OSMOSIS Eve Oram.

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Presentation transcript:

OSMOSIS Eve Oram

Water molecules move randomly with a certain amount of kinetic energy… Osmosis is the net movement of water molecules across a Partially-permeable membrane. Water molecules move randomly with a certain amount of kinetic energy…

Distilled water separated by a partially-permeable membrane: Water molecules are moving from one side of the membrane to the other but there is no net osmosis

If a substance is dissolved in water, the kinetic energy of the water molecules is lowered. This is because some water molecules aggregate on the surfaces of the other molecules…

For osmosis we talk about the potential water molecules have to move – the OSMOTIC POTENTIAL. Distilled water has the highest potential (zero). When water has another substance dissolved in it, the water molecules have less potential to move. The osmotic potential is NEGATIVE.

Water molecules always move from less negative to more negative water potential. Net osmosis = LN MN

The osmotic potential of a cell is known as its WATER POTENTIAL The osmotic potential of a cell is known as its WATER POTENTIAL. For animal cells, the water potential is the osmotic potential of the cytoplasm.

An animal cell with water potential –50 is placed in a solution…

Water potential of cytoplasm = -50 Osmotic potential of solution= -20 If the osmotic potential of the solution is less negative than the water potential of the cytoplasm(the solution is hypotonic), net endosmosis will occur, i.e. water will move into the cell from the solution. The result will be haemolysis (the cell will burst) Water potential of cytoplasm = -50 Osmotic potential of solution= -20

Water potential of cytoplasm= -50 Osmotic potential of solution = -80 If the osmotic potential of the solution is more negative than the water potential of the cytoplasm (the solution is hypertonic), net exosmosis will occur. The result will be crenation (the cell will shrivel up) Water potential of cytoplasm= -50 Osmotic potential of solution = -80

If the osmotic potential of the solution is the same as the water potential of the cytoplasm (the solution is isotonic), there will be no net osmosis. Water potential of cytoplasm= -50 Osmotic potential of solution= -50

In animal cells, the water potential is equal to the osmotic potential of the cytoplasm, but this is different in plant cells… Plant cells have a cell wall, which exerts an inward pressure when the cell is turgid. This is known as the pressure potential. The water potential of an animal cell is equal to the osmotic potential of the cytoplasm plus the cell wall pressure: W.P.= O.P. + P.P.

A plant cell with water potential –50 is placed in a solution…

If the solution is hypotonic, net endosmosis occurs and the cell becomes fully turgid. Water potential of cytoplasm = -50 Osmotic potential of solution = -20

If the solution is hypertonic, net exosmosis occurs and causes plasmolysis (the cell membrane pulls away from the cell wall. The cell wall stays intact). Water potential of cytoplasm = -50 Osmotic potential of solution = -80

If the solution is isotonic, no net osmosis occurs If the solution is isotonic, no net osmosis occurs. The cell is not plasmolysed, but it is not fully turgid either. Water potential of cytoplasm = -50 Osmotic potential of solution = -50