Transport across membranes and osmosisEdexcel International A Level Biology: Subtopic test

1. 1
   A student places a cylinder of potato tissue in a large volume of concentrated sucrose solution. The water potential of the sucrose solution is −800 kPa and the water potential of the potato tissue is −500 kPa. The cell surface membranes of the potato cells are partially permeable.

   (a)

   What is the direction of net movement of water?

   [1 mark]

   • AFrom the potato tissue into the sucrose solution
   • BFrom the sucrose solution into the potato tissue
   • CThere is no net movement because the membrane is partially permeable
   • DWater does not move; instead sucrose moves out of the potato tissue into the solution

   (b)

   Which statement describes the situation when the tissue and solution have reached equilibrium?

   [1 mark]

   • AWater molecules no longer cross the membrane
   • BThe water potential of the tissue is still higher than that of the solution
   • CWater molecules still cross the membrane in both directions but there is no net movement
   • DSucrose molecules have moved into the cells by osmosis

   (c)

   Explain, in terms of free water molecules, why water moves in this direction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Root hair cells absorb nitrate ions from the soil. When the nitrate concentration in the soil water is higher than inside the cell, uptake is rapid. When the concentration in the soil water is lower than inside the cell, uptake still continues. When the root cells are treated with a respiratory inhibitor that stops them producing ATP, the uptake of nitrate ions at low soil concentration stops.

   (a)

   Which process explains the uptake of nitrate ions when the soil concentration is lower than the concentration in the cell?

   [1 mark]

   • AFacilitated diffusion
   • BActive transport
   • COsmosis
   • DSimple diffusion

   (b)

   Which statement about the carrier proteins involved in this uptake is correct?

   [1 mark]

   • AA carrier protein forms a water-filled pore that is permanently open
   • BA carrier protein can transport any ion that is present
   • CCarrier proteins are only found in the cytoplasm of the cell
   • DA carrier protein changes shape when it moves an ion across the membrane, using energy from ATP

   (c)

   Explain why the respiratory inhibitor stops the uptake of nitrate ions at low soil concentrations.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Pancreatic cells make the enzyme amylase, which is a large protein, and release it from the cell in vesicles. White blood cells called phagocytes take in whole bacteria. Neither amylase nor a bacterium crosses the cell surface membrane through a channel or carrier protein.

   (a)

   Describe how the pancreatic cell releases amylase and how a phagocyte takes in a bacterium.

   [3 marks]

   (b)

   Explain why neither the amylase nor a bacterium can cross the membrane through a protein channel or carrier, and why these processes need ATP.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student wants to estimate the water potential of potato tissue. She cuts six identical potato cylinders, blots each one and weighs it. She places one cylinder in each of six sucrose solutions (0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 mol dm⁻³) for 30 minutes, then blots and reweighs each cylinder. The percentage changes in mass, in the order of the solutions, were +12.4, +6.8, +1.5, −3.9, −8.7 and −13.2.

   (a)

   Explain the results and describe how they can be used to estimate the water potential of the potato tissue.

   [6 marks]

   (b)

   Evaluate the student's method and suggest improvements that would give a more reliable estimate of the water potential of the tissue.

   [6 marks]

   Total for question 4: 12 marks

End of questions