Required Practical 4: Membrane Permeability in Beetroot Discs
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of membrane structure and permeability. You can test your knowledge on these below.
What is the structure of a phospholipid bilayer and how does it contribute to membrane permeability?
The phospholipid bilayer consists of hydrophilic phosphate heads facing outward and hydrophobic fatty acid tails facing inward. It acts as a barrier to water-soluble substances but allows small, non-polar molecules to pass.
How does temperature affect membrane permeability?
Higher temperatures increase kinetic energy, making the phospholipids move more and the bilayer become more fluid- making it more permeable. Extreme temperatures can denature membrane proteins, causing leakage of cell contents.
What is the effect of alcohol on membrane permeability?
Alcohol disrupts the phospholipid bilayer by dissolving phospholipids, increasing membrane permeability and leading to the leakage of cellular contents such as pigments.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains required practical 4: membrane permeability in beetroot discs or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview of Practical
This practical investigates how different factors (temperature and alcohol concentration) affect the permeability of cell membranes, using beetroot discs.
Scientific Background
- Beetroot cells contain red betalain pigments in the vacuole, which do not normally pass through an intact membrane.
- Factors such as temperature and solvents like alcohol affect membrane integrity and can cause leakage of these pigments.
Example Methods
Method 1: Investigating the Effect of Temperature (Independent Variable)
Apparatus:
- Beetroot discs (cut to equal size).
- Distilled water.
- Water baths at different temperatures (e.g., 0°C, 20°C, 40°C, 60°C, 80°C).
- Colorimeter & cuvettes.
- Pipettes and test tubes.
Procedures:
- Cut beetroot discs to equal length and diameter so they are the same size (e.g., 1 cm³) and rinse under water to remove excess pigment released from cutting.
- Place each disc into a test tube containing 5 cm³ distilled water.
- Place test tubes into water baths at different temperatures for 10 minutes.
- Remove beetroot discs and transfer the liquid into cuvettes.
- Measure the absorbance of pigment release using a colorimeter at 520 nm (red light filter).
- Record results and plot a graph of absorbance vs. temperature.
Expected Results:
- Low temperatures: Low pigment release due to intact membrane. With less pigment released, the absorbance of the solution will be low (as more light will be transmitted through).
- High temperatures: More pigment release as phospholipids become more fluid and proteins denature. With more pigment released, the absorbance of the solution will be high (as less light will be transmitted through).
Method 2: Investigating the Effect of Alcohol Concentration (Independent Variable)
Apparatus:
- Beetroot discs.
- Distilled water.
- Ethanol solutions of different concentrations (e.g., 0%, 20%, 40%, 60%, 80%).
- Colorimeter & cuvettes.
- Pipettes and test tubes.
Procedures:
- Prepare a dilution series of ethanol solutions at different concentrations.
- Cut beetroot discs to equal length and diameter so they are the same size (e.g., 1 cm³) and rinse under water to remove excess pigment released from cutting.
- Place beetroot discs into separate test tubes, each containing 5 cm³ of ethanol solution.
- Leave for 10 minutes at room temperature.
- Remove beetroot discs and transfer the liquid into cuvettes.
- Measure the absorbance using a colorimeter.
- Record results and plot a graph of absorbance vs. ethanol concentration.
Expected Results:
-
Higher ethanol concentrations dissolve phospholipids, increasing permeability and pigment leakage. With more pigment released, the absorbance of the solution will be higher (as less light will be transmitted through).
Key Terms
- Membrane permeability: How easily substances pass through a membrane.
- Denaturation: Loss of protein function as tertiary structure changes due to high temperature or pH changes.
- Betalain: A pigment stored in the vacuole of beetroot cells.
- Colorimeter: A device measuring the absorbance of a solution to determine pigment concentration.
No answer provided.
Exam Tips
Use control variables (e.g., same beetroot type/ age, same size/ mass discs, same exposure time, same volumes of solution) to get valid results.
Repeat 3 times- this is so any anomalies can be identified and discarded. This means a reliable mean can be calculated.
Repeating experiments allows you to check for repeatability (the results should be very similar if not the same for each temperature/ concentration when the same method and equipment is used.
No answer provided.
In an investigation into membrane permeability, beetroot discs were placed into the following extracts for 1 hour. Beetroot cells contain a red coloured pigment.
E – Ethanol, water and acid
F – Ethanol and water
G – Water
The discs were then removed and the solutions were put into a colorimeter to measure the light absorbance. The results are shown in the graph. Use your knowledge of membrane structure to explain the results in the graph. (4 marks)
- Higher absorbance indicates more membrane damage / increased permeability.
- More membrane damage / increased permeability results in more pigment release.
- E and F greater than water because phospholipids dissolve in ethanol.
- E greater than F because acid denatures membrane proteins. (Accept description of denaturation in terms of change in tertiary structure or breaking of hydrogen/ionic bonds).
- Some pigment found in water / solution G due to pigment released when cutting beetroot discs.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!