OSMOSIS Osmosis is a form of passive transport where the solvent water molecules move from an area of high water concentration to an area of low water concentration across a semi-permeable cell membrane. The solutes can not pass through the membrane.

Direction of Water Flow The direction of osmosis, or water flow, depends on the water concentration [H 2 O] on both sides of the membrane and the inability of the solute molecules to pass the membrane. Which way will water flow? ICF – 90% water ECF – 95% water The water will flow into the cell because ICF [H 2 O] < ECF [H 2 O]

Calculating Water Concentration Sometimes the [H 2 O] must be calculated by subtracting, % [H 2 O] = 100% - solute%. Which way will water flow? Cell parts - 10% ECF – 85% water The water will flow out of the cell as ICF [H 2 O] = 90% and ECF [H 2 O] = 85%

And now which way will water flow, assuming that salt can not pass the semi-permeable membrane? Cell parts - 20% 60% salt solution Cell parts - 20% 60% salt solution What happens to the shape of the cell? From inside ICF [H 2 O] = 80% > ECF [H 2 O] = 40% The cell will shrink.

TONICITY Tonicity is a term to describe what happens to cells immersed in an external solution. Tonicity is influenced only by solutes that can not pass through the semi-permeable cell membrane. An isotonic solution has the same water and solute concentrations as the cell. The same amount of water flows in as out. Animal cells are normal but plant cells are soft or flaccid in an isotonic solution.

Hypertonic Solution A hypertonic solution means that there are more solutes in the external solution and a lower [H 2 O]. A cell will lose water if immersed in a hypertonic solution. An animal cell will crenate or shrink. A plant undergoes plasmolysis as the cell membrane shrinks away from the cell wall.

Hypotonic Solution A hypotonic solution has less solutes than the cell and a higher [H 2 O] than the cell. Water flows into the cell. Animal cells expand and can burst open or lyse. This is from the Latin lysis for “rupture”. Plant cells are comfortable in a hypotonic environment as the cell is full of water and turgid.