The solutions in the two arms of this U-tube are separated by a membrane that is permeable to water and glucose but not to sucrose. Side A is half-filled with a solution of 2 M sucrose and 1 M glucose. Side B is half-filled with 1 M sucrose and 2 M glucose. Initially, the liquid levels on both sides are equal.

-Initially, in terms of tonicity, the solution in side A with respect to that in side B is
A) hypotonic.
B) plasmolyzed.
C) isotonic.
D) saturated.
E) hypertonic.

Question

The solutions in the two arms of this U-tube are separated by a membrane that is permeable to water and glucose but not to sucrose. Side A is half-filled with a solution of 2 M sucrose and 1 M glucose. Side B is half-filled with 1 M sucrose and 2 M glucose. Initially, the liquid levels on both sides are equal.
 
-Initially, in terms of tonicity, the solution in side A with respect to that in side B is
A) hypotonic. 
B) plasmolyzed. 
C) isotonic. 
D) saturated. 
E) hypertonic.

Quizplus · Accepted Answer

Initially, the concentration of solutes on both sides of the membrane is equal, so the solutions are isotonic. The fact that the membrane is permeable to both water and glucose means that both of these molecules will move down their concentration gradients to try to reach equilibrium on both sides of the membrane. The fact that the membrane is impermeable to sucrose means that the concentrations of sucrose will remain unequal on both sides of the membrane. Over time, water and glucose will move from side B to side A, resulting in a higher concentration of solutes on side B and a lower concentration on side A. This will eventually reach equilibrium, but at that point the concentrations of glucose and sucrose on both sides of the membrane will be different, so the solutions will not be isotonic anymore. However, at the initial point in time described in the question, the solutions are isotonic.

The Solutions in the Two Arms of This U-Tube Are