Biology: Solutions equilibrium

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Discussion Overview

The discussion revolves around a homework problem involving a U-tube with solutions separated by a differentially permeable membrane. The focus is on the behavior of sodium chloride (NaCl) and glucose concentrations over time, particularly addressing the implications of the membrane's permeability and the resulting osmotic pressures.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that side A will experience a decrease in NaCl concentration, an increase in glucose concentration, and a decrease in water level, questioning the reasoning behind these changes given the membrane's properties.
  • Another participant suggests that since the membrane is permeable to NaCl, the concentrations on both sides would equilibrate, potentially negating osmotic pressure from NaCl, but raises uncertainty about the glucose side's level changes.
  • A third participant asserts that all changes occur in side A, implying a focus on that side's dynamics.
  • One participant emphasizes the principle of mass preservation, suggesting that changes in solution levels must occur consistently across both sides of the U-tube.
  • Another participant reiterates the question of why glucose levels change if the membrane is impermeable to it, prompting a deeper exploration of what substances can move through the membrane and how this affects concentration differences.

Areas of Agreement / Disagreement

Participants express differing views on the changes in solution levels and concentrations, particularly regarding the implications of the membrane's permeability. There is no consensus on the outcomes for glucose levels or the water levels in the U-tube.

Contextual Notes

The discussion highlights uncertainties regarding the movement of substances through the membrane and the resulting osmotic pressures, with participants questioning the assumptions underlying the problem without resolving these issues.

future_vet
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Homework Statement


The solutions in the arms of a U-tube are separated at the bottom of the tube by a differentially permeable membrane. The membrane is permeable to sodium chloride but not to glucose. Side A is filled with a solution of 0.4 molar glucose and 0.5 molar sodium chloride (NaCl) and side B is filled with a solution containing 0.8 molar glucose and 0.4 molar sodium chloride. Initially the volume in both arms is the same. If you examine side A after three days, you should find...?

The Attempt at a Solution


The solution is: a decrease in the concentration of NaCl, an increase in the concentration of glucose and a decrease in the water level.

However, since the membrane is not permeable to glucose, why does the glucose level change? Why does the water level decrease?

Thanks!
 
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You didn't mention which arm of the U tube would see a decrease in level.

I would think that since the membrane is permeable to NaCl that the concentration of NaCl in both sides of the U tube would equilibrate and there would be no osmotic pressure from that component. The glucose side would exert an osmotic pressure since it cannot cross the membrane but it is not clear, from the answer you gave, if the glucose side would see an increase in level or a decrease in level. You can figure that out, I'm sure.
 
All the changes are in side A..
 
You have either changes of the solution level on both sides or on none. This is simple mass preservation.
 
future_vet said:
However, since the membrane is not permeable to glucose, why does the glucose level change? Why does the water level decrease?
Let's try backing up a step in the thought process and see if these questions help you answer the above questions. Besides NaCl, what else in the tube can move through the membrane? And, how does that "other" substance moving through the membrane change if concentrations of either NaCl or glucose differ on either side of the membrane?
 

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