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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.
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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 [H2O] 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 • The water will flow into the cell because ICF [H2O] < ECF [H2O] ECF – 95% water
Calculating Water Concentration • Sometimes the [H2O] must be calculated by subtracting, % [H2O] = 100% - solute%. • Which way will water flow? • The water will flow out of the cell as ICF [H2O] = 90% and ECF [H2O] = 85% Cell parts - 10% ECF – 85% water
And now which way will water flow, assuming that salt can not pass the semi-permeable membrane? • From inside ICF [H2O] = 80% > ECF [H2O] = 40% Cell parts - 20% 60% salt solution • What happens to the shape of the cell? • The cell will shrink. Cell parts - 20% 60% salt solution
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 [H2O]. • 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 [H2O] 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. • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_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. • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
