MCAT Biology Question — Red Blood Cell | Next Step Test Prep MCAT Biology Question — Red Blood Cell | Next Step Test Prep

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A red blood cell (RBC) placed in distilled water will burst because:

 

a) osmotic pressure pulls water through the membrane to the interior of the cell.

 

b) the electrochemical gradient favors the inside of the RBC.

 

c) non-polar molecules cross the cell membrane into the water down their concentration gradient.

 

d) solute molecules rapidly diffuse into the water exterior to the RBC.

Explanation

A red blood cell (RBC) placed in distilled water will burst because the osmotic pressure causes a rapid influx of water. In fact, so much water will enter the cell that the RBC will burst. This is an extreme case of the RBC being placed in a severely hypotonic solution. Conversely, when placed in hypertonic solution, the RBC will shrivel as water diffuses out.

 

a) osmotic pressure pulls water through the membrane to the interior of the cell. Correct.

 

b) the electrochemical gradient favors the inside of the RBC. Incorrect, Osmolarity, osmotic pressure, and water diffusion have nothing to do with the electrochemical gradient.

 

c) non-polar molecules cross the cell membrane into the water down their concentration gradient. Incorrect, water diffuses across the cell membrane to cause the cell to burst. If solutes crossed the membrane to create an isotonic environment, the cell would not burst.

 

d) solute molecules rapidly diffuse into the water exterior to the RBC. Incorrect, See explanation c).

 

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