Reviewed by: BD Editors

Hypotonic Definition

Hypotonic can mean several things in biology. Its many definitions apply to both human biology and biochemistry.

When referring to humans or animals, hypotonic signifies a muscle’s having less tone, or shape, compared to a normal comparison model or when compared to another muscle in the same human body.

The biochemical fields take this concept to a molecular level. In these arenas, hypotonic refers to a solution’s having less osmotic pressure, or concentration, than another solution between a semi-permeable membrane. In more simpler terms, hypotonic can mean a solution that has a lower concentration of solutes than other solutions, made of the same solutes.

Examples of Hypotonic

Building Your Triceps

When beginning weightlifting (or, in more extreme cases, bodybuilding) you usually start with one of a trainer’s most basic exercises: bicep curls. However, you may find that, despite having larger biceps, you’ve neglected your triceps and failed to rid yourself of the dreaded fat that lurks there.

You may then decide to look for exercises that build your triceps up to the same glory as your biceps but, until they reach that state, they will have less tone than their counterparts. Because they have less tone, they are considered hypotonic to your bicep muscle.

The Salt in Sports Drinks

Ah, the classic debate: sports drinks versus water. While we will compare the two, we will look beyond which drink offers better hydration.

However, we must go to the debate to get to the crux of our matter. The big argument for the hydrating benefits of sports drinks, like Gatorade and PowerAde, is that they provide electrolytes, mainly in salts, to restore essential minerals lost in sweat. In so doing, they do more than hydrate because they balance more than the water in your body.

We will look at this factor in simpler terms. Because sports drinks are essentially a mixture of salt, vitamins, and minerals in water, they are a solution. The mixture is called the solute.

Plain water does not have the solute found in sports drinks, however. While science considers water a solution because it is a mixture of H2O molecules, it does not contain a mixture of extra salts, vitamins, and minerals. It is by lacking these extras that water is hypotonic to sports drinks.

Rehydrating a Raisin

When you dehydrate, or remove the water from, a grape, it becomes a raisin. The process appears irreversible when comparing the difference in size and shape of raisins and grapes, but simple science suggests otherwise.

Grapes have semipermeable membranes, because you can extract water from them through their skin. Because grape skin allows water to flow out, water can also flow back in.

Since water can flow back into the skin of a grape, you can rehydrate raisins by placing them in a glass of water. Ideally water will flow into the raisin until it reaches grapey roundness, or an internal water pressure equal to the pressure its external surroundings exert on it.

Of course, things don’t begin that way. When you first place a raisin into water, it is hypotonic to water in that its internal water pressure is far less than the external water pressure than surrounds it.

  • Solute – The part of a solution that is dissolved into a liquid.
  • Solution – A liquid with a substance dissolved in it, to the degree that the substance evenly occupies the whole liquid.
  • Osmotic pressure – The difference in concentration of a solution on either side of a semipermeable membrane.


1.Two saltwater solutions contain different concentrations of salt. The solution with the lower concentration of salt is:
A. Hypertronic
B. Hypoglycemic
C. Hypotonic
D. Supersonic

Answer to Question #1
C is correct. When two saltwater solutions are placed side-by-side, the solution with the lower concentration of salt is hypotonic to the solution with the higher concentration of salt.

2. You follow the same workout routine for several months. Since this routine focused on your upper body and not your lower body, you find yourself carrying groceries with ease but struggling to climb stairs. Why?
A. Your legs are hypertonic to your arms, which means they have more tone and are possibly stronger.
B. Your gym membership was a rip off.
C. You’re carrying groceries the wrong way by lifting only with your arms and not your legs.
D. Your legs are hypotonic to your arms, which means they have less tone and are possibly weaker.

Answer to Question #2
D is correct. Since you haven’t been working your legs as rigorously as your arms, they have less tone and are therefore hypotonic.

3. Why do dried fruits rehydrate after being placed in water?
A. Dried fruits have a semipermeable membrane. While they are hypertonic to water at first, water passes through the semipermeable membrane until equal pressure on either side of the membrane is achieved.
B. Dried fruits have mouths. They drink the water in the solution to rehydrate themselves.
C. Dried fruits don’t rehydrate.
D. Dried fruits have a semipermeable membrane. While they are hypotonic to water at first, water passes through the semipermeable membrane until equal pressure on either side of the membrane is achieved.

Answer to Question #3
D is correct. The inside of a raisin is hypotonic to exterior water solutions until enough water solution passes through the raisin’s semipermeable membrane so as to make the pressure equal on either side of the membrane.

Cite This Article

Biologydictionary.net Editors. "Hypotonic." Biology Dictionary, Biologydictionary.net, 23 Dec. 2016, https://biologydictionary.net/hypotonic/.
Biologydictionary.net Editors. (2016, December 23). Hypotonic. Retrieved from https://biologydictionary.net/hypotonic/
Biologydictionary.net Editors. "Hypotonic." Biology Dictionary. Biologydictionary.net, December 23, 2016. https://biologydictionary.net/hypotonic/.

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