Diffusion and facilitated diffusionAQA A-Level Biology: Subtopic test

1. 1
   Glucose enters red blood cells through carrier proteins in the cell-surface membrane, and no ATP is used. Oxygen also enters red blood cells, but without the help of any membrane protein. In both cases the concentration outside the cell is higher than inside.

   (a)

   Which statement correctly describes how glucose enters the red blood cell?

   [1 mark]

   • AFacilitated diffusion down a concentration gradient through carrier proteins
   • BActive transport against a concentration gradient using ATP
   • CSimple diffusion through the phospholipid bilayer
   • DOsmosis through the phospholipid bilayer

   (b)

   Why can oxygen cross the membrane without a protein, whereas glucose cannot?

   [1 mark]

   • AOxygen molecules are larger than glucose molecules
   • BOxygen molecules are charged, so they pass between the heads
   • CGlucose is non-polar and so is repelled by the membrane surface
   • DOxygen is small and non-polar so it dissolves in the hydrophobic core; glucose is large and polar

   (c)

   Describe how a carrier protein moves glucose into the cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cell takes up substance X by facilitated diffusion through carrier proteins in its cell-surface membrane. The concentration of X outside the cell is higher than inside the cell.

   (a)

   Which change would increase the rate of uptake of X?

   [1 mark]

   • ADecreasing the concentration of X outside the cell
   • BReducing the number of carrier proteins in the membrane
   • CIncreasing the number of carrier proteins in the membrane
   • DRaising the concentration of X inside the cell to match the outside

   (b)

   What is the main difference between channel proteins and carrier proteins?

   [1 mark]

   • AChannel proteins use ATP, whereas carrier proteins do not
   • BCarrier proteins change shape to move a molecule, whereas channel proteins form water-filled pores
   • CChannel proteins move molecules against the gradient
   • DCarrier proteins are found only in the membranes of organelles

   (c)

   The rate of uptake of X increased as the outside concentration rose from 0 to 6 mmol dm⁻³, but stayed constant at higher concentrations. Explain why the rate stayed constant.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cells lining the small intestine have microvilli, which are folds of the cell-surface membrane. A researcher compared one cell with microvilli and a similar cell without. The absorptive membrane area is 1500 µm² for the cell without microvilli and 18 000 µm² for the cell with microvilli. Both membranes have 40 carrier proteins per µm².

   (a)

   Explain how the adaptations of the cell with microvilli allow rapid transport across its membrane.

   [3 marks]

   (b)

   Calculate how many times larger the absorptive membrane area is in the cell with microvilli, and how many more carrier proteins this cell has. Explain what this shows about the effect on the rate of facilitated diffusion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student measured the initial rate of uptake of two substances by cells placed in solutions of different concentrations. Substance X is small and lipid-soluble. Substance Y is polar and enters through carrier proteins. Both move down their concentration gradients.

   (a)

   Compare simple diffusion and facilitated diffusion, and explain, in terms of membrane structure, why X and Y cross the membrane by different routes.

   [6 marks]

   (b)

   Uptake of X increased in direct proportion to concentration from 1 to 12 mmol dm⁻³. Uptake of Y rose steeply from 1 to 6 mmol dm⁻³ and was then constant above 6 mmol dm⁻³. When a substance that denatures proteins was added, uptake of Y fell almost to zero but uptake of X did not change. Explain these results.

   [6 marks]

   Total for question 4: 12 marks

End of questions