A level-readyAll subjects · Biology · WBI11 (Unit 1)
Core practical 3: membrane permeability
Core practical 3 uses beetroot to test how temperature and alcohol affect membranes. Damaged membranes leak red pigment, which you measure with a colorimeter.
Method (temperature)
1. Cut equal-sized cylinders of beetroot with a cork borer and scalpel; rinse in water to remove pigment from cut cells.
2. Put one piece in a fixed volume of water at each temperature (for example 20 to 70 °C) in water baths for the same time.
3. Remove the beetroot and measure the colour of the liquid with a colorimeter (absorbance) using a blue-green filter, zeroed against distilled water.
For alcohol, keep the temperature constant and vary the ethanol concentration (for example 0 to 100%).
Variables
- Independent: temperature (or alcohol concentration).
- Dependent: absorbance of the solution (more pigment leaked = higher absorbance).
- Control: size and surface area of beetroot pieces, volume of liquid, time, rinsing, beetroot source, colorimeter filter.
Repeat at each value and calculate means.
Explaining results
The red pigment (betalain) is in the vacuole, inside two membranes: the tonoplast and the cell surface membrane.
- Higher temperature: phospholipids gain kinetic energy and the membrane becomes more fluid; at high temperatures membrane proteins denature, leaving gaps, so much more pigment leaks out.
- Alcohol: ethanol dissolves lipids, disrupting the phospholipid bilayer, so higher concentrations increase permeability.
Worked example
Absorbance rises slowly from 20 to 40 °C, then sharply above 50 °C. Explain.
- Up to 40 °C, increased fluidity lets a little more pigment through.
- Above 50 °C, membrane proteins denature, creating holes.
- Pigment leaks out rapidly, so absorbance rises sharply.
Answer: Fluidity rises slowly at first; protein denaturation at high temperature makes the membrane leak much more.
Check yourself
- What is the dependent variable when investigating the effect of temperature on beetroot membranes?
- Temperature of the water bath
- Size of the beetroot pieces
- Volume of water
- Absorbance of the surrounding liquid
Show answer
Absorbance of the surrounding liquid. Absorbance measures how much pigment leaked.
- Why are beetroot pieces rinsed before the experiment?
- To cool them down
- To remove pigment released from cells cut during preparation
- To add pigment
- To denature the proteins
Show answer
To remove pigment released from cells cut during preparation. Cut cells release pigment that is not due to the treatment.
- Why does a lot more pigment leak above about 50 °C?
- Phospholipids become solid
- Pigment molecules become smaller
- The cell wall dissolves
- Membrane proteins denature, leaving gaps in the membrane
Show answer
Membrane proteins denature, leaving gaps in the membrane. Denatured proteins can no longer keep the membrane intact.
- How does ethanol increase membrane permeability?
- It dissolves lipids in the phospholipid bilayer
- It cools the membrane
- It adds extra proteins
- It increases cholesterol
Show answer
It dissolves lipids in the phospholipid bilayer. Ethanol is a lipid solvent.
- Which must be kept the same for each beetroot piece?
- Temperature
- Absorbance
- Surface area of the piece
- Alcohol concentration in an alcohol experiment
Show answer
Surface area of the piece. Surface area affects how much pigment can leak.
- You mix 20 cm3 of pure ethanol with 80 cm3 of distilled water. What is the percentage concentration of ethanol?
Show answer
20 %. 20 / 100 x 100 = 20%.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.