(ii) practical investigations into factors affecting membrane structure…

The idea

Practical work on membrane permeability uses beetroot tissue: pigment (betalain) leaks from damaged vacuoles, and the colour intensity of the bathing solution indicates how much membrane damage has occurred. You vary one factor — temperature, ethanol concentration or detergent concentration — while keeping tissue mass, piece size, volume of solution, timing and rinsing constant. Pieces are blotted and rinsed to remove pigment released at the cut surfaces, then placed in water or the test solution for a fixed time. The solution is decanted or the tissue removed, and colour is compared by eye against a colour standard or measured with a colorimeter, whose absorbance you convert to a transmission or concentration value. Higher temperature, higher ethanol concentration and higher detergent concentration each increase permeability by disrupting the phospholipid bilayer and membrane proteins.

What earns the marks

• Beetroot pigment leakage is used as an indicator of membrane damage, because the pigment is normally contained within the vacuole and only escapes when the tonoplast and cell-surface membranes lose integrity.
• Control variables include mass or size of beetroot pieces, volume of bathing solution, time of exposure, temperature where not the independent variable, and rinsing or blotting of pieces before immersion.
• Colour intensity is quantified objectively, for example by colorimeter absorbance or by comparison with a colour standard, rather than by subjective description alone.
• Replicates are used and a mean calculated so that anomalous results can be identified and the reliability of the trend improved.
• An increase in temperature, ethanol concentration or detergent concentration increases membrane permeability by disrupting the phospholipid bilayer and denaturing or altering membrane proteins.

Common mistakes and how to put them right

• Assuming the beetroot pigment is released from the cell wall.

  Put it right: The pigment is in the vacuole and leakage reflects damage to the tonoplast and cell-surface membranes, not the wall.

• Failing to rinse or blot pieces before immersion, so pigment from cut surfaces is measured.

  Put it right: Rinse and blot so only pigment crossing intact membranes is recorded.

• Changing several factors at once, such as temperature and ethanol concentration together.

  Put it right: Vary only the independent variable and control the rest.

• Describing colour change only in vague words such as 'darker'.

  Put it right: Use a colorimeter or a defined colour standard to give a measurable value.

Exam tips

• State clearly what the pigment leakage is measuring and why it indicates membrane permeability.
• Name the controlled variables explicitly and explain why each must be kept constant.
• Explain the mechanism linking the factor to membrane disruption, referring to the phospholipid bilayer and membrane proteins.