Red blood cells with an internal osmolarity of 300 mOsM are placed in the following solutions. Designate each solution according to its osmolarity and tonicity, and explain what happens to the cells and why.

Question

A. 200 mOsM NaCl
B. 400 mOsM urea
C. 100 mOsM urea plus 200 mOsM NaCl
D. 300 mOsM urea
E. 300 mOsM NaCl
F. 200 mOsM urea plus 300 mOsM NaCl
G. 400 mOsM NaCl

Answer 1

A. Hyposmotic, hypotonic. The cell swells. By Rule 5 in Table 5.8 in the chapter, hyposmotic solutions are always
hypotonic, because the intracellular solutes are mainly nonpenetrating (Rule 1) thus there will be a net flow of
water into the cell.
B. Hyperosmotic, hypotonic. The cell swells. Urea is a penetrating solute, so some urea will move into the cell down
its concentration gradient. This will increase the osmolarity inside the cell, causing a net flow of water into the cell.
C. Isosmotic, hypotonic. The cell swells. Urea is a penetrating solute, so there will be a net movement of urea into
the cell, raising the osmolarity and causing a net flow of water into the cell.
D. Isosmotic, hypotonic. The cell swells. Urea will penetrate the cell, raising the osmolarity and causing a net flow
of water into the cell.
E. Isosmotic, isotonic. No change in cell size. Sodium and chloride are nonpenetrating solutes, so there will be no
net ion flow across the membrane. Because there is no osmotic pressure, there will also be no net flow of water.
F. Hyperosmotic, isotonic. No change in cell size at equilibrium. Initially water leaves the cell due to the higher
osmolarity outside the cell. Then, because there is a concentration gradient for urea, urea will enter the cell,
increasing its osmolarity, and bringing some water into the cell. The nonpenetrating solute concentrations in cell
and solution initially are equal, therefore there will be no net movement of water at equilibrium.
G. Hyperosmotic, hypertonic. The cell shrinks. There are no penetrating solutes, and water exits due to the higher
osmolarity.