(c) (i) factors affecting membrane structure and permeability

The idea

Membrane structure is described by the fluid mosaic model: a phospholipid bilayer with proteins, glycoproteins and glycolipids, held together by hydrophobic interactions. Permeability depends on this structure. Temperature raises kinetic energy, so phospholipids move more and gaps open; moderate warming increases permeability, while high temperature denatures membrane proteins and disrupts the bilayer. Solvents such as ethanol dissolve lipids, disrupting the bilayer and increasing permeability. Increasing the proportion of unsaturated fatty acids creates kinks that increase fluidity. Cholesterol fits between phospholipids, restricting movement at moderate temperatures but preventing rigid packing at low temperatures. Small non-polar molecules cross most easily; large or charged molecules need channels or carriers.

What earns the marks

• The fluid mosaic model describes a phospholipid bilayer containing proteins, glycoproteins and glycolipids.
• Raising temperature increases phospholipid movement and can open gaps, increasing permeability; very high temperature denatures proteins and disrupts the bilayer.
• Organic solvents such as ethanol dissolve membrane lipids and disrupt the bilayer, increasing permeability.
• More unsaturated fatty acids in the phospholipids increase membrane fluidity because the kinks prevent tight packing.
• Cholesterol restricts phospholipid movement at moderate temperatures and prevents rigid packing at low temperatures, moderating permeability.
• Permeability to a substance also depends on its size and polarity, with small non-polar molecules crossing more readily than large or charged ones.

Common mistakes and how to put them right

• Saying heat only denatures proteins.

  Put it right: Also explaining increased phospholipid movement and bilayer disruption at high temperature.

• Claiming cholesterol always increases fluidity.

  Put it right by stating it restricts movement at moderate temperatures but prevents rigid packing when cold.

• Assuming all molecules cross equally.

  Put it right by linking permeability to molecule size and polarity as well as to membrane structure.

Exam tips

• For each factor, state the structural change first, then the resulting change in permeability.
• Use the terms fluid mosaic, bilayer and phospholipid movement precisely rather than vague wording.
• Check whether the option describes an effect at moderate or high temperature, since the outcomes differ.