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2.5 Biological Membranes (2.5.3 Osmosis (Water potential (Pure water has…
2.5 Biological Membranes
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2.5.3 Osmosis
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In a solution, the liquid in which solute molecules are dissolved is called the solvent
Some membranes also have protein channels known as aquaporins which allow water molecules to cross the membrane more rapidly
Water potential
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When solute molecules are added, they lower the water potential of the solution. The more solute molecules in the solution, the lower the water potential
If two aqueous solutions are separated by a partially permeable membrane, such as a plasma membrane or a cell organelle membrane, water molecules will move from the solution with the lower water potential
If and when the water potential on both sides of the membrane, becomes equal, there will be no net osmosis, although water molecules will continue to move randomly
Water potential is measured in kilopascals (kPA), pure water has the highest water potential possible, and is given the value of 0kPa
AS solute molecules are added, the water potential of the solution is lowered, in numerical value so it becomes more negative. The more negative the value, the lower the water potential
The water potential inside cells is lower than that of pure water as there are solutes in solution, in the cytoplasm and inside the large vacuole of plant cells
When cells are placed in a solution of higher water potential then water molecules move by osmosis, down the water potential gradient, across the plasma membrane, into the cell
In animal cells, if a lot of water molecules enter, the cell will swell and burst as the plasma membrane breaks - this is cytolysis
When a cell is swollen, this is turgid due too much water in the cell. It will eventually burst
When animals are shriveled, they are known as crenated
Plasmolysis happens when the cytoplasm of plant cells shrinks and the membrane pulls away from the cellulose cell wall
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