Tonicity

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Tonicity or effective osmolality is the ability of a solution to cause water movement. It is in reference to hypertonic, hypotonic and isotonic cellular states.

Tonicity differs between animal cells and plant cells. Since plants cells have a cell wall, instead of shrinking or bursting the plant cell is considered to be plasmolyzed if it is placed in a hypertonic solution and is considered to be in its normal state if it is placed in an isotonic solution. In a hypotonic solution the plant cell is considered to be in a turgid state.

It is very important to remember that tonicity is dependent on the water concentration(or any solvent), not the solute concentration. If a solution is hypotonic, the water concentration is greater outside the cell and so water rushes into the cell in order to equal out the water concentration. This causes animal cells (without a cell wall) to burst and for plant cells (which do have a cell wall) to become turgid. If the solution is isotonic, the water concentration is the same on either side of the cell membrane and so, while water does move from inside to outside the cell and vice versa, there is no net movement of water. If the solution is hypertonic, the water concentraion inside the cell is greater. This leads to water rushing out of the cell. Animal cells shrivel up and plant cells become plasmolysed (This is where the cell membrane pulls away from the cell wall in places as the cytoplasm shrinks. Note that the cell wall does not change shape.)

It is important also to note that tonicity applies to the impermeant solutes within a solvent, as opposed to osmolarity which takes into account both permeant and impermeant solutes. For example, urea is a permeant solute, meaning it can pass the membrane freely, and will not be factored when determining the tonicity of a solution. On the other hand, NaCl is impermeant, and cannot pass the membrane without the help of a concentration gradient, and will therefore contribute to a solution's tonicity. It is then possible for a solution to be hypotonic, and yet be isoosmotic.

The elastic tension of living muscles, arteries, etc. that facilitates response to stimuli. Clinically, this is referred to as tonus (See muscle tone).