Tonicity refers to the affect of a solution on a cell. A solution is a mixture of two or more substances in which one (a solute) is dissolved in the other (a solvent). So a solute is the substance that is dissolved in a solvent to form a solution. This solution is homogeneous, meaning that it is a single phase and none of the solute is visible in the solvent. Common solutes would be salt or sugar. A common solvent would be water.
That solute can exert a certain amount of pressure referred to as tonicity. This pressure can affect the fluid volume and the pressure in a cell by affecting the movement of water down its concentration gradient. Yes, water, as other molecules will move down it’s concentration gradient.
Tonicity has three classifications; hypertonicity, hypotonicity and isotonicity. These refer to the concentration of a solution in reference to a living cell (a membrane bound structure).
Hypotonic –refers to a solution that is lower in solutes than that of the fluid inside of a cell. As a result, water will flow across the cell membrane into that cell (down its concentration gradient) from the surrounding environment eventually causing the cell to swell and burst (c).
Hypertonic – refers to a solution that is higher in solutes than that inside of a cell. As a result, water will flow out of the cell into the surrounding fluid eventually causing the cell to shrink or crenate (d).
Isotonic – refers to a solution whose concentration of solutes is equal to the concentration of solutes inside of a cell. The water flows equally across the membrane in both directions causing no change in the volume or shape of the cell (a,b).
