Question

The cell membranes of mammalian red blood cells are permeable to urea. If red blood cells are dropped into a solution of urea that is identical in osmotic pressure (isosmotic) to the cytoplasm of the cells, although the cells do not swell and burst as quickly as when they are dropped simply into pure water, they eventually swell and burst. Explain. Also discuss how you would design a solution into which red cells could be placed without ever swelling. (Hint: Think about whether urea will stay on the outside of the cells and the implications for osmotic pressures.)

Answer 1

For this exercise you must take into account that isotonic and isoosmotic are concepts that are used interchangeably (which is wrong) The term isoosmotico refers to there is the same number of particles in solution, which isotonic refers to that there is same concentration (the concentration does not necessarily depend on the number of particles) of particles in solution. The typical example of this phenomenon is because the liver stores glycogen instead of glucose. A glycogen molecule has the same osmotic level as a glucose molecule (which saves energy). In the following diagram I illustrate this phenomenon.

Now think

At the beginning of the process the erythrocytes do not have urea inside them. The urea begins to enter by the difference of concentrations between the external medium and the internal medium (facilitated diffusion) the erythrocytes have their own solutes inside them. Now, if urea enters, which has a certain affinity for water, it will enter through the membrane into the cell and the urea will pull water into the cell, so that in the end they swell and burst. So that the cells do not burst, a solution must be made that is not only isoosmotic but also that should be isotonic. Or If you want to keep the urea inside the solution, it would have to be a hypoosmotic but isotonic solution of urea.