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Biological membranes

Use beetroot to show how membrane integrity is affected by treatments that damage either the protein or the phospholipid component of the membrane.

The cell membrane, (plasma membrane), is composed primarily of a bilayer of phospholipid molecules with a mosaic of protein molecules embedded in and attached to it. Both these components are necessary for the membrane to fulfil its role of maintaining the integrity of the cell contents while allowing selected substances to cross the membrane. The effect of treatments that damage either the phospholipids or the protein components can be investigated using beetroot. The red pigment of beetroot is retained in the cells and only escapes into the surrounding medium if the membrane is damaged.

Safety

There are no hazards associated with the investigations described here.

Good laboratory practices should be observed.

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Results

Temperature

    Incubating the pieces of beetroot at increasing temperatures will denature the membrane proteins and allow the red pigment to leak out of the cells.
    These results were obtained after 20 minutes incubation.
    membrane temperature

Detergent

    Adding detergent, ( e.g. washing up liquid), will damage the phospholipid component of the membrane, allowing the red pigment to leak out of the cells.
    These results were obtained after the beetroot pieces had stood in the detergent solutions for 20 minutes.
    membrane detergent

Alcohol

    Adding alcohol, ( e.g. ethanol), will damage the phospholipid component of the membrane, allowing the red pigment to leak out of the cells.
    These results were obtained after the beetroot pieces had stood in the ethanol solutions for 20 minutes.
    membrane alcohol
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Methods

The experiments described here use small pieces of beetroot cut using a cork borer. Cylinders about 10mm long with a diameter of about 5mm are ideal. (If you do not have a cork borer just cut pieces about this size with a knife.) The pieces must be thoroughly washed in running water to remove all surface pigment released from damaged cells.

The amount of pigment leaking through the membrane into the surrounding liquid can be observed qualitatively by eye or measured accurately using the blue LED of the colorimeter.

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