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Osmosis is the passage of water from a region of high water concentration, through a semi-permeable membrane, to a region of low water concentration.

This is a passive processes of membrane transport.

Equivalently, osmosis is the way that some particles (like dissolved salt) flow towards water.


Semi-permeable membranes are very thin layers of material – that allow some things to pass through, but prevent other things from passing through.

Cell membranes are semi-permeable membranes.  They allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but they do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.

Osmosis cells

Below we see two compartments, separated by a semi-permeable membrane.

There are more water molecules on one side than on the other.

Molecules flow from the side with a high concentration of water, to the side with a lower concentration.

This continues until the concentration of water on both sides is equal (dynamic equilibrium)

Osmosis 2


Adding sugars to water results in a decrease in the water concentration (because the sugar molecules displace the water molecules)

osmosis water flows uphill

osmosis water flows uphill

Concentration of dissolved particles is super important in biology

solute conc. inside cell   <   solute conc. outside cell                    Hypertonic

solute conc. inside cell  =    solute conc. outside cell                    Isotonic

solute conc. inside cell    >  solute conc. outside cell                    Hypotonic

Osmosis three cases

Diffusion and Osmosis <- document



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