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2.1.3Unit 2: Cells, Development, Biodiversity and Conservation

Core Practical 3: Investigating Membrane Structure and Permeability — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 10 of 26 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Principle and Method

Beetroot pigment
  • Beetroot pigment Beetroot cells contain a red pigment, betalain, stored in the vacuole, which leaks out if the membranes become more permeable.
Preparing samples
  • Preparing samples Cut equal-sized cylinders of beetroot with a cork borer, and rinse them to remove pigment released from damaged cells.
Treatments
  • Treatments Place identical pieces in equal volumes of water at different temperatures, or in different ethanol concentrations, for the same time.
Measuring
  • Measuring Remove the beetroot and measure the colour of the surrounding liquid with a colorimeter, using a suitable filter after zeroing with distilled water.

Part 2 of 4Expected Results and Explanation

1
At low temperatures the membrane is intact, so only a small amount of pigment leaks out and the absorbance is low.
2
As the temperature rises, phospholipids gain kinetic energy and move more, so gaps appear and permeability increases.
3
At higher temperatures, proteins in the membrane denature, so channel and carrier proteins lose their shape and create holes.
4
Ethanol dissolves lipids and disrupts the phospholipid bilayer, so the pigment leaks out in greater amounts as concentration rises.
5
Plotting absorbance against temperature or concentration shows a steady increase followed by a sharp rise once the membrane is damaged.

Part 3 of 4Control and Evaluation

Control and Evaluation
Control variables
  • Control variables Use the same size and number of beetroot pieces, the same volume of liquid and the same time for every treatment.
Repeats
  • Repeats Repeat each treatment and calculate a mean to reduce random error and spot anomalous results.
Safety
  • Safety Use a scalpel or cork borer carefully on a cutting tile, and keep ethanol away from flames.
Improvements
  • Improvements Use a thermostatically controlled water bath, a narrower range of temperatures,
    • and a calibration of the colorimeter to give more reliable results.

Part 4 of 4Exam Checklist & Tips

2.1.3 – Key Points for the Exam

  • Permeability How easily substances pass through a membrane.
  • Phospholipid bilayer The double layer of phospholipids that forms the basis of cell membranes.
  • Colorimeter An instrument measuring the absorbance of a coloured solution.
  • Denaturation A permanent change in the shape of a protein.

Common Mistakes

  • Not rinsing the beetroot, so surface pigment gives a falsely high reading in all samples.
  • Using pieces of different size, which changes the surface area from which pigment leaks.
  • Forgetting to zero the colorimeter with distilled water.
  • Saying that heat melts the membrane, instead of explaining the effects on phospholipids and proteins.

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