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.

Sodium – Potassium Pump This form of active transport is used by cells to form a concentration gradient with more sodium on the outside of the cell. The is crucial for nerve impulses and animal cell function. hill.com/sites/ /student_view0/chapter2/animation__how_the_sodium_potassiu m_pump_works.html

1. Start the NaK Pump Three Na+ ions from the ICF attach to a special transmembrane enzyme protein. Also an ATP energy molecule is hydrolysed providing energy.

2. Pushing out sodium The energy released by the ATP causes the enzyme protein to change shape and push out the 3 Na+ ions to the ECF against the concentration gradient.

3. Moving K+ Now two K+ ions enter the enzyme protein from the ECF. The protein again changes shape and discharges the two K+ ions to the ICF. As a result there are more positive charges outside the cell. This ionic charge difference pulls water out of the cell and keeps the cell from swelling and breaking. hill.com/sites/ /student_view0/chapter2/animation__how_the_sodium_potassium_pu mp_works.html