Tonicity
In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane-impermeable solutes across a cell membrane which determine the direction and extent of osmotic flux. It is commonly used when describing the swelling-versus-shrinking response of cells immersed in an external solution.
Unlike osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an effective osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always equilibrate with equal concentrations on both sides of the membrane without net solvent movement. It is also a factor affecting imbibition.
There are three classifications of tonicity that one solution can have relative to another: hypertonic, hypotonic, and isotonic.A hypotonic solution example is distilled water.

Hypertonic solution
A hypertonic  is when the concentration inside the cell is high and the environment is dilute. In biology, the tonicity of a solution usually refers to its solute concentration relative to that of another solution on the opposite side of a cell membrane; a solution inside of a cell is called hypertonic if it has a greater concentration of solutes than the cytosol inside the cell and its environment is dilute. So when the cell is hypertonic(concentrated) the cell needs more water, so water is forced to enter inside the cell and the cell will swell up and eventually burst.
When plant cells lose water they are hypotonic, because their inside is diluted(with more water) and the environment is concentrated so it needs water then the flexible cell membrane pulls away from the rigid cell wall, but remains joined to the cell wall at points called plasmodesmata. The cells often take on the appearance of a pincushion. In plant cells the terms isotonic, hypotonic and hypertonic cannot strictly be used accurately because the pressure exerted by the cell wall significantly affects the osmotic equilibrium point.

Hypotonic solution
A hypotonic is when the concentration inside of the cell is lower than a solution outside of a cell is called hypotonic. If it has a lower concentration of solutes inside a cell then water is forced to move out. Due to this, water is removed out of the cell causing shrinking of the cell membrane.
For cells without a cell wall such as animal cells, they lose water and shrivel.
When plant cells are in a hypotonic solution, the central vacuole loses water and the cell membrane sticks against the cell wall.

Isotonicity

A solution is isotonic when its effective osmole concentration is the same as that of another solution. In biology, the solutions on either side of a cell membrane are isotonic if the concentration of solutes outside the cell is equal to the concentration of solutes inside the cell. In this case the cell neither swells nor shrinks because there is no concentration gradient to induce the diffusion of large amounts of water across the cell membrane. Water molecules freely diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction, the cell will neither gain nor lose water.


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