Edexcel A Level (IAL) Biology -2.3 Core Practical 3: Investigating Membrane Structure & Permeability- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -2.3 Core Practical 3: Investigating Membrane Structure & Permeability- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -2.3 Core Practical 3: Investigating Membrane Structure & Permeability- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 2.3 nvestigate membrane properties including the effect of alcohol and temperature on membrane permeability.
CORE PRACTICAL 3 – Investigate Membrane Properties
🌱 Introduction
Cell membranes are partially permeable, controlling the passage of molecules. Structure: phospholipid bilayer with proteins → gives fluidity & selective permeability. Membrane integrity can be disrupted by temperature or alcohol, causing leakage of cell contents. Beetroot cells are used because red pigment (betalain) is trapped inside intact membranes. Leakage of pigment is a visible sign of membrane damage.
🎯 Aim
To study how temperature and alcohol concentration affect the permeability of beetroot cell membranes.
🔬 Materials
- Fresh beetroot
- Test tubes / beakers
- Water baths (0°C, 25°C, 50°C, 70°C)
- Alcohol solutions (10%, 30%, 50%, 70%)
- Pipettes / droppers
- Knife / scalpel
- Measuring cylinder
- Colorimeter (optional)
- Stopwatch / timer
📝 Principle
- Intact membranes retain pigments inside cells.
- Stress (heat/alcohol) disrupts membrane → pigment leaks.
- Amount of pigment in solution = degree of membrane damage.
- High temperature → denatures membrane proteins.
- Alcohol → dissolves lipids, increases permeability.
⚡ Method
Step 1: Sample Preparation
- Cut beetroot into equal-sized pieces → uniform surface area.
- Rinse to remove surface pigments → avoids false readings.
Step 2: Setting Up Tests
- Temperature Test: Place pieces in water baths (0°C, 25°C, 50°C, 70°C).
- Alcohol Test: Place pieces in test tubes with alcohol solutions (10%, 30%, 50%, 70%).
Step 3: Incubation
- Leave beetroot in solutions for 10–15 min.
- Keep other conditions constant (same tube, same volume).
Step 4: Observation
- Remove beetroot pieces.
- Observe color change.
- Optional: Measure absorbance with colorimeter → higher absorbance = more pigment.
📊 Observations & Expected Results
| Factor | Condition | Observation (Color Leakage) | Explanation |
|---|---|---|---|
| Temperature | 0°C | Very faint / no color | Low temp → membranes stable |
| Temperature | 25°C | Slight pink | Mild temp → some permeability increase |
| Temperature | 50°C | Moderate pink | Proteins start denaturing |
| Temperature | 70°C | Dark red | Membrane severely damaged, max leakage |
| Alcohol | 10% | Faint pink | Low alcohol → minor disruption |
| Alcohol | 30% | Light pink | Moderate effect on lipid bilayer |
| Alcohol | 50% | Medium pink | High alcohol → more leakage |
| Alcohol | 70% | Dark red | Membrane lipids dissolve, max leakage |
✅ Conclusion
- Temperature and alcohol concentration ↑ → membrane permeability ↑.
- Extreme conditions → cell membrane breakdown → pigment leaks out.
- Beetroot pigment = visual marker of membrane integrity.
⚠️ Precautions
- Use equal-sized beetroot pieces → fair comparison.
- Rinse beetroot thoroughly → avoid false readings.
- Maintain same volume of solution in all tubes.
- Do not overheat → avoid burning beetroot.
- Use fresh alcohol solutions → concentration matters.
🧠 Quick Recap
Membrane intact → pigment stays inside → solution colorless
Heat effect → proteins denature → leakage ↑
Alcohol effect → lipids dissolve → leakage ↑
Mnemonic: “Hot & Drunk membranes leak red”
Observation tip: More red = higher permeability