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Plasmolysis in Onion Cells

Middle schoolBiologyImpermeant-solute osmotic equilibrium with nonlinear wall stress and exponential volume relaxation

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About this experiment

Exchange the solution around a virtual onion-cell section and observe the protoplast withdraw from the wall or recover. Compare water potentials and distinguish the fixed wall from the changing cell contents.

Learning goals

  • Use solute concentration and pressure to compare water potentials.
  • Identify plasmolysis as loss of membrane contact and turgor.
  • Explain why rinsing must preserve the starting volume of a shrunken cell.

How to use it

  1. 1Run the starting concentrated-bath trial for ten minutes.
  2. 2Record protoplast volume, turgor and membrane contact.
  3. 3Rinse with water and verify that volume is continuous at the instant of exchange.
  4. 4Continue recovery and compare the cell and bath water potentials near equilibrium.

The model

Pressure and total osmolarity determine the simplified water potential.
A bath exchange changes the target equilibrium while preserving initial volume.

Assumptions

  • The wall volume is fixed. Model solute remains inside the membrane while water can move. Controls use total osmolarity, not salt formula-unit molarity.
  • Solute potential follows the ideal van't Hoff relation. Wall stress is zero below volume 0.7 and increases as k(V-0.7)/(1-V) above that threshold.
  • Equilibrium is found by balancing osmotic and wall pressures. Transient volume relaxes exponentially toward that equilibrium; this is a phenomenological approximation, not a resolved membrane-flow law.
  • Below 0.7 reference volume the membrane withdraws. Above it the membrane remains against the wall while internal volume and turgor change.
  • Changing the bath preserves current volume and restarts elapsed exchange time. Changing cell parameters begins a fresh water-equilibrated cell trial.

Try this in class

  1. Q1Why does turgor become zero before the fixed wall disappears?
  2. Q2Why can water enter a cell whose solute concentration is already lower than before rinsing?

Frequently asked questions

Is the osmolarity control identical to NaCl molarity?

No. Osmolarity counts dissolved particles. Salt dissociation makes a formula-unit molarity a different quantity.

Why does the membrane not shrink away while turgor is positive?

The geometry keeps the membrane in wall contact until the specified relaxed-volume threshold is crossed.