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Active transport, co-transport and moving things in bulk questions

Active transport against a concentration gradient using carrier proteins and ATP, the sodium-potassium pump and its three-to-two stoichiometry, co-transport of glucose and sodium in the ileum worked through as a sequence, endocytosis and exocytosis, and a comparison of all the transport mechanisms.

5 original questions · 18 marks · the active transport, co-transport and moving things in bulk notes · Membranes and transport across cells

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• 4 marks · Describe

  Describe how a carrier protein moves a substance across a membrane by active transport, from the substance binding to the protein returning to its original shape.

  Mark scheme

  1. B1 the substance binds to a specific binding site on the carrier protein, on the side it is starting from
  2. B1 ATP binds to the protein and is hydrolysed to ADP and inorganic phosphate
  3. B1 the phosphate group attaches to the protein and changes its tertiary structure, so the binding site now faces the other side of the membrane
  4. B1 the substance is released, the phosphate detaches, and the protein returns to its original shape ready to repeat the cycle

• 4 marks · Explain

  Explain why the absorption of glucose from the lumen of the ileum is described as active, even though the co-transporter protein that carries the glucose hydrolyses no ATP.

  Mark scheme

  1. B1 glucose is moved into the epithelial cell against its own concentration gradient, which diffusion alone could never achieve
  2. B1 sodium-potassium pumps in the membrane facing the blood hydrolyse ATP to move sodium ions out of the cell, keeping the concentration of sodium inside the cell low
  3. B1 sodium ions then diffuse in from the lumen down the gradient this creates, through the co-transporter protein, which carries glucose at the same time
  4. B1 the ATP was therefore spent earlier, at a different protein and on a different ion; the energy is held in the sodium gradient rather than used at the co-transporter

• 4 marks · Compare

  Compare endocytosis with exocytosis, referring to the direction of movement, what the membrane does, and the energy required.

  Mark scheme

  1. B1 in endocytosis material is brought into the cell, whereas in exocytosis material is released out of the cell
  2. B1 in endocytosis the cell surface membrane invaginates around the material and pinches off to form a vesicle, whereas in exocytosis a vesicle fuses with the cell surface membrane
  3. B1 both move material in bulk, with no channel or carrier protein involved
  4. B1 both require ATP, for moving the vesicle along the cytoskeleton and for the pinching off or fusion of the membrane

• 4 marks · Suggest

  A tissue takes up two substances, X and Y. When a respiratory inhibitor is added, uptake of X falls almost to zero within minutes while uptake of Y continues unchanged. Suggest what this shows about how each substance crosses the membrane, and suggest one further observation that would support your conclusion about X.

• 2 marks · State

  State how many sodium ions and how many potassium ions the sodium-potassium pump moves for each molecule of ATP it hydrolyses, and state the direction of each movement.

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