What is tonicity?

Tonicity refers to the relative concentration of solutes in two solutions separated by a semipermeable membrane, such as a cell membrane. It is a measure of the osmotic pressure difference between the two solutions and determines the direction of water movement across the membrane.

When two solutions with different tonicity are separated by a semipermeable membrane, water molecules tend to move from the region of lower solute concentration (lower tonicity) to the region of higher solute concentration (higher tonicity) in order to equalize the solute concentration on both sides of the membrane. This process is known as osmosis.

There are three possible tonicity scenarios:

Isotonic: When two solutions have the same tonicity, there is no net movement of water across the membrane. The concentration of solutes is equal on both sides, resulting in no change in cell volume. In an isotonic solution, the cell maintains its shape and size.

Hypotonic: If the solution outside the cell has a lower tonicity (lower solute concentration) compared to the cytoplasm inside the cell, water moves into the cell by osmosis. This can cause the cell to swell and potentially burst or lyse.

Hypertonic: When the solution outside the cell has a higher tonicity (higher solute concentration) than the cytoplasm inside the cell, water moves out of the cell by osmosis. This can cause the cell to shrink or undergo crenation.

Understanding tonicity is important in various biological and medical contexts, such as maintaining the proper balance of fluids in the body, administering intravenous fluids, and understanding the effects of different solutions on cells and tissues.