AS Biology 9700: Temperature, Membrane Fluidity and Permeability

A student cuts equal-sized discs of beetroot tissue, whose vacuoles contain a red pigment, and rinses each disc thoroughly in distilled water to remove any pigment released by the cutting. Each disc is then placed into its own tube of distilled water, and the tubes are held at a different temperature: 10 °C, 30 °C, 50 °C and 70 °C. After 20 minutes, the student compares the colour intensity of the water in each tube.

The water in the 10 °C and 30 °C tubes stays almost colourless, the water in the 50 °C tube develops a faint red colour, and the water in the 70 °C tube develops a strong, deep red colour.

(a) Explain, in terms of the fluid mosaic model, why the pigment leaks out of the beetroot cells more at 70 °C than at 30 °C. [3]

(b) The pigment molecules are too large to cross the phospholipid bilayer by simple diffusion. Suggest how they are able to leave the cell in large amounts at 70 °C. [2]

(c) Predict and explain what would happen to the colour intensity of the water if the experiment were repeated with beetroot discs held at 95 °C instead of 70 °C. [2]

Part (a): Why more pigment leaks out at 70 °C than at 30 °C

The cell-surface membrane is built from a phospholipid bilayer described by the fluid mosaic model, with proteins embedded within it. Raising the temperature gives the phospholipid molecules more kinetic energy, so they move about more and the bilayer becomes progressively more fluid. At 70 °C the phospholipids are far more disordered and loosely packed than at 30 °C. At the same time, the membrane proteins, which normally control what crosses the membrane, have a precise tertiary structure held together by bonds that are disrupted by high temperature; at 70 °C these proteins are likely to be denatured, losing their specific shape. Both the increased fluidity of the bilayer and the denaturation of proteins make the membrane far less able to regulate what passes through it, so it becomes considerably more permeable at 70 °C than at 30 °C, allowing far more pigment to leak out.

Part (b): How the large pigment molecules escape

The pigment molecule is too large and polar to pass directly through the hydrophobic core of an intact phospholipid bilayer by simple diffusion. At 70 °C, however, the combination of a highly disordered bilayer (with larger, transient gaps between disorganised phospholipids) and denatured membrane proteins (which lose their normal, controlled shape and can leave gaps where they once sat, or form irregular openings) creates unregulated routes across the membrane. The pigment molecules move down their concentration gradient (from the high concentration inside the vacuole/cytoplasm to the low concentration in the surrounding water) through these gaps, rather than through the intact bilayer or a functioning, specific transport protein.

Part (c): Predicting the result at 95 °C

By 70 °C, the membrane’s phospholipids are already highly disordered and its proteins are largely denatured, so the membrane has already lost most of its ability to control permeability. Increasing the temperature further, to 95 °C, would disrupt the membrane structure only a little more, since there is little intact structure left to break down. The colour intensity of the water would therefore be expected to be similar to, or only slightly greater than, at 70 °C, rather than showing another large increase. Permeability approaches a maximum once denaturation is essentially complete, so the relationship between temperature and pigment leakage levels off at very high temperatures.

Final answers

(a) At 70 °C the bilayer is far more fluid and membrane proteins are more likely to be denatured than at 30 °C, so the membrane is far more permeable and leaks more pigment.
(b) Disordered phospholipids and denatured proteins create unregulated gaps in the membrane, through which the large pigment molecules can pass down their concentration gradient.
(c) Colour intensity would be similar to, or only slightly greater than, at 70 °C, since the membrane is already extensively disrupted and permeability is close to its maximum.