Explain what happens when an RBC is placed into distilled water and 5% salt solution and tell which are hypertonic and hypotonic to the cell.

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bandmanjoe | Middle School Teacher | (Level 2) Senior Educator

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To start with, the terms "hypertonic and hypotonic" refer to the salinity of the solution with respect to the internal environment of the cell.  Most cells are slightly saline, less than 1%.  So there is a small salt concentration in the internal environment of the cell.  If you place the cell in distilled water, there is 0% salt concentration in the distilled water.  The water will diffuse by osmosis across the cell membrane into the cell, in an attempt to equalize the water content inside and outside the cell.  The result will be the RBC will swell and burst, causing the cell to die.  The distilled water would represent a hypotonic solution.  On the other hand, the 5% saline solution would represent a hypertonic solution.  Placing the RBC in this solution would cause the opposite effect, having water diffuse by osmosis out of the cell into the 5% saline solution, all in an attempt to equalize the water concentration inside the cell and out.  The result would be the RBC would wither and die.  This is why you can not drink salt water from the ocean as a substitute for fresh water, because it causes the cells in your body to dehydrate.

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