MEDizzy - Which of the following best describes the changes in cell

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Membrane Physiology

Which of the following best describes the changes in cell volume that will occur when red blood cells (previously equilibrated in a 280-milliosmolar solution of NaCl) are placed in a solution of 140-millimolar NaCl containing 20-millimolar urea, a relatively large but permeant molecule?

ExplanationA solution of 140-millimolar NaCl has an osmolarity of 280 milliosmoles, which is iso-osmotic relative to “normal” intracellular osmolarity. If red blood cells were placed in 140-millimolar NaCl alone, no change in cell volume would occur because intracellular and extracellular osmolarities would be equal. The presence of 20-millimolar urea, however, increases the solution’s osmolarity and makes it hypertonic relative to the intracellular solution. Water will initially move out of the cell, but because the plasma membrane is permeable to urea, urea will diffuse into the cell and equilibrate across the plasma membrane. As a result, water will re-enter the cell, and the cell will return to its original volume.