SAT Subject Biology Practice Question 368: Answer and Explanation
18. 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.
Why does the mass of Tube 1 remain relatively unchanged throughout the experiment?
A. The dialysis tubing in Tube 1 is defective and does not allow water to cross.
B. There is no concentration gradient to drive the movement of sucrose.
C. The dialysis tubing broke, allowing the tube contents to mix with the beaker contents.
D. There is no concentration gradient to drive the movement of water.
E. The experimenter failed to record the data properly.
Correct Answer: D
D Because movement across the membrane relies strictly on concentration gradients, the fact that there is no gradient in Tube 1 would prevent the movement of water into or out of the tube. Thus there would be no change in mass.