# Membrane permeability

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u4-membrane-permeability/

This sub-topic explains how and why cell membrane permeability changes in response to different environmental conditions, including temperature, pH and organic solvents. It also covers common experimental investigations and exam question expectations for this frequently tested topic.

**Prerequisites:** [Fluid Mosaic Model of Cell Membranes](https://www.owlsprep.com/study/cie-9700-u4-fluid-mosaic-model/)

## Learning objectives

- Describe what is meant by membrane permeability
- Explain how temperature, organic solvents and pH affect permeability
- Interpret experimental data from membrane permeability investigations
- Predict how permeability changes in different environmental conditions

## Basics of Membrane Permeability

**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.

**Check your understanding**

Test your baseline understanding:

1. 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

   *Why:* 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.

**Worked example:** Classify the permeability of an intact healthy cell membrane to oxygen gas (O₂) and glucose.

1. 1. Oxygen is a small, non-polar, uncharged molecule:
2. 2. It can pass freely through the hydrophobic core of the phospholipid bilayer, so the membrane is highly permeable to oxygen.
3. 3. Glucose is a large, polar molecule:
4. 4. It cannot pass through the hydrophobic core, so the membrane has low permeability to glucose unless specific carrier proteins are present.

## Effect of Temperature on Permeability

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.

**Worked example:** 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. 1. At temperatures above 50°C, high kinetic energy disrupts chemical bonds in membrane proteins:
2. 2. This causes permanent denaturation of membrane proteins, which breaks gaps in the membrane structure:
3. 3. The phospholipid bilayer also becomes disorganized as proteins that held it in place are denatured:
4. 4. Betacyanin pigment leaks out of the vacuole and cell into the surrounding water:
5. 5. Higher pigment concentration increases absorbance, so higher temperature gives higher absorbance and higher permeability.

> **tip**
>
> In CIE practical questions, you must link absorbance trends to membrane structure changes, not just state 'heat makes it permeable'. Always mention denaturation of proteins and bilayer damage.

## Effect of Organic Solvents and pH

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.

**Worked example:** Predict and explain the change in membrane permeability when plant cells are placed in 70% ethanol.

1. 1. 70% ethanol is a high concentration of non-polar organic solvent:
2. 2. Ethanol dissolves the hydrophobic fatty acid tails of the phospholipid bilayer:
3. 3. The bilayer structure breaks down completely, creating large gaps in the membrane:
4. 4. Membrane permeability increases dramatically, allowing all dissolved molecules to leak out of the cell.

**Exam command terms**

CIE uses consistent command terms for this topic. Key cues:

- **Describe** — State the trend or relationship, no explanation required *(Describe the effect of temperature on membrane permeability between 10°C and 80°C)*

- **Explain** — Link observations to membrane structure, use biological mechanisms *(Explain why permeability increases above 50°C)*

## Common pitfalls

- **Wrong:** Claiming that increasing temperature always increases membrane permeability
  - Why it fails: 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: Describe the trend in stages: low permeability at low temperature, slow increase to optimum, sharp increase after denaturation at high temperature
- **Wrong:** Attributing organic solvent effect only to protein denaturation
  - Why it fails: The primary mechanism of organic solvent disruption is dissolving the phospholipid bilayer, not denaturing proteins (though denaturation can also occur)
  - Correct: Always mention that organic solvents dissolve the lipid bilayer as the main mechanism of increased permeability
- **Wrong:** Saying all permeability increases are permanent
  - Why it fails: Small temperature increases just above optimum (before denaturation) cause reversible increases in fluidity and permeability
  - Correct: Distinguish between reversible changes from increased fluidity and permanent changes from denaturation or solvent disruption
- **Wrong:** Claiming intact membranes are completely impermeable to all molecules
  - Why it fails: Many students forget that intact membranes are selectively permeable, not impermeable
  - Correct: State that intact healthy membranes allow small non-polar molecules through, and only block large, polar or charged molecules

## 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 |

## 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.

- [Passive Transport (Diffusion and Osmosis)](https://www.owlsprep.com/study/cie-9700-u4-passive-transport/)
- [Active Transport and Bulk Transport](https://www.owlsprep.com/study/cie-9700-u4-active-transport/)
- [Bulk transport](https://www.owlsprep.com/study/cie-9700-u4-bulk-transport/)

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