Osmosis is the passive movement of water through a semipermeable membrane from a compartment of relatively low osmotic pressure to a compartment of relatively high osmotic pressure, toward equilibrium.  A semipermeable membrane, as it is used here, is defined as a membrane that is permeable to water, but impermeable to certain solutes.  The osmotic pressure responsible for osmosis is generated by these impermeable solutes.  The greater the concentration of impermeable solutes, the greater the osmotic pressure.

The semi-permeable nature of the membrane is a critical component of an osmotic system.  Lets consider, for example, two compartments separated by a freely-permeable membrane, across which is a solute concentration gradient (fig. a), below).  If the membrane is freely permeable to the solutes, then equilibrium is reached by the simple diffusion of the solutes.  However, if the membrane is semipermeable, that is, impermeable to those solutes but permeable to water, then the only way equilibrium can be reached is if water moves passively to bring the system to equilibrium - that's osmosis (fig. b), below).

Biological membranes are inherently semipermeable.  They are permeable to water and other molecules, but they are impermeable to a whole host of solutes, including all charged molecules (ions).  Thus biological membranes are impermeable to all those inorganic ions found in the extracellular and intracellular fluids, such as Na+, K+, Cl- and Ca++, just to name a few.  These ions, as well as other inorganic ions, organic ions and large polar molecules that cannot penetrate biolgical membranes, all contribute to the osmotic pressure of the extracellular and intracellular fluids.

Osmotic pressure is often expressed in relative terms, that is, by comparing the osmotic pressure of one solution to some standard solution.  As it applies to living systems, solutions that have an osmotic pressure equal to that of body fluids or to intracellular fluid, are said to be isotonicHypotonic fluids have an osmotic pressure less than that of cellular fluids, and hypertonic fluids have an osmotic pressure greater than that of cellular fluids.

NaCl solutions can be formulated to meet these definitions.  For example, a 0.85% NaCl solution is isotonic to mammalian plasma and mammalian cells. Any NaCl solution that has a NaCl concentration less than 0.85% is considered hypotonic, and any NaCl solution that has a NaCl concentration greater than 0.85% is considered hypertonic.


Osmosis Across the Red Blood Cell Plasma Membrane

The mammalian red blood cell plasma membrane is very permeable to water, and osmosis across the red blood cell plasma membrane occurs very rapidly.  Therefore, mammalian red blood cells are sensitive to osmotic pressure gradients.  Under normal in vivo conditions, mammalian red cells are bathed in isotonic plasma, in which case water movement into the cell is equal to water movement out of the cell, and there is no net osmosis.  When red blood cells are taken out of their natural environment and placed in a sufficiently-strong hypertonic fluid (e.g. NaCl solution >> 0.85% NaCl) there is a net outward osmosis and the cells shrink (see figure below).  Conversely, when red blood cells are placed in a sufficiently-strong hypotonic fluid (e.g. NaCl solution << 0.85% NaCl), there is an uncontrolled net inward osmosis, resulting in the continuous uncontrolled swelling of the cells until the cells break open, a process known as cell lysisHemolysis is the lysis of red blood cells.

The percentage of red blood cells that hemolyze in hypotonic fluids depends on the degree of hypotonicity of the fluid.  A plot of % red blood cells hemolyzed (% hemolysis) vs. % NaCl solution is shown below.  The data generates a reverse sigmoidal (s-shaped) curve.  The plot shows that a very low percentage of normal mammalian red blood cells hemolyze in mildly hypotonic solutions above 0.6% NaCl.  Significant hemolysis doesn't occur until cells are bathed in stronger hypotonic solutions below 0.6%, with % hemolysis rising sharply between 0.6 and 0.4% NaCl to nearly 100%.

Cell Biology OLM | Authored by Stephen Gallik, Ph. D. | URL: cellbiologyolm.stevegallik.org | Copyright © 2011 Stephen Gallik, Ph. D.