Tonicity is a measure of "blood capacity" or "effective osmolality" in cell biology. Osmolality and osmolarity are properties of a particular solution, independent of any membrane. Tonicity is a logical measurment for complex compounds with provides immunity to the body, and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across that membrane. Tonicity, also, depends on solute permeability (permeant solutes do not affect tonicity; impermeant solutes do affect tonicity). Tonicity is generally classified in three ranges; hypertonicity, hypotonicity and isotonicity. Hypertonic, isotonic and hypotonic solutions are defined in reference to a cell membrane by comparing the tonicity of the solution with the tonicity within the cell.


A cell is surrounded by a higher concentration of impermeable solute than exists in the inside of the cell. Osmotic pressure directs a net movement of water out of the cell, causing it to shrink.effective osmolarity or osmolality > 290mOsm

In animal cells, being in a hypertonic environment results in crenation, where the shape of the cell becomes distorted and wrinkled as water leaves the cell. Some organisms have evolved methods of circumventing hypertonicity; for example, saltwater is hypertonic to the fish that live in it. Since they cannot isolate themselves from osmotic water loss, because they need a large surface area in their gills for gas exchange, they respond by drinking large amounts of water, and excreting the salt. This process is called osmoregulation.

In plant cells, the effect is more dramatic. The cell membrane pulls away from the cell wall, but the cell remains joined to the adjacent cells at points called plasmodesmata. Thus, the cell takes on the appearance of a pincushion, with the plasmodesmata almost ceasing to function because they have become so constricted. This condition is known as plasmolysis. The terms isotonic, hypotonic and hypertonic cannot be accurately used in plant cells however as the pressure potential exerted by the cell wall affects the equilibrium point significantly.


The opposite of a hypertonic environment is a hypotonic one, where the net movement of water is into the cell. If the cell contains more impermeable solute than its surroundings, water will enter it. In the case of animal cells, they will swell until they burst; plant cells do not burst, due to the reinforcement their cell wall provides effective osmolarity or osmolality < 290 mOsm

ee also

*Osmole (unit)

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Look at other dictionaries:

  • Tonicity — To*nic i*ty, n. (Physiol.) The state of healthy tension or partial contraction of muscle fibers while at rest; tone; tonus. [1913 Webster] …   The Collaborative International Dictionary of English

  • tonicity — [tō nis′ə tē] n. the quality or condition of having good or normal muscular tone …   English World dictionary

  • tonicity — 1. A state of normal tension of the tissues by virtue of which the parts are kept in shape, alert, and ready to function in response to a suitable stimulus. In the case of muscle, it refers to a state of continuous activity or tension beyond that …   Medical dictionary

  • tonicity — noun Date: 1824 1. the property of possessing tone; especially healthy vigor of body or mind 2. muscular tonus …   New Collegiate Dictionary

  • tonicity — /toh nis i tee/, n. 1. tonic quality or condition. 2. the state of bodily tone. 3. Physiol. the normal elastic tension of living muscles, arteries, etc., by which the tone of the system is maintained. [1815 25; TONIC + ITY] * * * …   Universalium

  • tonicity — noun a) The normal presence of tone or tension in a muscle or organ; tonus b) The ability of a solution to exert an osmotic pressure upon a membrane See Also: hypertonic …   Wiktionary

  • tonicity — to·nic·i·ty || təʊ nɪsÉ™tɪ n. tension of a muscle …   English contemporary dictionary

  • tonicity — [tə(ʊ) nɪsɪti] noun 1》 muscle tone. 2》 Linguistics the pattern of tones or stress in speech. 3》 Biology osmotic pressure …   English new terms dictionary

  • tonicity — to·nic·i·ty …   English syllables

  • tonicity — n. 1) the normal state of slight contraction, or readiness to contract, of healthy muscle fibres. 2) the effective osmotic pressure of a solution. See: hypertonic, hypotonic, osmosis …   The new mediacal dictionary

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