Hypertonic Definition and Examples

What Is Hypertonicity and What Is Its Effect?

Hypertonic refers to a solution with higher osmotic pressure than another solution. In other words, a hypertonic solution is one in which there is a greater concentration or number of solute particles outside a membrane than there are inside it.

Hypertonic Example

Red blood cells are the classic example used to explain tonicity. When the concentration of salts (ions) is the same inside the blood cell as outside of it, the solution is isotonic with respect to the cells and they assume their normal shape and size.

If there are fewer solutes outside the cell than inside it, such as would happen if you placed red blood cells in fresh water, the solution (water) is hypotonic with respect to the interior of the red blood cells. The cells swell and may burst as water rushes into the cell to attempt to make the concentration of the interior and exterior solutions the same. Incidentally, since hypotonic solutions can cause cells to burst, this is one reason why a person is more likely to drown in fresh water than in salt water. It's also a problem if you drink too much water.

If there is a higher concentration of solutes outside of the cell than inside it, such as would happen if you placed red blood cells in a concentrated salt solution, then the salt solution is hypertonic with respect to the inside of the cells. The red blood cells undergo crenation, which means they shrink and shrivel as water leaves the cells until the concentration of solutes is the same both inside and outside the red blood cells.

Uses of Hypertonic Solutions

Manipulating the tonicity of a solution has practical applications. For example, reverse osmosis may be used to purify solutions and desalinate seawater.

Hypertonic solutions help to preserve food. For example, packing a food in salt or pickling it in a hypertonic solution of sugar or salt creates a hypertonic environment that either kills microbes or at least limits their ability to reproduce.

Hypertonic solutions also dehydrate food and other substances, as water leaves cells or passes through a membrane to try to establish equilibrium.

Why Students Get Confused About the Definition of Hypertonic

The terms "hypertonic" and "hypotonic" often confuse students because they neglect to account for the frame of reference. For example. if you place a cell in a salt solution, the salt solution is hypertonic (more concentrated) than the cell plasma. But, if you view the situation from the inside of the cell, you could consider the plasma to be hypotonic with respect to the saltwater.

Also, sometimes there are multiple types of solutes to consider. If you have a semipermeable membrane with 2 moles of Na+ ions and 2 moles of Cl- ions on one side and 2 moles of K+ ions and 2 moles of Cl- ions on the other side, determining tonicity can be confusing. Each side of the partition is isotonic with respect to the other if you consider there are 4 moles of ions on each side. However, the side with sodium ions is hypertonic with respect to that type of ions (another side is hypotonic for sodium ions). The side with the potassium ions is hypertonic with respect to potassium (and the sodium chloride solution is hypotonic with respect to potassium).

How do you think the ions will move across the membrane? Will there be any movement?

What you would expect to happen is that sodium and potassium ions would cross the membrane until equilibrium is reached, with both sides of the partition containing 1 mole of sodium ions, 1 mole of potassium ions, and 2 moles of chlorine ions. Got it?

Movement of Water in Hypertonic Solutions

Water moves across a semipermeable membrane. Remember, water moves to equalize the concentration of solute particles.

  • If the solutions on either side of the membrane are isotonic, water moves freely back and forth.
  • Water moves from the hypotonic (less concentrated) side of a membrane to the hypertonic (less concentrated) side. The direction of the flow continues until the solutions are isotonic.
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Helmenstine, Anne Marie, Ph.D. "Hypertonic Definition and Examples." ThoughtCo, Feb. 23, 2018, thoughtco.com/hypertonic-definition-and-examples-605232. Helmenstine, Anne Marie, Ph.D. (2018, February 23). Hypertonic Definition and Examples. Retrieved from https://www.thoughtco.com/hypertonic-definition-and-examples-605232 Helmenstine, Anne Marie, Ph.D. "Hypertonic Definition and Examples." ThoughtCo. https://www.thoughtco.com/hypertonic-definition-and-examples-605232 (accessed March 21, 2018).