Effect of Temperature on Membrane Permeability

The structural features of biological membranes described in Lesson 1 have direct consequences for how they behave when environmental conditions change. Temperature is one of the most important variables, and it is also the focus of a required practical. This lesson covers OCR A-Level Biology A specification point 2.1.5 (c) — the movement of molecules across membranes — and (d) — the factors that affect membrane structure and permeability, with the beetroot practical as the worked example.

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1. Why Temperature Affects Membrane Permeability

Membrane permeability depends on the tight packing of phospholipids and the functional integrity of membrane proteins. Temperature affects both.

1.1 Low temperatures (below about 0 °C)

• Phospholipids have less kinetic energy and pack tightly together.
• Fatty acid tails are relatively rigid — the membrane becomes gel-like and less fluid.
• Channel and carrier proteins may become deformed; they are held in a rigid matrix and cannot change conformation easily.
• Ice crystals can form inside the cell, physically piercing membranes and causing large holes.
• Paradoxically, permeability to some molecules increases at very low temperatures because the membrane cracks or proteins no longer seal properly.

1.2 Moderate temperatures (0–45 °C)

• Membranes are fluid and selectively permeable.
• Phospholipids move laterally; proteins can carry out their functions (transport, signalling, catalysis).
• Permeability increases gently with temperature — more kinetic energy means faster diffusion.

1.3 High temperatures (above about 45 °C)

• Phospholipids have high kinetic energy and move apart — gaps open in the bilayer.
• Water and solutes can leak through these gaps.
• Membrane proteins denature: the hydrogen bonds and ionic interactions maintaining their tertiary structure break, the proteins unfold and their shape changes irreversibly.
• Channels may become permanently open (or closed); receptors stop functioning; enzymes are inactivated.
• Permeability rises sharply.

graph LR
  A[Temperature] -->|Low| B[Reduced fluidity<br/>Protein deformation<br/>Ice crystal damage]
  A -->|Moderate| C[Normal fluidity<br/>Functional proteins<br/>Controlled permeability]
  A -->|High| D[Gaps in bilayer<br/>Protein denaturation<br/>Increased permeability]

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2. Other Factors Affecting Permeability

Before we look at the practical, two other factors are examinable:

2.1 Solvents

Non-polar solvents such as ethanol, propanone (acetone) and methanol dissolve the hydrophobic core of the membrane:

• At low concentrations they disrupt phospholipid packing, increasing fluidity and permeability.
• At high concentrations they dissolve membranes entirely, causing catastrophic leakage and cell death.
• This is why ethanol above ~70% kills bacterial cells (and why it is used in hand sanitiser).

2.2 pH

Extremes of pH denature membrane proteins and alter the ionisation of phospholipid heads. Both effects increase permeability.

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3. Required Practical — Investigating Membrane Permeability Using Beetroot

This is a classic OCR required practical. Beetroot is an ideal material because its vacuoles contain a red-purple water-soluble pigment called betacyanin (also written betalain). Betacyanin is normally trapped inside the tonoplast (the vacuolar membrane) and plasma membrane. When these membranes are damaged, betacyanin leaks out into the surrounding water and can be quantified using a colorimeter.

3.1 Hypothesis

As temperature increases, membrane permeability increases, so more betacyanin will leak out and the colour intensity of the surrounding water will increase.

3.2 Method

1. Use a cork borer to cut several beetroot cylinders of equal diameter from a single beetroot.
2. Cut the cylinders into equal-length discs (e.g. 1 cm) using a ruler and scalpel — a control for surface area.
3. Rinse the discs thoroughly in cold distilled water to remove betacyanin released from cells damaged during cutting. Blot dry.
4. Place one disc into a labelled test tube containing 5 cm³ of distilled water at a known temperature. Use a water bath to control temperature.
5. Test a range of temperatures, e.g. 10, 20, 30, 40, 50, 60, 70 °C.
6. Leave each tube for a fixed time (e.g. 30 minutes).
7. Remove the disc. Shake the tube gently to mix the contents.
8. Transfer the solution to a cuvette and measure absorbance at a suitable wavelength (around 480 nm or a green filter — because betacyanin is red, it absorbs strongly in the green/blue region).
9. Calibrate the colorimeter with distilled water first.
10. Repeat each temperature at least three times and calculate a mean absorbance.

3.3 Variables

Type	Variable	How controlled
Independent	Temperature	Water bath
Dependent	Absorbance (pigment release)	Colorimeter
Controlled	Disc size	Same cork borer; equal-length discs
Controlled	Volume of water	5 cm³ per tube
Controlled	Time in bath	30 min timer
Controlled	Beetroot	One beetroot cut on the same day
Controlled	Rinsing	Cold distilled water for set time

3.4 Risk Assessment