SAT Subject Biology Practice Question 370: Answer and Explanation

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Question: 370

20. Dialysis tubing is a semipermeable membrane. It allows small molecules, such as water, to pass through easily, while larger molecules, such as sucrose, are restricted. Movement of molecules across the tubing is due to concentration gradients. In an experiment designed to study osmosis, several pieces of dialysis tubing were filled with sucrose solutions of varying concentration and placed in beakers containing distilled water. The rate and direction of water movement was determined by weighing the bags before and after placing them in the distilled water. The data are recorded below.

Table 1

Cell membranes are also semipermeable, allowing water but not other substances to cross easily. A red blood cell placed in a 0.9% NaCl solution will neither swell nor shrivel. Based on this knowledge, and the information presented in Table 1, what would happen to a red blood cell placed in a 20% NaCl solution?

A. Water would be drawn out of the cell and the cell would swell.
B. Water would be drawn into the cell and the cell would swell.
C. Water would be drawn out of the cell and the cell would shrivel.
D. Water would be drawn into the cell and the cell would shrivel.
E. No change would occur to the cell.

Correct Answer: C

Explanation:

C The fact that the cell does not swell or shrivel in 0.9% NaCl implies that there is no concentration gradient. A cell in a 20% NaCl solution would experience similar stresses to a dialysis tube filled with water sitting in a beaker filled with a more concentrated solution, such as Tube 4. The data indicate that Tube 4 lost mass, thus water exited the tube, and the same would happen to the red blood cell.

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