Osmosis

Osmosis is the net movement of freely moving water molecules down a concentration gradient through a partially permeable membrane (permeable to water and to certain solutes).

The pressure exerted by freely moving water molecules in a system is called the water potential, measured in kilopascals (kPa). By convention, the water potential of pure water is 0 kPa. The more solute the solution has, the less water potential. The contribution of solutes to the water potential of a system is called the solute potential. Since it always lowers water potential, it is always negative.

In a situation in which a solution with a higher water potential is separated by a partially permeable membrane from a solution with a lower water potential, more water molecules move from the first solution to the second than in the other direction. The movement continues until both sides of the membrane are equal. Water molecules keep moving, but with no net movement.


Osmosis in animal and plant cells

-Isotonic solution: the water potential is equal in both sides of the cell membrane.

• Animal and plant cells: no net water flow in or out of the cell.

-Hypotonic solution: the water potential is higher in the outside of the cell.

• Animal cells: water tends to enter the cell, increasing its volume. If the cell doesn't eliminate the excess, it will burst. However, if it is the natural environment of the cell, it will have a mechanism to eliminate it.
• Plant cells: water enters the cell, fills the vacuole, which pushes the cell surface membrane against the cell wall making the cell turgid. The pressure restricts the inflow of water.

-Hypertonic solution: the water potential is higher in the inside of the cell.

• Animal cells: the cell may lose water, causing it to shrink and shrivel.
• Plant cells: the water moves out, the cell vacuole shrinks and the cell surface membrane pulls away from the cell wall. The cell becomes flaccid because the contents are no longer pushing against the cell wall: Plasmolysed cell.

Pressure potential

The contribution made by the cell wall is the pressure potential. Turgid (positive value) or flaccid (0 kPa).

SEE RELATED VIDEOS IN THE "USEFUL LINKS" GADGET AT THE RIGHT