Required Practical 4: Membrane Permeability
How to investigate the effect of temperature on membrane permeability using beetroot discs and a colorimeter, and how to explain the results. Includes the 4-mark exam question, a worked model answer and the examiner trap on explaining why pigment leaks out.
Chapters (10)
Exam question [4 marks]
Describe how you would investigate the effect of temperature on the permeability of the cell-surface membranes of beetroot, and explain the results you would expect.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Cut discs of the same size / surface area (e.g. cork borer, ruler) and rinse to remove pigment already released | "same diameter and thickness", "same mass", "wash until water clear" | "cut beetroot" with no mention of equal size |
| 2 | Same number of discs in equal volumes of water, at a range of temperatures (water baths), for the same time | "at least four or five temperatures", "20, 30, 40, 50, 60 degrees", "time controlled", "check temperature with thermometer" | "heat the beetroot" with no range; temperatures with no control of volume or time |
| 3 | Measure pigment released using a colorimeter (blue-green filter), calibrated / zeroed with distilled water; higher absorbance = more pigment | "absorbance", "transmission falls", "repeat and calculate a mean" | "see how red it goes" by eye only |
| 4 | Higher temperature denatures membrane proteins and disrupts the phospholipid bilayer, so permeability increases and more pigment leaks out (accept: low absorbance at low temperatures, rising sharply at high temperatures) | "channel / carrier proteins change shape", "phospholipids move more", "membrane damaged", "betalain leaks" | "the heat makes pigment come out" with no reason; "cells burst" with no membrane reference; "pigment evaporates" |
This is a required practical, so marks reward a controlled method (mark points 1 to 3) and a reasoned explanation (mark point 4). Control variables must be named, not just implied. Expect low absorbance at low temperatures, then a sharp rise at higher temperatures. A named variable other than temperature (for example ethanol concentration) is also valid; the same logic applies. Extra credit-worthy points that do not add marks beyond 4: repeating and calculating a mean for reliability.
Model answer (4/4)
Cut beetroot into discs of the same size, using a cork borer and ruler so the surface area is the same, and rinse them to remove pigment already released. Place the same number of discs in equal volumes of water in tubes in water baths at a range of temperatures, such as twenty to sixty degrees, for the same length of time. Remove the discs and measure the pigment in the water with a colorimeter, calibrated with distilled water, where a higher absorbance means more pigment has leaked. At higher temperatures, membrane proteins denature and the phospholipid bilayer is disrupted, so the membrane is more permeable and more betalain leaks out.
Examiner trap
- Not controlling surface area and volume
- Not saying why pigment leaks out (membrane damaged/proteins denatured)
- Not using a calibrated colorimeter reading
Key terms
- beetroot
- betalain pigment
- permeability
- colorimeter
- temperature
- solvent
- denatured membrane protein
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:29Beetroot and betalain pigment
Start with the idea. Beetroot cells contain a red pigment called betalain. In a healthy cell it stays inside, because the membranes are a barrier. Permeability describes how easily substances cross that membrane. If the membrane is damaged, it becomes more permeable, and the pigment leaks out.
We cannot see the membrane itself, but we can measure what escapes through it. So the amount of pigment in the water is our measure of membrane damage.
0:58Membrane structure
Why would temperature do that? Two reasons. The membrane proteins are proteins, so at high temperature they denature. Their shape changes, and the channels stop working properly. The phospholipids also gain kinetic energy, so they move more, and the bilayer becomes disrupted. Gaps open, the membrane is more permeable, and the betalain leaks out.
So the higher the temperature, the more damage, and the more pigment we expect in the water.
1:28Cutting and preparing discs
Now the method. Cut the beetroot into equal discs, using a cork borer and a ruler. The discs must be the same size, so the same surface area and volume. Otherwise a bigger disc could leak more pigment for reasons that have nothing to do with temperature.
1:45Rinsing the discs
Cutting damages the cells at the edge, so pigment is already leaking. Rinse the discs in water until the water runs clear, so you only measure the leakage caused by the temperature.
1:57Setting up temperatures
Put one disc in each tube, with the same volume of water. Place the tubes in water baths at a range of temperatures, for example twenty, thirty, forty, fifty and sixty degrees. Leave them for the same length of time, and use a thermometer to check each bath. Temperature is the independent variable. Pigment released is the dependent variable. Disc size, water volume and time are controlled.
2:24Using the colorimeter
Remove the discs and put the liquid in a cuvette. Measure it in a colorimeter. Use a blue-green filter, because that light is absorbed by the red pigment. First calibrate it, using distilled water to set the reading to zero. More pigment in the solution means a higher absorbance. So absorbance is a number for how leaky the membrane is. Repeat the whole thing, with several discs at each temperature, and calculate a mean, to make the results more reliable.
Always say how you would make the results reliable, because examiners like to see it.
3:00Reading the graph
Expect a low reading at the lower temperatures, then a sharp rise once the membranes are damaged. You can use a different variable, such as ethanol concentration, and the logic is the same. The solvent damages the membrane, and more pigment leaks out. Whatever you choose, change only that one variable, and keep everything else the same.
4:00Examiner trap and recap
The trap. Saying the pigment comes out when it is hot is not enough. You must say why. The membrane is damaged, because the proteins denature.
Recap. Equal discs, a range of temperatures, a calibrated colorimeter, and an explanation about the membrane.
Next, we meet antigens, and see how the immune system tells self from foreign.
Watch first
This lesson builds on:
- Fluid-Mosaic Model: Cell Membrane Structure · AQA 7402 · 3.2.3
- Simple vs Facilitated Diffusion · AQA 7402 · 3.2.3
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-040.