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Better Equipped Practical Teaching Guides
A complete classroom guide for investigating how temperature affects beetroot cell membrane permeability, with apparatus, method, theory, calculations, risk assessment, troubleshooting, technician notes and exam support.
Teacher note: This resource supports practical teaching and equipment planning. Teachers should adapt the procedure and risk assessment to match their exam board specification, local laboratory rules and departmental policies.
This A Level Biology practical investigates how temperature affects the permeability of beetroot cell membranes. Beetroot contains a red-purple betalain pigment, mainly stored in the vacuole. When membranes are damaged by heat, more pigment diffuses into the surrounding water. The intensity of the solution can then be measured using a colorimeter or compared using a visual colour scale.
Beetroot cells contain betalain pigment, which is retained by the tonoplast and cell surface membrane. At low temperatures, beetroot cell membranes remain relatively stable and only small amounts of betalain pigment leak into the surrounding water. As temperature increases, phospholipids gain kinetic energy and move more freely, causing the membrane to become more fluid and permeable. This allows increasing amounts of pigment to diffuse out of the cells. At higher temperatures, some membrane proteins may denature, further increasing membrane permeability. Consequently, pigment leakage increases with temperature. Greater pigment leakage indicates greater membrane permeability and can be quantified by measuring absorbance with a colorimeter.
Greater pigment leakage indicates greater membrane permeability or membrane damage. Absorbance is therefore used as an indirect measure of membrane disruption.
Use equal-sized beetroot discs, equal water volumes, identical incubation times and the same colorimeter wavelength for every sample. Include a room temperature sample as a comparison.
| Variable | Example in this investigation | Why it matters |
|---|---|---|
| Independent variable | Temperature | This is deliberately changed to test its effect. |
| Dependent variable | Absorbance or colour intensity | This indicates the concentration of leaked pigment. |
| Control variables | Disc size, number of discs, water volume, incubation time, rinsing method and wavelength | These must be kept constant to make valid comparisons. |
Students should record repeat absorbance readings for each temperature, calculate a mean and plot temperature against mean absorbance. A line graph or scatter graph with a suitable line of best fit is usually appropriate. Error bars can be added if repeat data are available.
| Temperature (°C) | Trial 1 | Trial 2 | Trial 3 | Mean absorbance |
|---|---|---|---|---|
| 20 | ||||
| 30 | ||||
| 40 | ||||
| 50 | ||||
| 60 | ||||
| 70 |
Mean = total results ÷ number of trials. Example: 0.46, 0.50 and 0.48 gives a mean of 0.48.
Percentage change = ((final value - initial value) ÷ initial value) x 100.
At low temperatures, beetroot membranes remain mostly intact and only a small amount of pigment leaks into the surrounding water. As temperature increases, membrane permeability increases and the solution becomes more strongly coloured. At very high temperatures, protein denaturation and membrane damage cause much greater pigment leakage.
Conclusion: Pigment leakage increases as temperature rises because cell membranes become more permeable. As temperature increases, phospholipids gain kinetic energy and move more freely, making the membrane more fluid. This allows increasing amounts of betalain pigment to diffuse out of beetroot cells. At higher temperatures, some membrane proteins may also denature, further increasing membrane permeability and pigment leakage.
| Problem | Possible cause | Solution |
|---|---|---|
| No colour change | Beetroot not heated long enough | Increase incubation time consistently for every sample. |
| Very dark colour in all tubes | Samples not rinsed after cutting | Rinse gently before testing. |
| Inconsistent results | Unequal disc size or mass | Use a cork borer, ruler and consistent number of discs. |
| Unexpected low absorbance | Colorimeter not zeroed | Blank with distilled water before readings. |
| Large fluctuations | Temperature unstable | Use a water bath and allow tubes to equilibrate. |
Beetroot pigment is produced by heat.
Correction: pigment is already present; heat allows it to leak out.
Colour intensity directly measures temperature.
Correction: it measures pigment concentration, which is affected by permeability.
Bigger discs always give better results.
Correction: equal and consistent discs give valid comparisons.
Theory introduction: 10 mins | Teacher demonstration: 10 mins | Experiment setup: 15 mins | Data collection: 25 mins | Analysis: 15 mins | Evaluation: 10 mins
Overall risk: low to moderate. Schools should complete their own risk assessment according to local procedures. Students should wear eye protection and laboratory coats, and should take particular care with hot water, glassware and cutting equipment.
| Hazard | Risk | Control measures |
|---|---|---|
| Scalpels or cork borers | Cuts during beetroot preparation | Teacher or technician preparation recommended. Cut on a white tile and keep fingers clear. |
| Hot water baths | Burns or scalds | Use suitable temperatures. Handle tubes carefully and use forceps where needed. |
| Glass test tubes and cuvettes | Cuts from broken glass | Inspect for cracks. Clear breakages using a brush and dustpan. |
| Beetroot pigment | Staining of skin, benches and clothing | Wear lab coat and clean spills immediately. |
| Electrical equipment | Electric shock if wet | Keep colorimeters and plugs dry. Dry hands before handling. |
| Spillages | Slip hazard | Wipe up immediately and keep work area tidy. |
Before students begin, demonstrate:
Ask students:
Assess students during the practical for evidence of the following A Level practical skills:
| Skill assessed | Teacher observation |
|---|---|
| Preparing samples accurately | Cuts equal-sized beetroot discs or cylinders and rinses them correctly before use. |
| Controlling variables | Maintains constant volume of water, incubation time, beetroot size and temperature for a fair test. |
| Practical technique | Uses water baths, pipettes, forceps and the colorimeter safely and correctly. |
| Recording data | Produces clear, organised tables with appropriate headings, units and repeat measurements. |
| Processing data | Correctly calculates mean absorbance and identifies anomalous results where appropriate. |
| Graphing skills | Selects suitable axes, labels with units, chooses an appropriate scale and plots data accurately. |
| Scientific explanation | Explains results using membrane fluidity, phospholipid movement, protein denaturation and diffusion. |
| Evaluation | Identifies sources of error and suggests realistic improvements linked to reliability, accuracy and validity. |
Beetroot contains a strong red-purple betalain pigment, making leakage easy to observe and measure.
Cutting damages some cells and releases pigment. Rinsing removes this surface pigment so later colour change is caused mainly by the treatment.
A colorimeter provides quantitative absorbance readings, making results more precise than visual colour comparison.
Higher absorbance shows more pigment in the solution, which indicates greater membrane permeability or membrane damage.
Explore more A Level Biology practical guides, equipment support and laboratory planning resources from Better Equipped.
This guide was written and reviewed by Better Equipped's technical team, drawing on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. Our technical team include ex-school laboratory technicians and are here to support schools, colleges and laboratories across the UK. If you have feedback on this guide, please contact us.
Last reviewed and updated: July 2026
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