The tonicity of a solution pertains to the osmotic pressure or tension of a solution, as in the cells would swell or shrink depending on the tonicity of the environment. It determines the direction of diffusion between two solutions. In cells, the tonicity is influenced by the concentration of solutes between solutions separated by a semi-permeable plasma membrane. Not all solutes can enter and leave the cell because of the plasma membrane. The plasma membrane is an essential part of the cell that regulates the passage of ions and molecules into and out of the cell. Thus, this creates differences in the concentrations of solutes between the cytosol of the cell and the solution surrounding the cell.
There are three types of solutions determined based on tonicity: (1) hypotonic solution, (2) hypertonic solution, and (3) isotonic solution. In hypotonic solution, the osmotic pressure is lower than the solution being compared to. The solutes in a hypotonic solution are also less (in concentration) than another solution. Thus, a hypotonic solution would rather have more water. For instance, a cell in a hypotonic solution would cause the water to enter (diffuse) into the cell. This, in turn, would cause the cell to swell.