Sucrose and NaCl cannot cross the plasma membrane while Urea can. Given this inf

Question

Sucrose and NaCl cannot cross the plasma membrane while Urea can. Given this information and assuming that the osmolarity of a red blood cell (RBC) is 300 mOsm, what is the tonicity of the following solutions (Hyper, Hypo or iso) relative to a RBC. (Assume for any salt, e.g., NaCl, that there is complete dissociation.) DO 400 mmol/L urea Choose... + 300 mmol/L sucrose +300 mmol/L urea Choose... + 200 mmol/L sucrose + 50 mmol/L. NaCl + 50 mmol/L urea Choose... + Sucrose and NaCl cannot cross the plasma membrane while Urea can. Given this information and assuming that the osmolarity of a red blood cell (RBC) is 300 mOsm, what is the direction of net water movement when a RBC is placed in the following solution (Into cell, Out of cell, No net movement)? (Assume for any salt, e.g., NaCl, that there is complete dissociation.) 400 mmol/L urea Choose... 300 mmol/L sucrose +300 mmol/L urea Choose... 200 mmol/L. sucrose + 50 mmol/L NaCl + 50 mmol/L urea Choose...

Explanation / Answer

Ans 1: given, osmolarity of red blood cells = 300mOsm

tonicity counts only non permeable solutes, but it is given that membrane is permeable to urea, so for urea it will be isotonic. 300mmol/L is given concentration of sucrose which equal to the omolarity of RBC so its also isotonic, in this case also we are not taking urea in count and in last option 200mmol/L + 50mmol/L of NaCl, NaCl will dissociate into Na+ and Cl- ions so it will be equal to 100mOsm, So total in this case will be 300mOsm, so it is isotonic.

Ans 2:

for 400 mmol/L urea no movement because it can cross membrane

for 300 mmol/L sucrose + 300 mmol/L urea, no net movement because for sucrose it is isotonic and for urea it permeable.

for 200mmol/L sucrose + 50mmol/L NaCl + 50 mmol/L urea, no net movement as explained in Ans: 1

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