Diffusion?
Whenever there is a concentration gradient difference of molecules exists, diffusion occurs. Molecules typically move from an area of higher concentration to lower concentration. For example, when you add a spoonful of salt in a test tube, which is separated by a permeable membrane, the salt diffuses across the water.
Solvent – water
Solute – salt
Net result – diffusion
Example in human body: Oxygen diffuses from an area of high concentration (artery) to an area of low concentration (cell).
Osmosis?
Now, in the above example, supposing if the membrane is selectively permeable , i.e., if the membrane allows only the movement of the solvent (water) and not the solute (salt), then osmosis results. Water will diffuse across the semi-permeable membrane from the low-solute concentration (no-salt) to the high solute concentration (salt), until solvent-equilibrium results. This net diffusion of water occurs passively, i.e., they do not require any energy.
For example, if you sauté any vegetables with salt, the water oozes out because of osmosis!
Example in plant kingdom: roots from trees suck up the water from soil using osmosis.
So, what happen if you place cell that is covered by a plasma membrane (which is selectively permeable) in hypo or hyper tonic solutions?
1. If you place a cell in a hypertonic solution (less water & more solutes), the cell will shrink in size.
2. If you place a cell in a hypotonic solution (more water & less solutes – example tap water), the cell will drink up the water & swell!
Check these videos to understand the concept better!
http://www.youtube.com/watch?v=K3wG337xmQc
http://www.youtube.com/watch?v=H6N1IiJTmnc&feature=related
