Osmolarity and tonicity
Total particle concentration and lasting cell-volume effect are different properties.
Key knowledge
Osmolarity counts osmotically active solute particles per solution volume. Tonicity describes a solution's lasting effect on cell volume relative to a specified membrane and timescale, principally through effectively nonpenetrating solutes. In the passive wall-less-cell model, a hypotonic external solution causes swelling, a hypertonic one shrinking, and an isotonic one no lasting volume change. Equal osmolarity alone need not imply equal tonicity.
Conditions and scope
- Specify the membrane, solute permeabilities and observation timescale; penetrating solutes can cause transient water movements without the same lasting effect.
- The simple comparison assumes passive exchange without an imposed pressure difference or active volume regulation. Cell walls and turgor add a mechanical pressure constraint.
Sources · 2
References
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Evidence and review
Inspected Biology §5.2's osmosis/tonicity and equal-pressure, nonpermeant assumptions, and Duke notes numbered pp.3–5 distinguishing all particles from nonpenetrating particles. Excludes universal glucose/urea permeability assumptions and instant-equilibrium claims.
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Entry details
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- foundation-bio-osmolarity-tonicity
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