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MCAT Question A Day - 7/26/13 - Answer!

Cell A is hypotonic to Cell B. In which direction will there be more of a tendency for water to move between these cells?

A.  A to B
B.  B to A
C.  Equal net movement in both directions
D.  No net movement in either direction

The correct answer is (A). If Cell A is hypotonic to Cell B, it has less solutes than Cell B. With less solutes, Cell A must have more water than Cell B. Therefore, water will move (by osmosis) from where there is more water to where there is less water (A to B). Choice (C) only applies after the percentages of water have equalized in both cells. Choice (D) is never true because molecules are constantly in random motion based on their own kinetic energy.