Practical: Membrane Permeability (SQA National 5 Biology): Revision Note

Investigating factors affecting the permeability of the cell membrane

• Cell membranes are made up mainly of a phospholipid bilayer with embedded proteins

• This structure controls the movement of substances into and out of cells, maintaining a stable internal environment

• Environmental factors, like temperature, can affect membrane permeability

  • At low temperatures, phospholipids are tightly packed and have low kinetic energy, so the membrane is less fluid and less permeable

  • As temperature increases, phospholipids gain kinetic energy and move more, increasing membrane fluidity and permeability

  • At very high temperatures, phospholipid bilayer loses its structure, and membrane proteins denature, causing large gaps in the membrane

    • This allows substances that are normally contained (like pigments, ions, or enzymes) to leak out of cells

• Beetroot can be used to investigate how different factors, such as temperature, affect membrane structure and permeability

• Beetroot cells contain a dark purple-red pigment

  • The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell

Apparatus

• Fresh beetroot cylinders (cut using a cork borer)

• Ruler and scalpel

• Test tubes and rack

• Thermometer or water baths (set at different temperatures) OR Ethanol (various concentrations, e.g. 0%, 20%, 40%, 60%, 80%)

• Stopwatch

• Colorimeter (or visual colour scale)

• Distilled water

• Pipette or measuring cylinder

• Forceps

• Paper towel

Method

1. Prepare beetroot samples: Cut equal-sized cylinders of beetroot (same length and diameter)

   • Rinse in distilled water to remove any pigment released from damaged cells

2. Set up test tubes:

   • For temperature experiment: Add equal volumes of distilled water to test tubes and place them in water baths at different temperatures (e.g. 0°C, 20°C, 40°C, 60°C, 80°C)

   • For ethanol experiment: Add equal volumes of ethanol of different concentrations to separate test tubes

3. Add beetroot pieces: Place one beetroot cylinder in each tube. Leave for a fixed time (e.g. 5–10 minutes).

4. Remove beetroot: Use forceps to remove the beetroot and shake the tube gently to mix the solution.

5. Measure pigment release: Pour the solution into a cuvette and measure absorbance using a colorimeter set to a red or blue filter

   • The higher the absorbance the more pigment has been released meaning greater membrane damage

Expected results

Limitations

• Cuvettes may differ in thickness (very slightly). A thicker (or scratched) cuvette will absorb slightly more light than a thinner, unscratched cuvette

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

• 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