The key difference between isotonic and isosmotic is that isotonic solutions contain only non-penetrating solutes whereas isosmotic solutions contain both penetrating as well as non-penetrating solutes. Another important difference between isotonic and isosmotic solutions is that the isotonic solutions have different osmotic pressures from the cells they surround whereas isosmotic solutions have same osmotic pressure as the cells they surround.
Isotonic and Isosmotic are types of solutions and terms that we often encounter in chemistry labs. Many remain confused about the difference between the two types of solutions believing that they are the same. However, they are not same and we will highlight their differences in this article.
CONTENTS
1. Overview and Key Difference
2. What are Solutes
3. What is Isotonic
4. What is Isosmotic
5. Side by Side Comparison – Isotonic vs Isosmotic in Tabular Form
6. Summary
What are Solutes?
Solutes are the substances that dissolve in a solution. To understand Isosmotic and isotonic solutions we have to realize that they are either penetrating solutes or non-penetrating 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.
What is Isotonic?
Isotonic is when a solution has the same salt concentration as blood and cells of human body. Isotonic solutions contain only non-penetrating solutes and it refers to solutions having same osmotic pressure as the cells they surround.
Figure 01: Isotonic Solution
Moreover, they do not absorb anything from the cell and vice versa (cells also do not absorb solutes from these solutions). For example, a solution that is 154 mMNaCl is isotonic for human beings.
What is Isosmotic?
Isosmotic is when two solutions have the same number of 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 equals on both sides. This may have an effect on the cell so that it may burst finally.
For example, sucrose is a solution that has no ions. A solution of sucrose that is 320 mM is isosmotic for human beings. Comparing this sucrose solution with 154mM NaCl solution, it reveals that it is 154 mMsodium (Na) and 154 mMchloride (Cl) or about 308 milliosmolar which is close to 320 milliosmolar for sucrose.
What is the Difference Between Isotonic and Isosmotic?
Isotonic refers to a solution having the same solute concentration as in a cell or a body fluid. Isosmotic refers to the situation of two solutions having the same osmotic pressure. Therefore, Isotonic solutions contain only non-penetrating solutes whereas Isosmotic solutions contain both penetrating as well as non-penetrating solutes.
With consideration to the relationship these two solution types have with the osmotic pressure, Isotonic solutions have different osmotic pressures from the cells they surround. On the contrary, Isosmotic solutions have same osmotic pressure as the cells they surround. Furthermore, Isotonic solutions do not cause the cells to absorb water from surrounding or to lose water from cells. However, Isosmotic solutions cause cells to absorb water from surrounding or to lose water from cells.
Summary – Isotonic vs Isosmotic
The terms isotonic and isosmotic are useful in describing the properties of body fluids. Both terms, isotonic expresses the idea of having same solute concentrations while the term isosmotic expresses the idea of having equal osmotic pressures. The difference between isotonic and isosmotic is that isotonic solutions contain only non-penetrating solutes whereas isosmotic solutions contain both penetrating as well as non-penetrating solutes.
Reference:
1. “Tonicity.” Wikipedia, Wikimedia Foundation, 8 Feb. 2018. Available here
2. “Isosmotic.” Biology Online. Available here
Image Courtesy:
1.’Blausen 0685 OsmoticFlow Isotonic’By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work, (CC BY 3.0) via Commons Wikimedia