Question

1. NaCl is a non-penetrating solute and urea is a penetrating solute for cells. The intracellular concentration of non-penetrating solute is 300 OsM. Red blood cells are placed in each of the solutions below. Please label each solution with all terms that apply. Assume 1M NaCl = 2 OsM for simplicity.

150mM NaCl + 100 mM Urea is osmolarity and tonicity

This is the answer but i dont know how... please explain----> 150mM NaCl + 100 mM Urea is (Hyperosmotic) osmolarity and (Isotonic) tonicity

Answer 1

NaCl concentrations in cell and solution are equal initially (hence, Isotonic). Water will be left out
in the cell at first because it high osmolarity outside the cell (and urea).
There is a concentration difference for urea, it will enter from a region of higher concentration
to lower concentration region of the cell resulting in increased osmolarity. This is the reason why
it has hyperosmotic osmolarity. One of the consequences of this process is that urea along with
water will be present in the cell and hence eventually it is observed that there will be no change
in cell size. At equilibrium you should remember that there will be no gradient or difference and
  hence there will be no transfer of water.