Answer to Question #196392 in Cell Biology for LeighAnna D Miller

Question

Imagine a patient has been given a solution containing 100mOsm urea and 200mOsm glucose.

Which of the solutes in the solution are penetrating or nonpenetrating?

Determine tonicity of the solution.

Answer 1

 As a general rule, urea and dissolved gases such as O2 and CO2 can diffuse across cell membranes, while glucose or ions in solution cannot. The tonicity is the same as the osmolarity except that it only measures solutes that cannot diffuse across the membrane.

Tonicity depends on the relative concentrations of nonpenetrating solutes in the solution and cell. Thus you can predict that the cell will become more concentrated, either because water moves out into the solution, or because penetrating solute moves into the cell.

In this case, the solution has 200-milliosmolar concentration of glucose and a 100-milliosmolar concentration of urea, we would exclude the urea since it can diffuse across the cell membrane, so the solution would be 200-milliosmolar for purposes of tonicity.