In this article, you will learn to describe the tonicity of various solutions, understand what osmosis is, predict the movement of the water, and explain features of osmotic pressure.
- Biological Transport
- Physical & Chemical Properties of Water
- Aqueous Solutions
- What is a Solution in Chemistry?
- What is a Solute?
Cells are the most basic unit of life. Cells are responsible for facilitating metabolic processes, communicating between different sections of the human body, and dividing to produce daughter cells. However, none of these would be possible without the solutes suspended inside a cell as shown below.
While this is not a representative sample of all solutes inside the cell, it is important to note that solutes are a diverse class of molecules which help the cell survive. However, the surrounding environment is just as important. This is because the plasma membrane is a semipermeable membrane. In other words, material can flow into and out of the cell. Thus, it is critical to examine how the external environment compares to the internal environment.
We can compare each environment in terms of the concentrations of solutes as described in the following section.
There are three ways to represent how the concentration of the environment compares to the concentration of the cell: isotonic, hypotonic, and hypertonic.
An isotonic solution is where the concentration of the environment is the same as the concentration of the cell. Due to the similar concentrations, there is no net movement of water and the volume of the cell will remain constant as shown below.
A hypotonic solution is where the concentration of the environment is less than the concentration of the cell. Due to differing concentrations, water will move into the cell and the volume will increase as shown below. This movement of water is what is referred to as osmosis.
A hypertonic solution is where the concentration of the environment is greater than the concentration of the cell. Due to differing concentrations, water will move out of the cell and the volume will decrease as shown below. This movement of water is what is referred to as osmosis.
The net movement of water from both hypotonic and hypertonic solutions is responsible for what is known as osmotic pressure.
While we can analyze osmosis with respect to a cell, we can also understand it in a laboratory. For instance, we can use a U-shaped tube filled with either pure water or red food coloring separated by a semipermeable membrane.
As time goes on, water will flow through osmosis from a region of low solute concentration to a region of high solute concentration in an effort to equalize the concentrations. The difference in height is what is known as osmotic pressure, which may be calculated as shown below. Note, the van’t Hoff factor represents the number of species produced by a given solute. For instance, while glucose may have a van’t Hoff factor of one (i = 1), NaCl will have a van’t Hoff factor of two (i = 2).
Osmosis Practice Problems
An undergraduate student is investigating the structure of the nephron, the smallest unit of organization for mammalian kidneys. One feature of the nephron is known as the descending loop of Henle, where water moves from the inside of the nephron to the outside of the nephron. Based on this information, would you classify the region outside of the nephron as isotonic, hypotonic, or hypertonic?
Predict what the van’t Hoff factor of magnesium chloride would be.
An erythrocyte, otherwise known as a red blood cell (RBC), is responsible for transporting oxygen gas throughout the human body. However, variations in diet can alter the concentrations of species in the blood the erythrocyte travels in. Assuming the concentration of the blood is quite high relative to the erythrocyte, determine which direction water would flow.
Osmosis Practice Problem Solutions
The region outside the nephron is hypertonic.
The formula for magnesium chloride is MgCl2, which would dissociate into three ions. Thus, the van’t Hoff factor would be i = 3.
Water would flow out of the erythrocyte and into the blood.
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