Isoosmotic VS Isotonic Solution

• Solutions having same osmolarity are known as isoosmotic solutions.
• Solutions having same tonicity are known as isotonic solutions.
• Solutions having tonicity like that of plasma are known as isotonic solutions with respect to plasma.
• If two solutions differ with respect to tonicity irrespective of their osrnolarity, water movement (osmosis) will occur from hypotonic to hypertonic solution, but if two solutions are isotonic to each other, then there will be no osmosis between two solutions whatever may be their
  osmolarity status.
• Two solutions, each containing protein 4 mol/L, will generate same osmotic pressure, so they are isoosmotic. They are also isotonic to each other because proteins are nonpermeable solutes and they will not produce transmembrane osmotic gradient by diffusion across the membrane, so there will be no osmosis.
• Two solutions, one with protein 4 mol/L and another with protein 4 mol/L along with urea 4mol/L are not isoosmotic because the osmotic pressure of the pure protein solution will be less than that of the solution containing mixture of protein and urea, but they are isotonic to each
  other because the non-permeable protein concentration in two solutions are equal. Since urea is permeable solute, at equilibrium 4 mol urea will be equalized on either side when the two solutions will be separated by a membrane. So finally both solutions will have same concentration of protein and urea (4 mol/L protein + 2 mol/L urea) causing no osmosis. 
• Two solutions, one with protein 4 mol/L and another with urea 4 mol/L, are iso-osmotic because both will generate same osmotic pressure, but they are not isotonic to each other because non-permeable protein concentration in two solutions are not equal. If the two solutions are
  separated by a semipermeable membrane, permeable urea will be equalized on both sides. Finally at equilibrium the concentration of one solution will be 4 moUL protein + 2 mol/L urea and that of other polution will be 2 mol/L urea. Because of this differential concentration of two
  solutions there will be osmosis from low concentrated solution to high concentrated solution.

 

• Plasma osmolarity is roughly about 290 mOsm/L and here both permeable and non-permeable solutes of plasma contribute. Among the different solutes,NaCl alone contributes about 92% of plasma osmolarity. Since NaCl is a non-permeable solute and it represents almost 100% of plasma osmolarity, so plasma tonicity is also regarded to be 290 mosm/L. Same tonicity (290mOsm/L) also prevails in ICF. Any solution having tonicity less than that of plasma is called hypotonic solution, more than that of plasma is called hypertonic solution and same as that of plasma is called isotonic solution.
• Tonicity of a solution has its effect on cell volume when cells with intracellular tonicity of 290 mosm/L are suspended in that solution. Cell volume increases because of water entry into cells if the solution is hypoionic; cell volume decreases because ofcellular water loss if the solution is hypertonic and cell volume remains normal because of no water movement if the solution is isotonic.