If a solution is placed in contact with a membrane that is permeable to molecules of the solvent, but not to molecules of the solute, the movement of solvent through the membrane is called osmosis. Such a membrane often is called semipermeable. As the several types of membranes of the body vary in their permeability, it is well to note that they are selectively permeable.
Most normal living-cell membranes maintain various solute concentration gradients. A selectively permeable membrane may be defined either as one that does not permit free, unhampered diffusion of all the solutes present or as one that maintains at least one solute concentration gradient across itself. Osmosis, then, is the diffusion of water through a membrane that maintains at least one solute concentration gradient across itself.
Osmolality (osmol/kg)
A solution has an osmolal concentration of 1 when it contains 1 osmol of solute/kg of water. A solution has an osmolality of n when it contains n osmol/kg of water. Osmolal solutions, like their counterpart molal solutions, reflect a weight-to-weight relationship between the solute and the solvent. Because an osmol of any nonelectrolyte is equivalent to 1mol of that compound, a 1 osmolal solution is synonymous to a 1 molal solution for a typical nonelectrolyte.
Osmolarity (osmol/l)
The relationship observed between molality and osmolality is shared similarly between molarity and osmolarity. A solution has an osmolar concentration of 1 when it contains 1 osmol of solute per liter of solution. Likewise, a solution has an osmolarity of n when it contains n osmols/L of solution. Osmolar solutions, unlike osmolal solution, reflect a weight in volume relationship between the solute and final solution.
Tonicity (Isotonic, Hypotonic and Hypertonic)
A solution is isotonic with a living cell if there is no net gain or loss of water by the cell, or no other change in the cell, when it is in contact with that solution. Physiological solutions with an osmotic pressure lower than that of body fluids, or of 0.9% sodium chloride solution, are referred to commonly as being hypotonic. Physiological solutions having a greater osmotic pressure are termed hypertonic.
Difference between Osmolality and Osmolarity
Osmolality is defined as the number of osmoles of solute in a kilogram of solvent, while osmolarity is defined as the number of osmoles of solute in a liter of solution.
The units of osmolality are osmole per kilogram of water (Osmol/kg) or milliosmole per kilogram of water (mOsmol/kg), while the units of osmolarity are osmole per liter of solution (Osmol/L) or milliosmole per liter of solution (mOsmol/L).
Difference among Osmolality, Osmolarity and Tonicity
The terms osmolality, osmolarity, and tonicity are often used to describe the number of solutes in the solution, however, it is incorrect to use these terms interchangeably.
The measurement of the number of solutes in the solution, using colligative properties, allows the direct determination of the osmolality of the solution with units of moles of solutes per kilogram of solvent.
This result can be converted into a value for osmolarity with units of moles of solutes per liter of solvent, typically water.
And the tonicity is a medical term that relates to the osmotic pressure difference between the internal and external sides of a cell membrane.
Osmolarity Calculation
