SAT Subject Biology Practice Question 8: Answer and Explanation

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Question: 8

3. A solute cannot move into a cell by crossing the phospholipid bilayer of the membrane. It can, however, enter through a membrane protein. The fact that solute molecules are entering the cell MOST LIKELY indicates that

A. energy is being expended by the cell
B. the solute is moving against its concentration gradient
C. the solute molecules are hydrophilic
D. the cell is in a hypotonic solution
E. solute concentration is higher inside than outside the cell

Correct Answer: C

Explanation:

The fact that solute molecules are entering a cell via a membrane protein does not necessarily mean that the cell is expending energy (active transport), that the solute is moving against its concentration gradient (which requires energy), or that the solute concentration is greater inside the cell than outside (implying that the solute is moving against its concentration gradient). If the cell were in a hypotonic solution, solutes would tend to move out of the cell while water moved into it. Facilitated diffusion uses membrane protein to allow polar, hydrophilic molecules or ions into the cell. These solutes cannot pass through the nonpolar, hydrophobic inner layer of the cell membrane.

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