Transport across membranesEdexcel A-Level Biology A: Subtopic test

1. 1
   Human red blood cells contain a mixture of dissolved solutes with the same overall concentration of free water molecules as a 0.9% sodium chloride solution. A technician places samples of red blood cells in pure water, in 0.9% sodium chloride solution and in 3.0% sodium chloride solution, and observes them under a microscope.

   (a)

   What happens to the red blood cells in 0.9% sodium chloride solution?

   [1 mark]

   • AWater enters the cells faster than it leaves
   • BNo water molecules cross the membrane
   • CThere is no net movement of water, because the concentration of free water molecules is the same inside and outside the cells
   • DWater leaves the cells and they shrink

   (b)

   Which statement describes what happens to red blood cells in 3.0% sodium chloride solution?

   [1 mark]

   • AFree water molecules move into the cells, down their concentration gradient
   • BSodium and chloride ions move into the cells by osmosis
   • CThere is no net movement because the membrane is partially permeable
   • DFree water molecules move out of the cells by osmosis, down their concentration gradient

   (c)

   Explain why the red blood cells in pure water swell and burst.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A root hair cell contains nitrate ions at a concentration of 40 mmol dm⁻³. The nitrate concentration in the surrounding soil water is 5 mmol dm⁻³. When a student adds a respiratory inhibitor to the solution around the roots, the uptake of nitrate ions falls to almost zero.

   (a)

   Which process moves nitrate ions into the root hair cell?

   [1 mark]

   • AActive transport by carrier proteins
   • BFacilitated diffusion through channel proteins
   • COsmosis
   • DSimple diffusion through the phospholipid bilayer

   (b)

   What is the role of ATP in the uptake of nitrate ions?

   [1 mark]

   • AIt binds to a nitrate ion and carries it across the membrane
   • BIt is hydrolysed to release energy, which changes the shape of the carrier protein
   • CIt forms a channel through which nitrate ions diffuse
   • DIt is made from nitrate ions inside the cell

   (c)

   Use the information to explain why the student can conclude that nitrate ions are taken up by active transport.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Researchers investigated the uptake of three substances, X, Y and Z, by a type of cell. For X and Y, the rate of uptake was measured at external concentrations of 0, 2, 4, 6, 8 and 10 mmol dm⁻³. The rates of uptake of X were 0, 3, 6, 9, 12 and 15 arbitrary units. The rates of uptake of Y were 0, 8, 14, 18, 19 and 19 arbitrary units. Adding a respiratory inhibitor did not change the uptake of X or Y. Substance Z, however, was found inside the cells at a higher concentration than outside, and its uptake stopped when the respiratory inhibitor was added.

   (a)

   Describe how the rates of uptake of X and Y change as the external concentration increases, and explain the pattern for Y.

   [3 marks]

   (b)

   Explain how the movement of substance Z across the membrane differs from the movement of substance Y.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Cells use several methods to move substances across their membranes. A pancreatic cell secretes insulin, a large protein hormone, into the blood. A phagocyte takes in whole bacteria. A muscle cell takes up oxygen and glucose from the blood, and it pumps sodium ions out of the cell, against their concentration gradient.

   (a)

   Explain how insulin and bacteria cross the cell surface membrane, and why they cannot cross by the methods used by oxygen and glucose.

   [6 marks]

   (b)

   Explain how oxygen, glucose and sodium ions each cross the membrane of the muscle cell, and compare the energy requirements of the three methods.

   [6 marks]

   Total for question 4: 12 marks

End of questions