Practical: Investigating Membrane Structure & Permeability

How can a colorimeter be used to investigate membrane permeability in beetroot?

• Beetroot cells contain a red-purple pigment called betalain.

• If the cell-surface membrane becomes more permeable, more pigment leaks out into the surrounding water.

• A colorimeter measures the absorbance of light by the coloured solution, using an appropriate colour filter.

• Higher absorbance indicates a higher pigment concentration, suggesting greater membrane permeability.

• Calibrate (zero) the colorimeter using distilled water before taking measurements.

Describe the method for investigating the effect of temperature on membrane permeability.

1. Use a cork borer and scalpel to cut equal-sized beetroot pieces.

2. Rinse the pieces to remove pigment released by cutting.

3. Place each piece into a test tube containing the same volume of water.

4. Place the tubes in water baths at different temperatures (e.g. 10°C, 20°C, 30°C, 40°C and 50°C) for the same length of time.

5. Remove the beetroot pieces and use a colorimeter to measure the absorbance of each coloured solution.

6. Compare absorbance at different temperatures. Use a suitable colour filter and a separate pipette for each sample.

What variables should be controlled, and why?

• Size / surface area of beetroot pieces: use equal-sized pieces because surface area affects pigment leakage.

• Volume of water: keep it the same so pigment concentration can be compared fairly.

• Time in the water bath: keep it the same because more time allows more pigment to leak out.

• Colorimeter settings: use the same suitable filter and consistent measurement procedure.

• Rinse the beetroot pieces to remove pigment released during cutting, which could affect results.

• Repeat measurements where possible and calculate a mean to improve reliability.

Explain why increasing temperature generally increases membrane permeability.

• Phospholipids gain kinetic energy and move more, making the bilayer less tightly packed and allowing more pigment to escape.

• At high temperatures, membrane proteins can denature, disrupting membrane structure and increasing permeability.

• Expansion of cell contents can also damage membrane components.

• At temperatures below 0°C, ice crystals may damage membranes; permeability can therefore increase after the cells thaw.

Expected result: as temperature increases, absorbance generally increases because more betalain leaks into the water.

explain 3 limitations each with a solution

• Cuvettes may differ in thickness; a thicker cuvette will absorb slightly more light than a thinner cuvette

  • Solution: use the same cuvette for every reading, or repeat the investigation many times and find a mean

  • Note that scratched cuvettes can have the same impact on absorbance as thicker cuvettes

• The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others

  • Solution: cut the discs as accurately as possible using a scalpel and ruler, and repeat each investigation several times to find a mean

• Some parts of beetroot tissue have more pigment in their cells than others

  • Solution: conduct several repeats, using different parts of the beetroot and find a mean

what’s the other way to investigate membrane permeability

• The impact of alcohol on membrane permeability could also be investigated

• The expected trend is

  • As alcohol concentration increases the permeability of the membrane increases

  • This is because alcohol dissolves lipids in the cell surface membrane so the membrane loses its structure and the beetroot pigment can leak