When a human red blood cell is placed in distilled water it will:

• A. Swell up and eventually burst
• B. Shrink in size
• C. Stay the same size
• D. Actively pump water out of the cell

Answer: (A)

Osmosis drives water across a semi-permeable membrane from a region of low solute concentration to a region of higher solute concentration. Distilled water has very few solutes, while the interior of a red blood cell contains dissolved salts and proteins, so the outside is hypotonic relative to the cell. Water therefore moves into the cell, causing it to swell; if too much water enters, the membrane can rupture. The cell isn’t actively pumping water out against the gradient, and in this situation the influx continues because the gradient favors entry of water.