How Will Water Molecules Move in a Isotonic Solution?

Biological Tonicity

• Biologists classify solutions in three categories according to their tonicity: hypertonic, hypotonic and isotonic. The amount of water present in the solutions inside and outside the cell and the cellular membrane separating the two are especially important in cytology, the study of cells and the cellular environment. The tonicity of the solution inside the cell is compared to that of the surrounding extracellular fluid. In a living system, water moves back and forth through the cell membrane to establish an equilibrium between the interior and exterior solutions.

Solution Types

• A hypertonic solution is one where the water molecules move out of the cell into the extracellular fluid, seeking to dilute the excess salt in the fluid surrounding the cell. As a result, the cell will shrink and wrinkle. In a hypotonic solution, water molecules move into the cell to equalize its tonicity with the cellular environment. The cell will swell and can break due to the volume of excess water it absorbs. When the concentration of solution outside the cell is the same as the concentration within the cell, an isotonic solution results. Because of this balance, water molecules will not pass through the cell membranes in either direction.

Osmosis

• The movement of water across a semipermeable membrane from an area of low solute concentration to one of higher concentration is called osmosis. The membrane surrounding cells is semipermeable or selectively permeable, allowing only certain molecules, such as water, to pass through. Water can pass from a hypotonic solution to a hypertonic solution, and the reverse. If the solutions both surrounding and inside the cell are equal in tonicity, they reach an equilibrium forming an isotonic state. The water molecules will not move through the membrane and remain in place.

Diffusion

• The diffusion process refers to the spontaneous movement of particles from an area of high concentration to one of a lower concentration. It does not require energy and takes place through random kinetic movement, even if the two areas are separated by a cell membrane. The particles cease to diffuse when a uniform distribution of particles reaches an equilibrium in an isotonic solution, wherein no water molecules move through the membrane.