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The fluid mosaic model: what a membrane is made of questions

The fluid mosaic model, why a phospholipid bilayer assembles itself, intrinsic and extrinsic proteins, channel and carrier proteins, cholesterol at high and at low temperature, glycoproteins and glycolipids in cell recognition, and how temperature and solvents change permeability, tested with the beetroot investigation.

5 original questions · 18 marks · the fluid mosaic model: what a membrane is made of notes · Membranes and transport across cells

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• 4 marks · Explain

  Explain why phospholipids arrange themselves into a bilayer when they are placed in water, and why a torn fragment of membrane reseals itself into a closed vesicle.

  Mark scheme

  1. B1 a phospholipid is amphipathic: the phosphate head is hydrophilic and the two fatty acid tails are hydrophobic
  2. B1 the heads face the water on both surfaces while the tails are buried between them, facing each other
  3. B1 no bonds join one phospholipid to the next; this arrangement is simply the one of lowest energy, because it hides the greatest number of tails from water
  4. B1 an open edge would leave hydrophobic tails exposed to water, so a fragment closes on itself rather than remaining as a sheet

• 4 marks · Describe

  Describe what cholesterol does to a cell surface membrane at high temperature and at low temperature, and describe the effect it has on permeability in each case.

  Mark scheme

  1. B1 at high temperature cholesterol binds to the fatty acid tails around it and restricts their movement
  2. B1 so the membrane is less fluid than it would otherwise be and does not become leaky
  3. B1 at low temperature cholesterol sits between the tails and stops them packing closely together
  4. B1 so the membrane keeps some fluidity instead of setting solid and cracking, and stays intact

• 4 marks · Compare

  Compare a channel protein with a carrier protein as routes across a cell surface membrane.

  Mark scheme

  1. B1 a channel protein is a pore lined with hydrophilic amino acids, whereas a carrier protein has a binding site shaped to fit one kind of molecule
  2. B1 a substance passes through the open channel, whereas a carrier protein changes shape to move the substance across
  3. B1 channels carry water-soluble ions such as sodium, potassium and chloride, whereas carriers take larger polar molecules such as glucose and amino acids
  4. B1 channels are used in facilitated diffusion only, whereas carrier proteins are used in both facilitated diffusion and active transport

• 4 marks · Suggest

  A student cuts discs of beetroot with a cork borer and places them directly into water baths at a range of temperatures without washing them first. Suggest how this affects her absorbance readings, and suggest one control variable she must still keep the same.

• 2 marks · Name

  Name the two kinds of molecule found on the outer surface of a cell surface membrane that carry short branched carbohydrate chains.

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