Osmosis

• Osmosis can be defined as the movement of water from a region of a higher (less negative) water potential to a region of lower (more negative) water potential across a selectively permeable membrane.
• Osmosis occurs across a selectively permeable membrane, whereby water molecules (and a few other small molecules) can pass through but not larger molecules.
• A solute is any substance that is dissolved in a solvent.
• Water potential is the pressure created by water molecules, and it is measured in kPa (kilopascals).
• Pure water has a water potential of 0kPa under standard conditions (25 degrees C and 100kPa pressure).
• The water potential of a solution will always be a negative value.
• In order to find the true water potential of cells, you can place them in a series of solutions of different water potentials. Where there is no net gain or loss of water from the cells, the water potential inside the cells must be the same as the external solution (i.e. they are isotonic).
• The process of osmosis begins with a lower concentration solution on one side of the selectively permeable membrane and a higher concentration solution on the other side. Both the solute and water molecules on either side are in constant random motion due to their kinetic energy.
• The selectively permeable membrane will only allow water molecules across it, not solute molecules, so water molecules diffuse from the side with a higher water potential to the side with a lower water potential. Eventually, a dynamic equilibrium will exist, and there is no further net movement of water.
• If an animal cell (e.g. red blood cell) is placed into pure water, it will absorb water via osmosis because it has a lower water potential. Cell-surface membranes are very thin, and not endlessly flexible, therefore the cell-surface membrane will break. This will cause the cell to burst and release its contents.
• In order to prevent haemolysis, red blood cells exist in the blood plasma, which has the same water potential as the RBCs. This prevents any net movement of water into or out of the red blood cells.