Hypertonic Solution Definition

A hypertonic solution contains a higher concentration of solutes compared to another solution. The opposite solution, with a lower concentration or osmolarity, is known as the hypotonic solution. In biology, scientist must describe cell contents compared to the environment. If a cell is placed in a hypertonic solution, the cell will be hypotonic. If the cytosol of the cell is a hypertonic solution, it means the environment is hypotonic, or more weakly concentrated. This is of great importance because solutes and water tend to flow or diffuse along their gradients. Two solutions mixed together will eventually become a single solution. If the solutions are separated by a permeable membrane that only allows water through, the solutions will become isotonic. Isotonic solutions have equal concentrations, although they may have different volumes.

The plasma membrane that surrounds cells is a special permeable membrane that separates the contents of the cell from the environment. The plasma membrane is embedded with special membrane transport proteins that help transport solutes across. It also has special protein channels called aquaporins that allow water to flow freely across the membrane. The cell must use energy to actively move solutes into and out of the cell. Too many solutes and the cytosol will become a hypertonic solution compared to the environment. Cells without cell walls can burst in this condition. Too few solutes and the environment will become the hypertonic solution. In this case, the opposite will happen, as water moves out of the cell. Water moves against the concentration gradient of solutes, moving from areas of low solute concentration to areas of high solute concentration. In another sense, water moves with the water concentration gradient, from areas of high water concentration to areas of low water concentration.

Organisms regulate the osmolarity of their cells are known as osmoregulators. Typically, cells try to maintain their cytoplasm as a hypertonic solution compared to the environment. While this does pose certain structural problems, it allows water to flow freely through the cell, and participate in many of the reaction in which it is necessary. If cells were hypotonic, they would eventually lose most of their water to the environment. Other organisms, osmoconformers, have the same osmolarity as the environment, although the exact solutes may be different. This ensures that they neither lose nor gain lots of water.

Examples of Hypertonic Solution

Human Kidney

To regulate the amount of water in the body, the human brain has special proteins called osmoreceptors, which can measure the osmolarity of the environment surrounding the cell. If the environment becomes a highly hypertonic solution, it is because there is not enough water in the blood to dilute the solutes. The hypothalamus releases hormones while increase the permeability of membranes in the kidney. The kidney resorbs the water that would have been excreted, and adds it back to the bloodstream. The blood becomes more isotonic compared to the cells, and normal processes can continue.

Plants in Hypertonic Solution

Generally, plants prefer to live in hypotonic environments. In a hypotonic environment, water easily floods plant cells and they can remain turgid, or rigid, due to pressures exerted on their cell walls by the influx of water. The plants use this water potential to give their bodies structure and move water from the roots to the top of the plant. However, many plants have adapted to live in hypertonic environments. Marshes by the sea, mangrove swamps, and other brackish waters contain a much higher salt content than fresh water. The soil becomes saturated with these salts, creating a much higher solute concentration in the soil. Most plants would shrivel up if they were transplanted to this habitat, but a special group of plants known as Halophytes have evolved to overcome this obstacle. By increasing the osmolarity of their roots, the plants are able to change from a hypotonic environment inside the cell compared to the environment to a hypertonic solution in the cytosol. This lowers the water potential of the root cells, and allows water to enter the cells. The cells either store the excess salts in the roots or transport the salts to the leaves, where they can be excreted out of glands.

Sea Turtle Osmoregulation

Compared to fresh water, salt water is a hypertonic solution. This means that for cells to function, they must contain a cytosol that is a more hypertonic solution than salt water. Sea turtles for example, live in a much more hypertonic solution compared to freshwater turtles. If you put a freshwater turtle in seawater, the hypertonic seawater will dehydrate the turtle. Instead of being hydrated by the water, the solute-dense ocean water will pull water from the body to balance the difference in osmolarity. To overcome this obstacle, sea turtles and other sea animals have developed unique pathways to remove excess salts. The salts move from the digestive tract into the bloodstream. When they reach the salt gland, they are removed. This creates an internal environment that is higher in solutes, but one that doesn’t lose excess amounts of water to the environment.

  • Hypotonic – A solution that is weaker, or less concentrated, than another solution.
  • Isotonic – Two solutions that exist with similar ionic concentrations, even if they are of different ions.
  • Osmolarity – The total concentration of solutes in a body of water.
  • Osmoregulation – The ability of some animals to regulate the salt content of their cells.


1. You start with plain tap water. You add the water to a machine that boils it and dumps it over ground beans. Some solutes are dissolved off the beans and the water gets a dark brown color. To finish it off, you add some granules of the polysaccharide sucrose, which also dissolve into the water. Compared to the tap water, what is happening to this solution?
A. It is becoming a more hypertonic solution
B. It is becoming a more hypotonic solution
C. The solution has the same osmolarity as the water

Answer to Question #1
A is correct. As the various solutes dissolve into the water, it becomes a more hypertonic solution compared to regular water. Although this might sound like a crazy experiment, most scientists do this procedure to start their day. Making coffee is a process that anyone can do which produces a hypertonic solution. Sucrose is simple table sugar, which many people take in their coffee. The other solutes include salts and sugars from the beans, as well as caffeine, which is a stimulant in humans.

2. Which of the following describes a cell in a hypertonic solution?
A. A shark egg sits at the bottom of the ocean. The egg is about the same concentration as sea water.
B. Your “friendly neighbor” just watered your plants with salt water. The roots now sit in a highly concentrated solution.
C. A scientist uses pure, distilled water for an experiment on an animal cell. Curiously, it dies.

Answer to Question #2
B is correct. When highly concentrated saltwater is used to water regular plants, their roots have no way to deal with the excess salt levels, and exist in a hypertonic solution permanently. The water will be drawn from the roots, which in turn will draw it out of the leaves. The whole plant will be drained as the water potential stays higher in the solution. The shark egg exists in an isotonic solution, as the concentration of the egg is the same as sea water. If distilled water is used in an experiment on animals unable to deal with the influx of water that will be created by the highly hypotonic environment, they will lyse, or break open.

3. The following are the osmolarities of different solutions. Compared to each other, which of the following is the MOST hypertonic solution?
A. 4 mg/L
B. 6 g/L
C. 8 g/kL

Answer to Question #3
B is correct. Look close! The units on the osmolarity of each solution are not the same. To compare the 3 values, they must be converted to the same units. 4 mg is only .004 g, so A = .004 g/L. Much smaller than B. C is in g/kL, which is equivalent to 1000 L. That means C = 8 g / 1000 L, or in other terms: .008 g/L. That means B is by far the most hypertonic solution.