International A Level Biology / Membranes and transport
Core practical 3: beetroot and membrane permeability
Edexcel International A Level Biology (YBI11), chapter 4: Membranes and transport, specification point 2.3. Free lesson.
Read the revision notes · International A Level Biology past papers
Beetroot pigment leaks out when the membranes around the vacuole and cell become more permeable. Measuring the colour of the surrounding water shows how temperature or ethanol affects membrane permeability.
Why leaked pigment shows membrane permeability
The red pigment is dissolved in the cell sap inside the vacuole. It must cross two membranes to leave the cell: the tonoplast around the vacuole, then the cell surface membrane around the cytoplasm. The cell wall is not a barrier because dissolved substances can pass through it. If the membranes are damaged, more pigment enters the surrounding water. A redder solution has a higher absorbance on a colorimeter. Higher absorbance means more pigment has been released, so the membranes have become more permeable.
How to test the effect of temperature
Change the temperature while keeping other conditions the same. Equal discs begin with similar surface areas and amounts of pigment, while washing removes pigment released by cells damaged during cutting.
How temperature changes the results
Absorbance stays low at cooler temperatures, then rises steeply at higher temperatures. Up to about 40 °C, little pigment is released because the membranes are mostly intact. At higher temperatures, the membranes are badly damaged, so they become more permeable and more pigment escapes. On a graph, label the vertical axis absorbance after 30 minutes, not just absorbance. Explain the result in terms of pigment released and membrane permeability, not colour alone.
How ethanol changes the results
To test ethanol, keep the temperature fixed and use different ethanol concentrations, for example from 0% to 50%. Use the same method otherwise. Absorbance rises as ethanol concentration rises because ethanol dissolves membrane lipids. The membranes become more permeable, so more pigment is released. Heat and ethanol damage membranes in different ways, but both lead to increased permeability. Heat gives phospholipids more kinetic energy, so they move more and the bilayer becomes more fluid. Ethanol dissolves lipids and disrupts the bilayer. High temperatures can denature membrane…
Explain the cause, not just the colour
Pigment leaks out because the membranes let it through. Enzymes are not the cause. At high temperatures, membrane proteins denature and phospholipids move more, making the membranes more permeable and allowing more pigment to escape. For ethanol, link the rising absorbance to lipids dissolving, increased membrane permeability and more pigment released.