Membrane Permeability

Learning Goals

After this lesson, you should be able to:

• explain selective permeability from the structure of the phospholipid bilayer;
• rank substances by expected direct permeability using size, polarity, charge, and lipid solubility;
• distinguish permeability, flux, net movement, and equilibrium;
• apply a simplified permeability relationship to qualitative and quantitative predictions;
• explain how channels change effective membrane permeability without removing selectivity;
• distinguish the roles of a cell wall and plasma membrane;
• analyze an original liposome data set; and
• design a controlled permeability investigation.

1. Selective Does Not Mean Completely Sealed

A selectively permeable membrane allows different substances to cross at different rates. A molecule may cross rapidly, slowly, only through a protein pathway, or not detectably under a particular set of conditions. Permeability is therefore a continuous, condition-dependent property rather than a simple “open” or “closed” label.

The same membrane can be highly permeable to one solute and effectively impermeable to another because those molecules interact differently with the aqueous solutions and hydrophobic bilayer core.

2. The Hydrophobic Core Is the Main Direct-Passage Barrier

To cross a protein-free bilayer, a molecule must leave favorable interactions with water, enter the nonpolar hydrocarbon core, move through it, and return to water on the opposite side. Small nonpolar molecules generally make this transition more readily than charged or strongly polar substances.

An ion is surrounded by oriented water molecules called a hydration shell. Moving the charged ion and disrupting those interactions to enter a nonpolar region has a high energetic cost. Small physical size therefore does not guarantee high permeability.

3. A Useful Permeability Ranking

This ordering is a starting model. Exact permeability also depends on membrane composition, temperature, molecule shape, and experimental conditions.

5. Size, Polarity, and Charge Must Be Considered Together

Molecular mass alone does not determine passage. A small ion can face a greater barrier than a somewhat larger nonpolar molecule because charge interacts strongly with water and poorly with the membrane core. Likewise, two molecules of similar size may differ greatly in permeability if one is nonpolar and the other is strongly polar.

A strong AP explanation states the relevant property and mechanism: “The ion crosses slowly because its charge and hydration shell are unfavorable in the hydrophobic core,” not simply “the ion is too large.”

6. Permeability Is Not the Same as Net Flux

Permeability describes how readily a particular substance crosses a particular membrane under specified conditions. Flux describes movement per unit area per unit time. In a simplified uncharged-solute model,

Here \(J\) is inward net flux, \(P\) is the permeability coefficient, and the concentration difference is the driving gradient. A membrane can have unchanged \(P\) while net flux decreases as the two concentrations approach one another.

7. Area and Thickness Affect Total Movement

If flux is uniform over membrane area \(A\), a simplified total transfer rate is

Increasing area increases total transfer capacity without necessarily changing \(P\). Increasing the diffusion path through an otherwise similar barrier generally reduces permeability. This is why membrane folds and thin exchange surfaces can change rates even when the chemical identity of the transported molecule is unchanged.

8. Dynamic Equilibrium Does Not Mean Motion Stops

For an uncharged molecule that can cross the membrane, diffusion in both directions continues at equilibrium. Net movement is zero because the opposing rates are equal, not because molecules become stationary.

If the membrane is impermeable to that molecule on the relevant timescale, concentration differences can persist instead of reaching diffusion equilibrium.

9. Ion Movement Depends on an Electrochemical Gradient

When an ion has an open pathway, both its concentration difference and the electrical potential difference across the membrane influence net movement. A concentration force and an electrical force may point in the same direction or oppose each other.

Do not predict ion direction from concentration alone unless the problem states that voltage is irrelevant. Permeability answers whether a route is available; the electrochemical gradient helps determine the favored net direction through that route.

10. Water Is Small but Polar

Water can cross a lipid bilayer to a limited extent despite its polarity, and many cell membranes contain aquaporins that greatly increase effective water permeability. Thus “polar” does not mean absolutely incapable of crossing, and the measured rate in a living membrane can differ sharply from a protein-free bilayer.

Aquaporins provide a selective hydrophilic path; they do not make the entire hydrophobic core freely permeable to ions and other polar solutes.

11. Ionization State Can Change Apparent Permeability

Some molecules can exist in charged and uncharged forms depending on pH. The uncharged form may partition into the bilayer more readily, while the charged form may remain mostly in water. Changing pH can therefore change observed passage even when molecular size is essentially unchanged.

This is not evidence that the membrane “detects pH.” The chemical equilibrium changes the fraction of molecules in each form, and those forms interact differently with the hydrophobic core.

12. Membrane Composition and Temperature Modify the Barrier

Tail length, saturation, cholesterol, temperature, and protein abundance influence lipid packing and membrane dynamics. Moderate changes can alter permeability differently for different solutes. Extreme heat or solvents may disrupt organization and cause broad leakage rather than regulated transport.

