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Osmosis and cell volume

Let's look at the net movement of water and the effect of the cell wall on cell volume.

    B02 · Osmosis and cell volumeReady
    Let's look at the net movement of water and the effect of the cell wall on cell volume. You can also experiment with the numbers and controls below.

    Life sciences, cells, and material movement / principles and exploration

    Osmosis and cell volume — principles and investigation

    As the concentration of the solution surrounding the cell changes, the net movement of water changes on either side of the membrane that selectively allows water to pass through. Animal and plant cells are placed in a hypotonic, isotonic, and hypertonic solution and the volume changes are compared. Take a look at how the presence or absence of a cell wall makes a difference in hemolysis, turgor, and plasmolysis.

    How does it work?

    In a hypertonic solution, net water loss shrinks the cell. In a hypotonic solution, net water uptake enlarges it. Animal cells can undergo hemolysis; plant cells develop turgor pressure.

    Key relationship

    Ideal equilibrium V/V₀ = Cᵢₙ,₀/Cₒᵤₜ

    Try this investigation

    1. Water moves in both directions. A concentration difference causes net water movement and changes the cell volume. Changing the initial internal concentration also changes which external solution is hypotonic, isotonic, or hypertonic.
    2. Run and record isotonic and hypertonic solutions of red blood cells and compare relative volumes.
    3. Compare animal and plant cells in a hypotonic solution. For plant cells in a hypertonic solution, observe plasmolysis: the cell membrane pulls away from the cell wall.

    Interpreting your results

    Why do plant cells not grow infinitely in hypotonic solution?

    Assume a membrane that only allows water to pass through. In plants, the cell wall and turgor pressure oppose the net influx of water, and this model limits V/V₀ to 1.15.

    Assumptions and limitations

    It is a dilute solution model in which only water passes through and the number of moles of solute is constant. Net migration approaches equilibrium exponentially. V/V₀>1.6 for red blood cells is the virtual hemolysis limit, after which volume calculation and water transfer stop. Plants use an artificial upper limit of 1.15. Turf pressure is a relative value proportional to the difference in concentration and is not actual pressure. It cannot be used for quantitative prediction of actual cells.