Why is it that when you buy a fresh bell pepper it is crisp, and yet a few days later it becomes soft? What makes a plant wilt when you forget to water it? These transformations have to do with the loss of water. The movement of water is very important in plant cells (and animal cells). The diffusion of water is called osmosis.
What Is Osmosis?
Osmosis is the diffusion of water across a semi-permeable membrane. Water flows down a concentration gradient and toward an area that has a higher solute concentration.
For example, if there were two regions connected by a membrane, and one (A) contained more solute than the other (B), then the water would flow from B to A.
Tonicity vs. Osmolarity
The ability of a solution to make water move is called tonicity. The tonicity of a solution is related to its osmolarity, which is the total concentration of solute.
A solution with a lower concentration of solute has lower osmolarity than one that contains a higher concentration of solute. Whenever two solutions are separated by a semipermeable membrane that is permeable to water but not to solute, water will always move from the side with low osmolarity to the side with higher osmolarity.
Types of Tonicity
There are three terms used to describe tonicity when comparing two solutions separated by a membrane: hypotonic, hypertonic and isotonic.
In the example discussed above, region A is hypertonic to region B because region A has a higher osmolarity. Region B, on the other hand, is hypotonic to region A. Tonicity can only be said in reference to another region. On its own, a solution cannot have tonicity.
Isotonic simply refers to two regions separated by a membrane that have the same osmolarity.
Tonicity and Cells
So far, tonicity has only been discussed in terms of two areas containing solute that are connected by a semipermeable membrane, but tonicity and the movement of water is very important for cells. Instead of two regions divided by a membrane, you can imagine a cell that is placed in a fluid. There are two regions: one inside the cell and one outside of the cell. The fluid outside of the cell is called the extracellular fluid.
What happens when you place an animal cell in a hypotonic solution?
- Remember, water moves from a region of low osmolarity to a region of high osmolarity. In this case, since the extracellular fluid has low osmolarity, the water would rush into the cell. The cell would then expand and eventually lyse or burst.
What happens when you place an animal cell in a hypertonic solution?
- In this case, water will leave the cell since the cell has a lower osmolarity than the extracellular fluid. As a result the cell would shrink in what is called plasmolysis.
What happens when you place an animal cell in an isotonic solution?
- The osmolarity of both fluids is equal. As such, though water diffuses in and out, there is no net change in the volume of the cell.
What happens when you place a plant cell in a hypotonic solution?
- The water moves from a region of low osmolarity (extracellular fluid) to a region of high osmolarity (inside the cell). The cell would then expand. Unlike an animal cell, the plant cell does not burst. This is because plant cells have a rigid cell wall around the plasma membrane. Upon swelling with water they become turgid.
- Hypotonic solutions keep vegetables like bell peppers crisp.
What happens when you place a plant cell in a hypertonic solution?
- Water will leave the cell since the cell has a lower osmolarity than the extracellular fluid. As a result, the cell would shrink.
What happens when you place a plant cell in a isotonic solution?
- The osmolarity of both fluids is equal. Though water diffuses in and out there is no net change in the volume of the cell.
About the Author
Riti Gupta holds a Honors Bachelors degree in Biochemistry from the University of Oregon and a PhD in biology from Johns Hopkins University. She has an interest in astrobiology and manned spaceflight. She has over 10 years of biology research experience in academia. She currently teaches classes in biochemistry, biology, biophysics, astrobiology, as well as high school AP Biology and Chemistry test prep.