Investigating Cell Membrane Permeability (AQA A-Level Biology): Revision Notes
Investigating Cell Membrane Permeability
Purpose and principle
This practical investigates how external variables affect the permeability of cell membranes. Cell membranes consist of a phospholipid bilayer that exhibits selective permeability, allowing some substances to pass through whilst restricting others. Variables such as temperature and solvent concentration can alter this permeability.
The investigation uses beetroot cells, which contain a purple pigment called betalain. When membrane permeability increases, more betalain leaks from the cells into the surrounding solution. The concentration of leaked pigment can be measured using a colorimeter, providing a quantitative measure of membrane permeability.
Apparatus and materials
- Beetroot
- Scalpel and cutting board
- Forceps and ruler
- Tongs
- Distilled water
- Boiling tubes and rack
- Colorimeter and cuvettes
- Philtre paper
- Timer
- Water bath and thermometer
- Ethanol
Method
Sample preparation
- Cut beetroot into 6-10 identical cubes using a scalpel to ensure consistent surface area.
- Rinse the cubes thoroughly to remove any pigment that may have been released during cutting.
Temperature investigation
- Place each beetroot cube in an equal volume of distilled water (5-15ml) in separate boiling tubes.
- Place test tubes in a water bath maintained at different temperatures ranging from 30-80°C.
Solvent concentration investigation
- For ethanol investigation, create a dilution series using distilled water with ethanol concentrations ranging from 0-100%.
- Leave all samples for 20 minutes to allow sufficient time for pigment leakage.
Critical Measurement Steps:
Proper measurement technique is essential for reliable results. The colorimeter must be properly calibrated and samples must be filtered to remove debris.
Measurement
- Set the colorimeter to use a blue philtre and zero the instrument using a cuvette containing distilled water.
- Philtre each sample through philtre paper into clean cuvettes to remove any beetroot fragments.
- Measure the absorbance of each solution. Higher absorbance readings indicate greater pigment concentration and therefore increased membrane permeability.
Risk assessment
Safety First: Always follow proper safety procedures when handling sharp instruments, glassware, chemicals, and hot liquids. Wear appropriate personal protective equipment throughout the investigation.
| Hazard | Risk | Safety precaution | Emergency action |
|---|---|---|---|
| Scalpel | Cuts from sharp blade | Cut away from fingers, use forceps to hold samples, keep blade away from desk edge | Elevate cuts, apply pressure, seek medical assistance |
| Broken glass | Cuts from sharp fragments | Handle glassware carefully, keep away from desk edge | Elevate cuts, apply pressure, do not remove glass from wounds |
| Ethanol | Eye/skin irritation, flammable | Wear eye protection, keep away from naked flames | Wash eyes and skin with cold water |
| Hot liquids | Scalding | Handle carefully, use tongs to remove tubes from water bath, wear eye protection | Run burns under cold water, seek medical assistance |
Data collection and processing
Plot a graph showing absorbance against the independent variable (temperature or ethanol concentration). The absorbance value directly correlates with pigment concentration, which indicates membrane permeability.
Analysis and interpretation
Temperature effects
As temperature increases, membrane permeability also increases. This occurs because:
Temperature Effects on Membrane Structure:
Understanding how heat affects membrane components is crucial for interpreting your results. Both proteins and phospholipids respond differently to temperature changes.
- Heat energy causes membrane proteins to denature, disrupting their normal structure and function
- Higher temperatures damage the bonds maintaining protein tertiary structure
- This creates gaps in the membrane, making it easier for molecules to pass through
- At low temperatures, phospholipids have limited kinetic energy and pack tightly together, creating a more rigid membrane structure that restricts molecular movement
At extremely low temperatures, ice crystals can form, physically piercing the cell membrane and dramatically increasing permeability.
Ethanol concentration effects
Ethanol acts as a solvent that disrupts membrane structure:
Solvent Effects on Membrane Integrity:
Ethanol's ability to dissolve lipid components makes it particularly effective at disrupting membrane structure. This creates a dose-dependent relationship between concentration and permeability.
- Ethanol molecules cause the membrane to rupture by dissolving phospholipid components
- Higher ethanol concentrations produce greater disruption and larger gaps in the membrane
- As ethanol concentration increases, membrane permeability increases proportionally
- This allows more betalain pigment to leak from the cells
Links to theory
This practical demonstrates key concepts in cell membrane structure and function, specifically how the fluid mosaic model explains membrane permeability. It connects to broader topics including enzyme activity (protein denaturation), transport mechanisms, and cellular responses to environmental stress.
Key Points to Remember:
- Higher absorbance readings indicate greater membrane permeability
- Temperature increases membrane permeability by denaturing proteins and increasing phospholipid movement
- Ethanol dissolves membrane components, creating gaps that allow pigment leakage
- Beetroot's betalain pigment provides a visible and measurable indicator of membrane damage
- Proper controls (distilled water, consistent timing) are essential for reliable results