osmosis: one construct, many systems

In physical chemistry, osmosis is the net movement of solvent through a semipermeable membrane from the side of lower solute concentration toward the side of higher concentration, driven by the difference in chemical potential of the solvent. Van't Hoff (1887) showed that the osmotic pressure needed to halt this flow follows a law formally like the ideal-gas law, tying a dilute solution's behaviour to the number of dissolved particles. The referent is the thermodynamic tendency of solvent to cross a selective barrier and the pressure that opposes it, and it is the same physics that later appears in a plant cell and in a drying fruit.[1]

Unit

The osmotic pressure, in pascals, that halts net solvent flow

Recovery means

Not applicable

Measured by

Determined by osmometry and predicted from solute molality via the van't Hoff relation

In biology the same construct governs how cells and tissues manage water: water crosses the partially permeable cell membrane along its osmotic gradient, swelling or shrinking the cell, which is why organisms regulate internal solute levels to keep that flow in check. Kedem and Katchalsky (1958) gave a thermodynamic treatment that separated true osmotic water flow from solute leakage through the same membrane, formalising how real, imperfect biological membranes behave. It is the identical solvent-across-a-barrier idea as in a chemist's osmometer, realised in a living system and measured through membrane water permeability and reflection coefficients.[2]

Unit

The net water flux across a living membrane under an osmotic gradient

Recovery means

Not applicable

Measured by

Characterised by the membrane's hydraulic permeability, the solute reflection coefficient, and observed cell volume change

Food scientists put the same mechanism to work: immersing fruit or vegetable pieces in a concentrated sugar or salt solution sets up an osmotic gradient across the cell membranes, so water leaves the food while some solute enters, partially dehydrating and sweetening or salting it before further processing. Rastogi, Raghavarao, Niranjan, and Knorr (2002) reviewed how this osmotic dehydration is driven and how the two-way mass transfer can be enhanced. It is the same osmotic construct as in chemistry and biology, here deliberately exploited as a gentle, low-heat way to remove water, measured by water loss and solute gain over time.[3]

Unit

The water loss and solute gain of the food over immersion time

Recovery means

Not applicable

Measured by

Followed by gravimetric water loss and solid gain, and by the water activity of the treated food