The problem is shown below. I don't understand how can the tonicity of a solution be determined if the solution has multiple solutes dissolved in it.

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Do I take the total sum of the mass of solutes and divide it by the volume of the solution in both outside and inside of the cell (total concentration) and determine the osmosis direction based on whichever is greater? Or should it be done in another way?

If it could be of use, here are the questions that were asked below the picture and their answers. This question was taken out of a test-bank for the 8th edition of the Campbell biology textbook.

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1 Answer 1


Osmotic pressure for non-electrolytic solutes is given by

$$\pi = CRT$$

where $C$ is the effective concentration of all the solutes. In our case, with multiple solutes, we simply add all their concentrations to obtain the effective concentration. This gives us

$$ \begin{align} \pi_\mathrm{cell} &= 0.05RT\\ \pi_\mathrm{environment} &= 0.03RT \end{align} $$

Since $\pi_\mathrm{environment} < \pi_\mathrm{cell},$ The environment solution is hypotonic.

  • $\begingroup$ Thanks for the answer, but the problem states that the membrane is permeable to fructose and glucose but not to sucrose; The 2 simple sugars should balance their concentration by diffusion only, and sucrose's is balanced by osmosis. Therefore, I speculate that the tonicity of the solution should be determined based on only the solutes which cannot pass through the membrane; Needless to say, osmosis wouldn't occur if the membrane was permeable to all solutes. Could this be the case? $\endgroup$
    – Saif Taher
    Jan 8, 2020 at 7:51

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