in many membranes. Its behaviour therefore defeats an overly simple rule that no polar molecule can cross lipids. Ask about the particular molecule, membrane and timescale before making an absolute claim.

Fluidity means movement within an organised boundary

Many membrane lipids and proteins can move laterally within the membrane. That movement helps explain the term fluid mosaic. It does not mean every component moves freely, that the membrane is a uniform liquid, or that molecules constantly switch between the two layers without constraint.

Temperature and lipid composition affect fluidity. Hydrocarbon tails with cis double bonds generally pack less closely than comparable saturated tails. Cholesterol influences animal-membrane packing and fluidity in ways that depend on temperature and composition. More cholesterol cannot be translated into one universal statement that fluidity always increases or always decreases.

Structure, fluidity and selectivity are related but distinct questions. A membrane may be fluid while remaining a barrier to an ion. A new channel may increase a substance's permeability without making the whole membrane dissolve. Identify the property actually tested rather than using the word flexible to explain every change.

Worked example

Change one route while preserving the membrane

Consider two hypothetical membrane systems with the same lipid composition. One has a selective sodium channel and the other lacks it. Both are placed under matching conditions, including an electrochemical gradient that favours sodium entry. Greater sodium entry in the channel-containing system would be consistent with a protein-mediated route, rather than sodium readily dissolving through the lipid core.

The comparison would need an intact barrier and controls for other differences. It does not establish that every protein speeds sodium transport, or that channels drive ions against their electrochemical gradient. A useful model narrows the proposed mechanism and the evidence needed to test it.

Check your understanding

Why is molecular size alone insufficient to predict membrane crossing?
Show reasoning
Charge, polarity, lipid solubility and available transport proteins also matter. A small charged ion encounters a different barrier from a small nonpolar molecule.
Does fluidity mean the membrane freely passes every dissolved substance?
Show reasoning
No. Movement of membrane components is different from movement across the membrane. A fluid lipid boundary can remain selectively permeable.
Is every membrane-spanning protein a channel?
Show reasoning
No. Proteins can act as carriers, receptors, enzymes or attachments as well as channels. Their function needs evidence.

What to carry forward

A membrane is a chemically selective, dynamic boundary whose crossing routes depend on both lipids and proteins.

References

Resources supporting this lesson’s explanations and examples.