2.7 Tonicity and Osmoregulation
- Syllabus
- 2025
- Topic
- 2.7
- Level
- —
Learning objectives
2.7A—Explain how concentration gradients affect the movement of molecules across membranesExplain how concentration gradients affect the movement of molecules across membranes.• External environments can be hypotonic, hypertonic, or isotonic to internal environments of cells. Movement of water can also be described as moving from hypotonic to hypertonic regions. Water moves by osmosis from regions of high water potential to regions of low water potential. Relevant equation: Water Potential: ps where: p = pressure potential s = solute potential2.7B—Explain how osmoregulatory mechanisms contribute to the health and survival of organismsExplain how osmoregulatory mechanisms contribute to the health and survival of organisms.• Growth and homeostasis are maintained by the constant movement of molecules across membranes.• Osmoregulation maintains water balance and allows organisms to control their internal solute composition and water potential. Water moves from regions of low osmolarity or solute concentration to regions of high osmolarity or solute concentration. Relevant equation: Solute Potential of a Solution: s=−iCRT where: i = ionization constant C = molar concentration R = pressure constant R Lb ars molK0.0831 = T = temperature in Kelvin (°C + 273)
Tonicity Predicts the Direction of Osmosis
Tonicity compares an external solution with a cell's internal environment. Across a membrane permeable to water, osmosis produces net water movement from higher water potential to lower water potential—often described as movement from a hypotonic region toward a hypertonic region.
| External environment relative to cell | Relative external solute concentration | Predicted net water movement |
|---|---|---|
| Hypotonic | Lower | Into the cell |
| Isotonic | Equal | No net movement |
| Hypertonic | Higher | Out of the cell |
\Psi=\Psi_p+\Psi_s
Ψ is total water potential, Ψp is pressure potential, and Ψs is solute potential. Compare total Ψ on both sides: water moves toward the side with the lower value. Pressure can therefore affect direction as well as solute concentration.
Hypotonic and hypertonic are relative terms, not fixed properties of a solution. Name what the solution is being compared with before predicting water movement.
Osmoregulation Stabilizes Water and Solute Balance
Osmoregulation maintains water balance by controlling an organism's internal solute composition and water potential. Constant movement across membranes supports growth and homeostasis, but unregulated gain or loss of water would disrupt cell volume and function.
Water moves from lower osmolarity (lower solute concentration) toward higher osmolarity (higher solute concentration), which corresponds to movement from higher water potential toward lower water potential. Osmoregulatory mechanisms alter solute handling, water movement, or both so internal conditions remain within a survivable range.
\Psi_s=-iCRT
For a supplied solution, substitute the ionization constant i, molar concentration C in molL−1, pressure constant R=0.0831Lbarmol−1K−1, and temperature T in kelvin (∘C+273). The negative result is Ψs in bars; increasing i, C, or T makes solute potential more negative and lowers total water potential if pressure potential is unchanged.
Water does not move toward 'more water.' It moves down a water-potential gradient; solute concentration and pressure both contribute to that gradient.