Key difference – Isotonic vs Hypertonic
It is important to understand the concept of Tonicity before analyzing the difference between isotonic and hypertonic. Therefore, let us first describe briefly the concept of tonicity and its importance. Tonicity is the variance in water concentration of two solutions divided by a semipermeable membrane. It can also be explained as the relative water concentration of solutions that decide the direction and amount of diffusion of water until it achieves equal concentrations on both sides of the membrane. By identifying the tonicity of solutions, we can determine in which direction water will diffuse. This phenomenon is commonly used when illustrating the response of cells immersed in an external solution. There are three classifications of tonicity that one solution can have relative to another.They are hypertonic, hypotonic, and isotonic. The key difference between Isotonic and Hypertonic is that the hypertonic solution contains more solvent than solute whereas the solute and solvent are equally distributed in the isotonic solution. However, memorizing the definition of hypertonic and isotonic solutions is not essential if we can understand the difference between isotonic and hypertonic solutions.
What is Hypertonic?
Hyper is another word for above or excessive. Hypertonic solutions will have a higher concentration of solute (glucose or salt) than the cell. Solutes are the elements that are dissolved in a solvent, thereby forming a solution. In a hypertonic solution, the concentration of solutes is greater outside the cell than inside it. When a cell is immersed in a hypertonic solution there will be an osmotic shift and water molecules will flow out of the cell in order to balance the concentration of the solutes and there will be shrinkage in the size of the cell.
What is Isotonic?
Iso is another word for equal and tonic is for the tonicity of the solution. Isotonic solutions will have a similar concentration of solute than the solution it’s being compared to. In an isotonic solution, the concentration of solutes is the same both inside and outside of the cell creating an equilibrium within the environment of the cellular organization. When a cell is immersed in an isotonic solution, there will not be an osmotic shift and water molecules diffuse through the cell membrane in both directions to balance the concentration of the solutes. This process will not create swelling or shrinking of the cell.
What is the difference between Isotonic and Hypertonic?
The differences between hypertonic and isotonic can be classified into following categories.
Definition of Isotonic and Hypertonic
Hypertonic: “hyper” is known as above or excessive + “tonic” is known as something along the lines of a solution. Thus, hypertonic suggests increased tonicity of the solution.
Isotonic: “iso” is known as same + “tonic” is known as something along the lines of a solution. Thus, isotonic suggests similar tonicity of the solution.
Characteristics of Isotonic and Hypertonic
Concentration of solute and solution
Hypertonic: Solution contains more solvent than solute.
Isotonic: Solute and solvent in the solution are distributed in an equal manner.
Hypertonic: Purified water, because no/less solute is dissolved in the purified water, and its concentration is very low compared to the cellular environment.
Isotonic: Saline solution is an isotonic to human blood plasma
Cells response in hypertonic and isotonic solution (See figure 1)
Hypertonic: When a biological cell is in a hypertonic environment, water flows across the cell membrane out of the cell, in order to balance the concentration of solutes in both cell and the environment around the cell. As a result, the cell will shrink as water leaves the cell to reduce the higher concentration of solute in the outer environment.
Isotonic: When a cell is in an isotonic solution, it will not create swelling or shrinking of the cell.
Water concentration gradient
Hypertonic: Water concentration gradient can be observed from the inside of cell to the hypertonic solution
Isotonic: Water concentration gradient does not exist
Solute concentration gradient
Hypertonic: Solute concentration gradient is seen from the hypertonic solution to inside of cell
Isotonic: Solute concentration gradient does not exist.
Hypertonic: osmotic shift exists.
Isotonic: osmotic shift does not exist
Movement of water
Hypertonic: Water molecules move or diffuse quickly from inside of the cell to outer solution directions, and thus the cell will lose water.
Isotonic: Water molecules move or diffuse in both directions, and the rate of water diffusion is similar in each direction. Thus cell will either obtain or lose water.
Isotonic: An isotonic drink comprises similar concentrations of salt, sugar carbohydrate and electrolytes as in the human body. Isotonic sports drink is often preferred as an oral rehydration solution. It usually have 4-8g of carbohydrate per 100 ml.
Hypertonic: Hypertonic drink comprises higher concentrations of salt, sugar carbohydrate and electrolytes as in the human body. It usually has approximately 8g of carbohydrate per 100 ml. A hypertonic solution is also used in osmotherapy in order to manage cerebral hemorrhage. Hypertonic sports drinks are ideal for those who need very high levels of energy.
In conclusion, there are three forms of solutions that are based on solute concentration and they are isotonic, hypotonic, and hypertonic. The concentration of solutes is the same both inside and outside of the cell in an isotonic solution. The concentration of solutes is greater inside the cell than the outside environment in a hypotonic solution whereas the hypertonic solution is one where the concentration of solutes is greater outside environment than inside cell.
References Mansoor, M. A., Beverly, J., and Sandmann. (2002). Applied Physical Pharmacy. McGraw-Hill Professional. pp. 54–57. Voet, D., Judith, G. V. and Charlotte, W. P. (2001). Fundamentals of Biochemistry (Rev.ed.). New York: Wiley. p.30. Image Courtesy: “Osmotic pressure on blood cells diagram” by LadyofHats – did it myself based on ,  , and ..(Public Domain) via Wikimedia Commons