Question

When RBCs are kept in isotonic NaCl solution 

• A. There will not be any movement of solutes across the RBC membrane
• B. The RBC shape and size will not change
• C. Because the osmotic pressure across the membrane is same the solutes will not cross across the RBC membrane
• D. All of these

Answer 1

The Correct Option is B

When considering the behavior of red blood cells (RBCs) in an isotonic NaCl solution, it is important to understand the concept of tonicity and osmotic pressure.

• Isotonic Solution: An isotonic solution has the same osmotic pressure as that inside the cell, meaning the solute concentration outside the cell is equal to that inside the RBC.
• Osmotic Pressure: The pressure required to prevent water from diffusing across a semipermeable membrane. In an isotonic solution, there is no net movement of water into or out of the cells, hence the osmotic pressure is balanced.

1. Since the concentration of solutes is equal inside and outside the RBC, there will be no net movement of water through the RBC membrane. This means there will be no change in cell size or shape.
2. Given no net movement of water, the volume of the RBC remains constant, ensuring that the cell neither swells nor shrinks.
3. The RBC shape and size will not change. This option accurately describes the effect of isotonic solutions on cells and is the correct answer to the question.

Other options can be addressed as follows:

• There will not be any movement of solutes across the RBC membrane: While the net movement is zero due to equilibrium, individual solute molecules may still move randomly across the membrane.
• Because the osmotic pressure across the membrane is the same the solutes will not cross across the RBC membrane: The osmotic pressure is balanced, but, similarly to the previous point, solutes can still move across the membrane but with no net change in concentration.

Therefore, the correct answer is that the RBC shape and size will not change, as the isotonic solution creates equilibrium conditions for the cell.