The membranes of red blood cells (RBCs) are permeable to water and urea, but not to sucrose. Osmosis would cause the RBCs to shrink when immersed in which of the following solutions (once they come to equilibrium)?
a) A hypertonic sucrose solution
b) A hypertonic urea solution
c) A hypotonic sucrose solution
d) A hypotonic urea solution
e) Pure water
The correct answer and explanation is:
The correct answer is a) A hypertonic sucrose solution.
Here’s why: The key to this question lies in understanding the effects of osmotic pressure and the behavior of the specific solutes involved—water, urea, and sucrose—and their permeability through the red blood cell (RBC) membrane.
Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Red blood cell membranes allow the passage of water and urea, but they do not allow sucrose to pass through.
In a hypertonic sucrose solution, the solution outside the RBC has a higher concentration of sucrose compared to the RBC’s interior. Since sucrose cannot enter the RBC, this creates a situation where the solution outside is more concentrated than the fluid inside the RBC, leading to an osmotic gradient. Water will leave the RBC to balance the osmotic pressure, causing the cell to shrink or crenate. This is the typical response when a cell is placed in a hypertonic solution that it cannot equalize by taking in the solute.
On the other hand, in solutions involving urea (whether hypotonic or hypertonic), urea can move freely across the RBC membrane. In a hypertonic urea solution, urea would enter the RBC to equilibrate the solute concentrations, and water would follow. If the urea concentration is very high, water might leave the cell to balance the concentrations, but the key difference is that urea can cross the membrane, unlike sucrose.
In hypotonic solutions (whether sucrose or urea), the concentration of solute outside the RBC is lower than inside, so water would enter the cell, causing it to swell rather than shrink. Pure water would have the same effect, as the cell would gain water and potentially burst.
Thus, a hypertonic sucrose solution is the correct choice for causing RBCs to shrink.