Explain how the properties of phospholipids determine the arrangement of these molecules in a plasma membrane.

The permeability of plasma membranes can be investigated using red cabbage (Brassica oleracea). Red cabbage cells contain water-soluble anthocyanin pigments within their large vacuoles. These pigments only leak out of the cells when the vacuolar and plasma membranes become damaged or more permeable.

Some students investigated the effect of ethanol concentration on the permeability of plasma membranes in red cabbage cells.

The students used a valid method, which is outlined below:

• Equal-sized discs of red cabbage were cut using a cork borer and thoroughly washed with distilled water to remove any surface pigment.
• The discs were each immersed in an equal volume of different concentrations of ethanol inside test tubes.
• The tubes were kept at a constant temperature of $25^{\circ }C$ for a set time of 30 minutes.
• After this time, the solutions were shaken and the absorbance measured using a colorimeter with a green filter.
• The procedure was replicated three times at each concentration of ethanol.

The students recorded their results in the format shown below:

• $0\%$ -- $0.01,0.00,0.02$; average = $0.01$
• $10\%$ -- $0.04,0.06,0.05$; average = $0.05$
• $20\%$ -- $0.18,0.22,0.20$; average = $0.20$
• $30\%$ -- $0.49,0.53,0.51$; average = $0.51$
• $40\%$ -- $0.82,0.85,0.82$; average = $0.83$
• $50\%$ -- $0.96,0.98,0.97$; average = $0.97$

Present the students' results in an appropriate table.

The students concluded that the anthocyanin pigment began to leak out of the red cabbage cells at ethanol concentrations above $10\%$. Suggest and explain why a high concentration of ethanol causes the pigment to leak out of the red cabbage cells.

Outline how the students could modify their investigation to get a more accurate value for the ethanol concentration at which the pigment begins to leak out.