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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

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
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
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

slide4

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
  • 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
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
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
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
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
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
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