A change in fluidity and a change in permeability are related but not identical measurements. An experiment should define which property is measured instead of assuming one automatically proves the other.

13. Channels Add Selective Routes

A channel's hydrophilic interior allows certain ions or polar molecules to avoid direct contact with lipid tails. Selectivity can depend on pore diameter, charge distribution, dehydration, binding sites, and gating. Adding one channel may greatly increase permeability to its preferred solute without changing passage of unrelated molecules.

Channels permit passive movement down an electrochemical gradient; they do not by themselves guarantee movement in one permanent direction. Pumps and coupled transporters use different mechanisms addressed in later transport topics.

14. Cell Walls and Plasma Membranes Have Different Roles

Plants, fungi, bacteria, and archaea can have cell walls, although wall composition differs among these groups. A wall provides mechanical support, influences shape, and can help resist osmotic lysis. It may also restrict some large materials.

The cell wall does not replace the plasma membrane. Many walls are porous to water and small solutes, whereas the membrane's hydrophobic core and transport proteins provide the principal selectively regulated boundary. In a plant cell, water can cross the wall before encountering the plasma membrane.

15. Original Protein-Free Liposome Data

Researchers create equal-sized liposomes with identical lipid composition. Each begins with zero internal solute and the same external solute concentration. Initial influx is normalized so that Solute A in the protein-free membrane equals 100 arbitrary units.

The protein-free results support the predicted permeability hierarchy. The channel selectively changes D influx while leaving A–C nearly unchanged, so the data do not support a claim that the entire bilayer became generally leaky.

16. Quantitative Reasoning from the Model

For two uncharged solutes measured across equal areas with equal concentration differences,

Question: If \(P_X=5P_Y\), compare their initial net fluxes. Then predict what happens if both concentration differences are halved.

Answer: Initially \(J_X=5J_Y\). Halving both gradients halves both fluxes, but their 5:1 ratio remains unchanged. The gradient changed; the permeability coefficients did not.

17. Investigation: Measure Relative Permeability

Prepare replicate liposomes containing the same nonreactive indicator and expose each group to one candidate solute under identical conditions. Use a calibrated signal to estimate internal solute concentration over time.

1. Keep liposome lipid composition, size distribution, area, temperature, pH, external concentration, and sampling time constant.
2. Include a no-solute blank for background signal and a fully disrupted liposome reference for the maximum signal.
3. Measure several early time points so an initial slope can be estimated.
4. Repeat each condition and display variation rather than only one mean.
5. Test membrane integrity with a marker known not to cross intact bilayers.

The initial slope estimates relative influx under the tested gradient. It does not alone distinguish direct bilayer passage from an unidentified contaminating protein pathway.

18. Worked Prediction: A Channel Mutation

Question: A mutation replaces negatively charged residues lining a cation channel with uncharged residues. The channel still reaches the membrane. Predict one possible effect.

Reasoning: The pore's electrostatic interactions help determine which hydrated ions can enter or pass. Changing those residues may alter selectivity or conductance without changing the phospholipid bilayer itself.

Answer: Effective permeability to the channel's cation substrate may decrease or its ion selectivity may broaden. Measuring currents for several ions under controlled gradients would distinguish these possibilities.

19. AP-Style Claim–Evidence–Reasoning

Prompt: Solute D crosses protein-free liposomes very slowly but crosses rapidly after one purified protein is inserted. Evaluate the claim that D has become lipid-soluble.

Claim: The evidence does not support increased lipid solubility as the main explanation.

Evidence: D's passage changes only when the protein is present, while the lipid composition and external concentration are controlled.

Reasoning: The inserted protein supplies an alternative pathway through the hydrophobic barrier. A lipid-solubility change would require a change in D's chemical state or its interaction with lipids, neither of which is described.

20. Common Misconceptions and Fast Checks

• Incorrect: useful molecules cross and harmful molecules do not. Check: identify molecular properties and pathways.
• Incorrect: all small substances cross rapidly. Check: charge can dominate size.
• Incorrect: high permeability guarantees net movement. Check: determine the driving gradient.
• Incorrect: equilibrium means molecular motion stops. Check: compare opposing rates.
• Incorrect: inserting a channel makes the membrane permeable to everything. Check: evaluate channel selectivity.
• Incorrect: cell walls control all entry into plant cells. Check: distinguish wall support from membrane selectivity.
• Incorrect: more fluid always means more permeable to every solute. Check: measure both properties and specify the solute.

21. Key Takeaways

• The hydrophobic bilayer core creates selective permeability.
• Small nonpolar molecules cross directly more readily than large polar molecules and ions.
• Permeability is a membrane–solute property; net flux also depends on the driving gradient.
• Channels supply selective hydrophilic pathways and can change effective permeability.
• Cell walls provide structure and protection but do not replace selectively permeable plasma membranes.
• Experimental conclusions require controlled area, concentration, time, composition, and membrane integrity.