From Costanzo's Physiology:

Osmosis of water is not diffusion of water: Osmosis occurs because of a pressure difference, whereas diffusion occurs because of a concentration (or activity) difference of water.

Also from Costanzo, describing two solutions 1, and 2 , seprated by a semipermeable membrane, and 1 having a higher concentration of solute:

The solute in Solution 1 produces an osmotic pressure and causes, by the interaction of solutes with pores in the membrane a reduction in hydrostatic pressure of Solution 1. The resulting hydrostatic pressure difference across the membrane then causes water to flow from Solution 2 into Solution 1.

My question is:

How does the solute produce an osomtic pressure, and what type of interactions with the membrane pores causes the reduction in hydrostatic pressure that ultimately drive water movement?


3 Answers 3


Osmotic pressure

  • is consequence of non zero net water diffusion,
  • which is consequence of non equal water activities on both sides of semipermeable membrane,
  • which is consequence of the fact dissolved solutes decrease activity of water.

Osmotic pressure of a free solution is formally an external pressure needed to be acting on this solution to keep diffusion equilibrium with pure solvent on the other side of a semipermeable membrane.

Applying external pressure then leads to the well known reverse osmosing, widely used in production of deionized water ( replacing expensive distillation ) from sweet water, or in desalination of sea water.

Osmosis continues until fully counteracted by extenal, e.g. atmospheric pressure. E.g. if there is osmotic pressure $\pu{1 atm}$, osmosis leads up to $\pu{10 m}$ high solution column ( eventually corrected by solution density ). Similarly, you would need to use external pressure $\pu{p > 1 atm}$ to start reverse osmosis.

If a book says the primary reason is hydrostatic pressure difference, it is wrong. We can have 2 solutions of the same density, but different osmotic pressures. Combining them on the opposite membrane sides would lead to disbalancing of solution levels, even if they would be initially the same and even if the hydrostatic pressure difference would be zero.

  • $\begingroup$ What about the hydrostatic pressure? $\endgroup$
    – Positron12
    Sep 30, 2020 at 7:11
  • $\begingroup$ But the book states that the hydrostatic pressure difference is what drives water flow, how is the hydrostatic pressure difference generated at first? $\endgroup$
    – Positron12
    Sep 30, 2020 at 7:23
  • $\begingroup$ @Positron12 The answer has been updated. $\endgroup$
    – Poutnik
    Sep 30, 2020 at 7:30
  • $\begingroup$ Thank you. Could you please give a look at this question chemistry.stackexchange.com/questions/140823/… @Poutnik $\endgroup$
    – Positron12
    Sep 30, 2020 at 7:36

In addition to Poutnik's answer, I would like to add some comments on the question:

The first statement 'Osmosis of water is not diffusion of water:....' is incorrect.

The cause of osmosis is simply diffusion: the solvent is able to diffuse through the semi-permeable membrane the solute is not. It is wrong to assume that diffusion occurs only in the direction of decreasing concentration it is rather the difference in chemical potential that drives this.

Statement 2 is also incorrect. There is no special interaction with the solute and the semi-permeable membrane that causes osmosis, other than that the solute is too big to pass through it but the solvent is not.

The pressure difference is not what causes osmosis but it is the equalisation of the chemical potential as equilibrium is reached. The pressure difference is a result of this process.

A solution made up with an involatile solute (say a protein) has a lower vapour pressure than that of the pure solvent due to the fact that some of the solution volume is now taken up with the solute. In thermodynamic terms this means that the chemical potential of the solution is less than that of the pure solvent (i.e Raoult's Law). To restore chemical equilibrium the chemical potentials have to become equal and this is achieved by solvent diffusing into the solution thereby diluting it (i.e. trying to make it pure solvent and so increase its chemical potential). The consequence is that the volume of the solution increases (e.g. as seen by pushing solution up a tube) and at a certain hydrostatic pressure, equilibrium is restored.

  • $\begingroup$ Regarding the first statement, the way I understood this -and correct me if I'm wrong- is that diffusion of water occurs when there's a difference in the amount of water between two solutions of constant concentration of solute, whereas osmosis occurs when the solute concentration is different, even if two solutions have the same amount of the solvent (water). @porphyrin $\endgroup$
    – Positron12
    Sep 30, 2020 at 16:51
  • $\begingroup$ If you have 'constant concentration of solute' the two solutions are the same no matter what the volume. Osmosis occurs only via a membrane that is permeable only to solvent. Its not the amount of pure solvent that is important but it is the concentration of solute in solution on the other side of the membrane. Diluting this tries to equalise the chemical potential. $\endgroup$
    – porphyrin
    Sep 30, 2020 at 17:54
  • $\begingroup$ "The diffusion model of osmosis is rendered untenable by the fact that osmosis can drive water across a membrane toward a higher concentration of water" - Wikipedia What are your thoughts ? @porphyrin $\endgroup$
    – Positron12
    Sep 30, 2020 at 18:29
  • $\begingroup$ Well, it is an experimental fact that the water does go into the higher concentration solution, call this diffusion or call this migration it makes no odds. There is no other way it can occur. It is the equalisation of chemical potential that drives the process. $\endgroup$
    – porphyrin
    Oct 1, 2020 at 7:23
  • $\begingroup$ continued... of course at equilibrium there is no net flow of water either way. Also as mentioned in my answer 'It is wrong to assume that diffusion occurs only in the direction of decreasing concentration it is rather the difference in chemical potential that drives this.' $\endgroup$
    – porphyrin
    Oct 1, 2020 at 7:49

How does the solute produce an osomtic pressure?

The solute creates a difference in the chemical potential of the solvent in two chambers separated by a selectively permeable membrane that allows passage only of the solvent. The system changes such as to reduce the imbalance in chemical potential and the only way for this to happen is by reducing the concentration of the solute in the high potential chamber through solvent diffusion from one chamber into the other.

Note that at the molecular level diffusion is a process that occurs all of the time in all fluid substances due to random molecular motion and irrespectively of chemical potential.

What type of interactions with the membrane pores causes the reduction in hydrostatic pressure that ultimately drive water movement?

In the case of osmosis, solvent movement is not caused by hydrostatic pressure. Rather the opposite. Increasing hydrostatic pressure in the more concentrated chamber increases the chemical potential of the solvent in the chamber, leading eventually to equilibrium.

The membrane is a selective filter. Interactions of the solute with the membrane impairs solute passage (the pores allow only solvent to pass), sustaining the concentration gradient that drives osmosis.


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