Skills applied: CP3 membrane permeability, CP4 enzyme rates

CP3: beetroot membranes

Method recap (see Core practical 3, Unit 1): equal-sized beetroot pieces (cork borer, same length) are rinsed to remove pigment from cut cells, then placed in equal volumes of water at different temperatures (or in different alcohol concentrations) for a fixed time. The solution's absorbance in a colorimeter (blue-green filter) measures how much red betalain pigment leaked out.

Pattern: absorbance rises slowly, then sharply above about 40 to 50 °C as membrane proteins denature and phospholipids become more fluid; alcohol dissolves phospholipids.

Skills: zeroing the colorimeter with distilled water, consistent timing, controlling surface area, and why higher absorbance means more permeable membranes.

CP4: initial rate of enzyme reactions

Method recap (see Core practical 4, Unit 1): vary temperature, pH, enzyme or substrate concentration. Measure product formed or substrate used over time, for example:

• amylase with starch: time until iodine no longer turns blue-black (rate = 1 / time), or colorimeter readings;
• catalase with hydrogen peroxide: volume of oxygen in a gas syringe or upturned cylinder against time;
• trypsin with milk protein: time for the cloudiness to clear (colorimeter).

Initial rate = gradient of the tangent at time zero. Use buffers for pH and a water bath for temperature, and equilibrate before mixing.

Exam favourites

• Why the beetroot pieces are rinsed (remove pigment from damaged cells so only leakage through membranes is measured).
• Why absorbance is used rather than 'redness by eye' (quantitative, not subjective).
• Why the substrate concentration affects rate until enzyme becomes limiting.
• Why results at high temperature are less repeatable (denaturation is time-dependent).
• Sketching and explaining the shape of rate against temperature or pH.

Worked example

At 30 °C, 24 cm³ of oxygen is collected from catalase in the first 60 s; the tangent at t = 0 passes through (0, 0) and (20, 12). Calculate the initial rate.

1. Initial rate = gradient of the tangent
2. = (12 - 0) / (20 - 0) = 0.6

Answer: 0.6 cm³ s⁻¹ (higher than the mean rate over 60 s, 24 / 60 = 0.4 cm³ s⁻¹)

Check yourself

1. Why are beetroot pieces rinsed before CP3?
   • To cool them down
   • To make the membranes more permeable
   • To remove pigment released from cells damaged by cutting
   • To add pigment to the water
   Show answer

   To remove pigment released from cells damaged by cutting. Only leakage through intact membranes should be measured.

2. In CP3, higher absorbance of the surrounding water means:
   • More pigment has leaked out, so membranes are more permeable
   • Less pigment has leaked out
   • The beetroot has absorbed water
   • The temperature was lower
   Show answer

   More pigment has leaked out, so membranes are more permeable. Absorbance increases with pigment concentration.

3. Why does membrane permeability increase sharply above about 45 °C?
   • The cell wall dissolves
   • Pigment molecules become larger
   • Membrane proteins denature and phospholipids become more fluid
   • Water stops moving
   Show answer

   Membrane proteins denature and phospholipids become more fluid. Gaps form as proteins lose shape and the bilayer becomes more fluid.

4. In a catalase experiment, 36 cm³ of oxygen was collected in 120 s. Calculate the mean rate over this time.
   Show answer

   0.3 cm³ s⁻¹. 36 / 120 = 0.3 cm³ s⁻¹.

5. A tangent drawn at t = 0 on a product-time curve passes through (0, 0) and (25, 15), with time in s and product in arbitrary units. Calculate the initial rate.
   Show answer

   0.6 a.u. s⁻¹. 15 / 25 = 0.6 a.u. s⁻¹.

6. Why is a buffer used in an enzyme experiment where temperature is the IV?
   • To provide the substrate
   • To keep the pH constant, since pH affects the enzyme's active site
   • To speed up the reaction
   • To stop the reaction
   Show answer

   To keep the pH constant, since pH affects the enzyme's active site. pH is a controlled variable.

7. In the alcohol version of CP3, why does permeability rise with ethanol concentration?
   • Ethanol adds pigment
   • Ethanol cools the membrane
   • Ethanol makes proteins more stable
   • Ethanol dissolves phospholipids, disrupting the membrane
   Show answer

   Ethanol dissolves phospholipids, disrupting the membrane. Lipid solvents damage the bilayer.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills

Last updated 8 November 2026. Spotted a mistake? Tell us.