Osmosis:

The diffusion of water across a semi-permeable membrane due to differences in solute concentrations

A way to think about it…

Let’s imagine a way to think about the direction of water movement. You have two glasses filled with marbles but one is filled with a lot more marbles than the other. If you added water to both glasses to reach a certain height, then the glass with more marbles would require less water to reach the height. There is a lower concentration of water in this glass than the other glass.

However, if the two glasses were connected by a semi-permeable membrane, osmosis would cause water to flow from the glass of high water concentration to the glass with low water concentration. It is important to note that this is not the actual mechanism of osmosis, just an easy way to think about it.

OSMOLARITY

Osmolarity (osmotic concentration)

• Measure of solute concentration (osmoles of solute per liter)

Ionic compounds

• Often dissociate in solution (NaCl becomes Na+ and Cl-)
• 2 moles of NaCl therefore become 4 osmoles of solute (2 Na+ and 2 Cl-)

Nonionic compounds

• Don’t dissociate in solution
• 2 moles of glucose therefore become 2 osmoles of solute

SEMI-PERMEABLE NATURE OF CELLULAR MEMBRANES

Isotonic solution

• Solution has same solute concentration as the inside of the cell

Hypertonic solution

• Solution has greater solute concentration than the inside of the cell

Hypotonic solution

• Solution has less solute concentration than the inside of the cell

If a red blood cell is placed in a:

• Isotonic solution – water freely diffuses in and out of the cell at equilibrium
• Hypertonic solution – water diffuses out of the cell and the cell shrivels up
• Hypotonic solution – water diffuses into the cell and the cell lyses