Transport across membranesEdexcel A-Level Biology A: Flashcards

Exam questions on Transport across membranes

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.
   Explain why the red blood cells in pure water swell and burst.2 marks
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.
   Use the information to explain why the student can conclude that nitrate ions are taken up by active transport.2 marks
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.
   Describe how the rates of uptake of X and Y change as the external concentration increases, and explain the pattern for Y.3 marks