Isotonic vs Isosmotic
Isotonic and Isosmotic are types of solutions and terms that are often encountered in chemistry labs. Many remain confused about the difference between the two types of solutions believing them to be same. However, they are not same and their differences will be highlighted in this article.
To understand Isosmotic and isotonic solutions we have to realize that they are either penetrating solutes or non penetrating solutes. Isotonic is when a solution have the same salt concentration as blood and cells of human body. Isosmotic is when two solutions have same number of solutes. Penetrating solutes are those which can pass through membranes of the cell affecting the osmotic pressure across the membrane. On the other hand, non penetrating solutes cannot pass through the membrane which is why they only affect tonicity.
The difference between Isosmotic and isotonic solutions is that isotonic solutions contain only non-penetrating solutes and they can be termed as solutions having same osmotic pressure as the cells they surround. They are neither absorbed nor do they absorb anything from the cell. On the other hand, isosmotic solutions contain both penetrating as well as non penetrating solutes. Thus despite the fact that they have the same osmotic pressure as the cell they surround. They contain penetrating solutes that can enter and increase the osmotic pressure of the cell. When the osmotic pressure of the cell increases causing the cell to absorb water from the medium, it reaches equilibrium and osmotic pressure is equalized on both sides. This may have an effect on the cell so that it may burst finally.
Taking a numerical example, a solution that is 154mM NaCl is isotonic for human beings. Sucrose is a solution that has no ions. A solution of sucrose that is 320mM is considered isosmotic for human beings. Comparing this sucrose solution with 154mM NaCl solution, it is found that it is 154mM Na and 154mM Cl or about 308 milliosmolar which is close to 320 milliosmolar for sucrose.