Explain how the structure of phospholipid molecules allows for the formation of plasma membranes.

The permeability of plasma membranes can be investigated using red cabbage (Brassica oleracea). Red cabbage cells contain water-soluble anthocyanin pigments within their large central vacuoles. These pigments are unable to pass through intact plasma membranes and tonoplasts but leak out if the membranes are damaged or disrupted.

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:

• Uniform discs of red cabbage were cut using a cork borer and thoroughly washed in distilled water to remove pigment from damaged surface cells.
• The discs were then immersed in equal volumes of different concentrations of ethanol.
• The tubes were kept at a constant temperature of $20^{\circ }C$ for 30 minutes.
• After this time, the solutions were stirred and the absorbance of the solution was measured using a colorimeter with a green filter.
• The procedure was replicated three times at each ethanol concentration.

The students recorded their results in the raw format shown below:

• $0\%ethanol$ -- $0.00,0.00,0.00$; average = $0.00$
• $10\%ethanol$ -- $0.03,0.01,0.02$; average = $0.02$
• $20\%ethanol$ -- $0.15,0.18,0.15$; average = $0.16$
• $30\%ethanol$ -- $0.38,0.41,0.41$; average = $0.40$
• $40\%ethanol$ -- $0.69,0.72,0.69$; average = $0.70$
• $50\%ethanol$ -- $0.84,0.86,0.85$; average = $0.85$

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 increasing ethanol concentration 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 of the cells.