Membrane permeability
Biology· 15 min read
1. Basics of Membrane Permeability★★☆☆☆⏱ 5 min
Membrane permeability
A semi-quantitative measure of the ability of molecules to cross the phospholipid bilayer of a cell membrane. More permeable membranes allow more molecules to cross faster; less permeable membranes restrict movement.
Example:
A healthy plant cell membrane is selectively permeable, allowing small non-polar molecules to cross freely but blocking large polar molecules.
Cell membranes are selectively permeable due to the hydrophobic core of the phospholipid bilayer. The hydrophobic fatty acid tails repel charged and polar molecules, while allowing non-polar molecules to diffuse through. Protein channels and carriers increase permeability to specific molecules that cannot cross the bilayer directly.
Test your baseline understanding:
Which molecule crosses an intact healthy cell membrane most easily by simple diffusion?
Large polar glucose
Non-polar carbon dioxide
Charged sodium ion
Polar amino acid
Reveal answer
Non-polar carbon dioxide —Correct! The hydrophobic core of the bilayer allows small, non-polar, uncharged molecules to pass easily. All other options are repelled by the hydrophobic core.
Classify the permeability of an intact healthy cell membrane to oxygen gas (O₂) and glucose.
- 1
- Oxygen is a small, non-polar, uncharged molecule:
- 2
- It can pass freely through the hydrophobic core of the phospholipid bilayer, so the membrane is highly permeable to oxygen.
- 3
- Glucose is a large, polar molecule:
- 4
- It cannot pass through the hydrophobic core, so the membrane has low permeability to glucose unless specific carrier proteins are present.
2. Effect of Temperature on Permeability★★★☆☆⏱ 6 min
Temperature affects membrane permeability by changing the fluidity of the phospholipid bilayer and the structure of embedded membrane proteins. At low temperatures, phospholipids have low kinetic energy, are packed tightly together, so the membrane is less fluid and less permeable to most molecules.
As temperature increases to the physiological optimum, phospholipids gain kinetic energy, move more freely, and the membrane becomes more fluid, slightly increasing permeability. Above ~50°C, increased kinetic energy breaks the hydrogen and ionic bonds that hold membrane proteins in their 3D shape, causing denaturation.
Beetroot cells store red betacyanin pigment in vacuoles. A student heats beetroot cubes to different temperatures and measures absorbance of the surrounding water. Explain why absorbance increases sharply above 50°C.
- 1
- At temperatures above 50°C, high kinetic energy disrupts chemical bonds in membrane proteins:
- 2
- This causes permanent denaturation of membrane proteins, which breaks gaps in the membrane structure:
- 3
- The phospholipid bilayer also becomes disorganized as proteins that held it in place are denatured:
- 4
- Betacyanin pigment leaks out of the vacuole and cell into the surrounding water:
- 5
- Higher pigment concentration increases absorbance, so higher temperature gives higher absorbance and higher permeability.
3. Effect of Organic Solvents and pH★★★☆☆⏱ 4 min
Organic solvents like ethanol are non-polar, so they dissolve the lipid components of the phospholipid bilayer. High concentrations of organic solvents completely disrupt the bilayer structure, leading to massive increases in permeability.
Extreme pH changes alter the charge on R-groups of amino acids in membrane proteins. This changes the interactions that hold the protein's tertiary structure together, causing denaturation, which disrupts the membrane and increases permeability, similar to high temperature.
Predict and explain the change in membrane permeability when plant cells are placed in 70% ethanol.
- 1
- 70% ethanol is a high concentration of non-polar organic solvent:
- 2
- Ethanol dissolves the hydrophobic fatty acid tails of the phospholipid bilayer:
- 3
- The bilayer structure breaks down completely, creating large gaps in the membrane:
- 4
- Membrane permeability increases dramatically, allowing all dissolved molecules to leak out of the cell.
4. Common Pitfalls
Wrong move:
Claiming that increasing temperature always increases membrane permeability
Why:
At low temperatures below 10°C, permeability is low because phospholipids are tightly packed, so the trend is not linear from 0°C upwards
Correct move:
Describe the trend in stages: low permeability at low temperature, slow increase to optimum, sharp increase after denaturation at high temperature
Wrong move:
Attributing organic solvent effect only to protein denaturation
Why:
The primary mechanism of organic solvent disruption is dissolving the phospholipid bilayer, not denaturing proteins (though denaturation can also occur)
Correct move:
Always mention that organic solvents dissolve the lipid bilayer as the main mechanism of increased permeability
Wrong move:
Saying all permeability increases are permanent
Why:
Small temperature increases just above optimum (before denaturation) cause reversible increases in fluidity and permeability
Correct move:
Distinguish between reversible changes from increased fluidity and permanent changes from denaturation or solvent disruption
Wrong move:
Claiming intact membranes are completely impermeable to all molecules
Why:
Many students forget that intact membranes are selectively permeable, not impermeable
Correct move:
State that intact healthy membranes allow small non-polar molecules through, and only block large, polar or charged molecules
5. Quick Reference Cheatsheet
Factor | Mechanism | Effect on Permeability |
|---|---|---|
Low temperature (<10°C) | Phospholipids tightly packed, low kinetic energy | Decreased |
Moderate temperature (10-40°C) | Phospholipids more fluid | Slightly increased |
High temperature (>50°C) | Membrane protein denaturation, bilayer damage | Greatly increased |
High organic solvent | Dissolves phospholipid bilayer | Greatly increased |
Extreme pH | Membrane protein denaturation | Greatly increased |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 2
Explain temperature effect on permeability
- 2023 · 1
Multiple choice on ethanol effect
- 2021 · 3
Beetroot permeability practical
Going deeper
- Practical guideCIE 9700 Practical Skills HandbookSection on membrane permeability experiments
What's Next
Understanding membrane permeability is foundational for all topics related to cell transport across membranes, and it is regularly assessed in both theory and practical papers for CIE A-Level Biology. This topic frequently appears in multiple choice, structured questions, and practical analysis tasks, so mastering the mechanisms of permeability change is critical for high marks. Building on what you have learned here, you will next explore how different molecules move across membranes, and how selective permeability enables essential cellular processes like osmoregulation, nutrient uptake and cell signalling. These concepts also underpin applied topics like food preservation and drug delivery in biological contexts.