AQA AS Biology Unit 1: Osmosis

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  • Created on: 22-08-13 17:03
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Osmosis only involves the movement of water molecules.
Osmosis The passage of water from a region where is has a higher
water potential to a region where is has a lower water
Cell-surface membranes and other plasma membranes such as those around organelles are
partially permeable.
Explanation of Osmosis
The solution on the left has a low concentration of solute molecules while the
solution on the right has a high concentration of solute molecules
Both the solute and water molecules are in random motion due to their kinetic
The partially permeable plasma membrane, however, only allows water molecules
across it and not solute molecules
The water molecules diffuse from the left-hand side, which has the high water
potential, to the right-hand side, which has the lower water potential
At the point where the water potentials on either side of the plasma membrane are
equal, a dynamic equilibrium is established and there is no net movement of water

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Animal Cells and Osmosis
Animal cells contain a variety of solutes dissolved in their watery cytoplasm.
If a cell is placed in pure water it will absorb it by osmosis because it has a low water
Cell-surface membranes are very thin and, although they are flexible, they cannot stretch to
any great extent.
The cell surface membrane will therefore break, bursting the cell and releasing its contents.…read more

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Plant Cells and Osmosis
Like animal cells, plants also contain a variety of solutes, mainly dissolved in the water of the
large cell vacuole that each possesses.
When placed in pure water they also absorb water by osmosis because of their lower water
Unlike animal cells, however, they are unable to control the composition of the fluid around
their cells.
Water entering a plant cell by osmosis causes the protoplast to swell and press on the cell
wall.…read more

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Solutions and Water Potential
Solute any substance that is dissolved in a solvent
Water potential is represented by the Greek letter psi
It is measured in units of pressure, usually kilopascals kPa.
Water potential is the pressure created by water molecules.
Water is said to have a water potential of zero.…read more


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