Osmosis (Section B 1.6 of Biology 2015 Syllabus)

What is Osmosis?
Osmosis is the spontaneous passage or diffusion of water or other solvents through a semipermeable membrane. The process was first thoroughly studies by Plant Physiologist Wilhelm Pfeffer in 1877.

How does the process occur?

 

An example of osmosis occurs when a sugar solution and water, top, are separated by a semipermeable membrane. The solution’s large sugar molecules cannot pass through the membrane into the water. Small water molecules move through the membrane until equilibrium is established, bottom.

(Credit: Encyclopædia Britannica, Inc.)

 

An very simple way of thinking of osmosis, is that it involves the movement of solvents (usually water) from an area of higher concentration (dilute solution) to an area of lower concentration (concentrated solution), until an equilibrium is established. 

Take for example the following situation

Inside the cell, the solution is very dilute (hypotonic) while the liquid in the beaker is concentrated (or hypertonic). Where would you find a higher concentration of water molecules? That's right, the dilute cell would have a higher water potential (concentration) whereas the concentrated solution in the beaker outside the cell would have a lower water potential. Therefore, water molecules will move out of the cell, and into the beaker, until both concentrations are almost equal. 

 

When this happens, the cell would shrink or becomes flaccid because it lost water. If the cell gained water it would swell, and become turgid. Because plant cells have cell walls, they would not burst if they take in excess water, since the cell wall acts as a protection. However, animal cells having no cells walls, will burst if they take in too much water due to osmosis.