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Membranes and transport

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Which part generally faces the cytosol?

  1. Hydrophobic fatty-acid tails
  2. Hydrophilic phospholipid heads
  3. Carbohydrate chains of glycoproteins
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Hydrophilic phospholipid heads

Heads face the aqueous cytosol and extracellular fluid; tails point inward.

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Which statements about the fluid mosaic model are correct? 1: Phospholipids can move sideways within each layer. 2: Glycoproteins face the cytoplasm. 3: Cholesterol helps regulate fluidity. 4: Channel proteins give a hydrophilic route for ions.

  1. 1, 2, 3 and 4
  2. 1 and 4 only
  3. 2, 3 and 4 only
  4. 1, 3 and 4 only
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1, 3 and 4 only

Lateral movement, cholesterol and channel proteins are all part of the model. Carbohydrate chains of glycoproteins face outwards, so 2 is false.

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At equal water potentials across a water-permeable membrane, what happens?

  1. Water molecules stop moving across the membrane.
  2. Water keeps moving into the more concentrated solution.
  3. Water moves both ways with no net movement.
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Water moves both ways with no net movement.

Opposing water fluxes balance.

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Uptake of solute S by cells rises with external concentration and then levels off. A respiratory inhibitor has no effect, and S is always more concentrated outside. Which conclusion is best supported?

  1. S enters by facilitated diffusion through carriers.
  2. S enters by simple diffusion through the bilayer.
  3. S enters by active transport using ATP.
  4. S enters by endocytosis in vesicles.
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S enters by facilitated diffusion through carriers.

The plateau shows a limited number of carriers, the inhibitor shows no ATP is needed, and movement is down the gradient.

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A cotransporter brings glucose uphill while sodium moves downhill. Which is best?

  1. Glucose must be diffusing directly through phospholipids.
  2. Glucose transport is passive because the carrier does not directly hydrolyse ATP.
  3. The sodium gradient supplies energy for coupled transport.
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The sodium gradient supplies energy for coupled transport.

Indirect ATP dependence differs from a direct ATP-driven glucose pump.

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What happens to a secretory vesicle's lumen during exocytosis?

  1. It opens to the extracellular space when membranes fuse.
  2. It opens into the cytosol before the cargo crosses the membrane.
  3. It stays sealed while the whole vesicle passes through the lipid bilayer.
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It opens to the extracellular space when membranes fuse.

Cargo can then be released outside.

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Question 1 of 5

Choose between transport models

4 marks

Solute X uptake is selective and saturates. Uptake remains downhill and is unaffected immediately by inhibiting ATP production. Which mechanism is consistent with the evidence, and what cannot yet be concluded? [4]

Use all observations, not just the plateau.

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