Hypotonic Solution

Hypotonic Solution Definition

A hypotonic solution is a solution that has a lower solute concentration compared to another solution. A solution cannot be hypotonic, isotonic or hypertonic without a solution for comparison. Hypotonic is a description of the solute content of one solution in relation to another solution. It is used in biology to help scientist describe cells. Knowing the osmolarity (concentration of a solution in number of solutes per liter) of different solutions can show scientists which way the water gradient and solute gradients will form.

Due to the properties of diffusion, every solute in a solution has the tendency to disperse away from each other until evenly distributed. In aqueous solutions, this is caused mainly by the interactions that polar water molecules have on the solutes. The opposite ends of the molecule have different charges, which form temporary bonds, called hydrogen bonds, with other charged portions of solute molecules. Water molecules cluster around solutes, move them away from the highest concentrations of solutes, and allow more water molecules to move in. Therefore, if you are to pour a hypotonic solution into a hypertonic solution, the solution will initially have areas of high and low concentration but will quickly reach equilibrium. If these two solutions are separated by a membrane which will only let water through, the water will move out of the hypotonic solution and into the hypertonic solution, until the two are isotonic with each other.

Cells are simply a solution surrounded by a semipermeable bag, the plasma membrane. The plasma membrane is able to keep solutes from diffusing across the cell membrane, while it allows water to diffuse by osmosis across the membrane into the cytoplasm. The membrane is embedded with special proteins, called membrane transport proteins that help transport specific solutes across the membrane. Other proteins, called aquaporins keep channels open that only water can pass through. All cells must regulate their solute content, to ensure they do not dry up or become too full of water. A cell with a cytosol that is a hypotonic solution to the environment will lose water to the more hypertonic environment that has more solutes. The water, driven to equalize the two solutions, is drawn from the cell. A cell whose cytosol is an extremely hypotonic solution compared to its environment will shrivel up, and is said to be plasmolyzed. This is almost always a bad state for cells, which need water for many chemical reactions.

In the opposite situation, the environment can be a hypotonic solution compared to the cell. In this case, water from the environment tends to diffuse into the cell. If the hypotonic solution of the environment is too strong, the cell may lyse (split open). Cells have many mechanisms for controlling this flow of water. In plant, fungi, and bacterial cells a cell wall is formed around the cell, which keeps it from bursting. This cell wall is composed of various polysaccharides, proteins, and other molecules. As water fills the cell and pushes against the cell wall, turgor pressure is created. This pressure helps force water back out of the cell, countering the inward flow of water. The image below shows a single plant cell in different environments. The cell on the far right represents a turgid plant cell in a hypotonic solution.

Examples of Hypotonic Solution

Plants and Fungi

Large plants and fungi control the environment around their cells, helping ensure the environment is always a hypotonic solution, compared to the cells. This creates cells that are turgid. The turgid cells push outward on their cell walls, which push against each other creating a rigid structure. The organisms are constantly cycling solutes, to keep the contents of their cells filled with water. If you’ve ever over-fertilized your garden, you know it is not good for plants. The added solutes in the soil turn the hypotonic solution around the roots into a hypertonic solution. Thus, the roots and entire plant are quickly drained of water. Organisms in this condition will quickly die because they cannot complete the reactions necessary to sustain life.

Animal Cells

Animal cells do not have a cell wall. Typically, animals rely on their skin to separate the outside environment from their inside organs. The fluid inside of their body cavity can then be regulated by a series of membranes and proteins. The fluid will thus remain an isotonic or slightly hypotonic solution in comparison to the cells, keeping them plump and healthy without destroying them. The process of maintaining the solute concentration in an organism is known as osmoregulation and occurs in all animals. Many animals that live in the ocean have salt glands which expel excess salt from their body. The animals must drink the salt water to get the water into their bodies, but the salts must be concentrated and excreted from the body to maintain it as a hypotonic solution.

Hypertonic Solution – When a solution has more solutes per liter than another solution.
Isotonic – When two solutions have the same concentration, and exchange water and solutes at the same rate.
Osmoregulation – The complex series of cellular mechanisms in organisms which regulates the amount of water and the concentration of each cell’s cytosol.
Aquaporin – A protein which allows the passage of water through the cell membrane.

Quiz

1. A cell has a concentration of solutes in its cytosol of 1 g/L. The environment has a concentration of 2 g/L. Which of the following describes the cell?
A. Hypotonic
B. Hypertonic
C. Isotonic

Answer to Question #1

A is correct. The cytosol of this cell represents a hypotonic solution in comparison to the environment. The solute concentration is lower, which means there is more water per unit of solute. Because the cell is hypotonic to the environment, water will tend to flow out unless the cell increases its solute concentration. It can do this by taking in salts through the plasma membrane across special channels. If the concentrations become balanced, the cell will be isotonic with the environment.

2. A plant cell is placed in a hypotonic environment. Which of the following statements is FALSE?
A. The cell will lose water, and become flaccid.
B. The cell is hypertonic to the environment.
C. The environment will lose water to the cell.

Answer to Question #2

A is correct. Only A or C can be true. If the environment is hypotonic, the cell is hypertonic. The solutes cannot move as fast as the water across the plasma membrane, therefore water will rush into the cell to try to balance the osmotic equilibrium.

3. Two vertical tubes containing an equal volume of solution are connected on the bottom in a “U” shape, as seen in the image below. A semipermeable membrane exists between the two solutions, as shown by the dotted line. As the solution sits, the water level in the left tube rises. Which words describe the right tube, in the initial then final state?
A. Isotonic; Hypotonic
B. Hypotonic: Isotonic
C. Hypertonic; Hypotonic

Answer to Question #3

B is correct. At first, the solution on the right is hypotonic compared to the solution on the left. The semipermeable membrane ensures that the large solute molecules cannot pass through the membrane. Much smaller water molecules can easily pass through the membrane. To balance the solute gradient that exists, water floods into the left solution. Eventually, the solutions will be the same concentration although they will contain different volumes of water.