Tonicity

Tonicity refers to the osmotic pressure of a solution, typically relative to the concentration of solutes in a cell. It's a measure of the effective osmotic pressure gradient, dictating the direction of water movement across a semipermeable membrane. Tonicity is crucial for maintaining cell volume and function, as it influences whether cells swell, shrink, or remain stable in size. This is influenced by solute concentrations that cannot freely cross the cell membrane, such as salts and proteins, determining the relative water flow to and from the cell.

Tonicity meaning with examples

• In a hypotonic solution, like pure water, the cell has a higher solute concentration, causing water to rush into the cell, leading to swelling and potentially bursting the cell membrane. Red blood cells in distilled water will lyse due to the high water influx.
• An isotonic solution has the same solute concentration as the cell's interior. There is no net movement of water; the cell maintains its normal size and shape. Saline solution (0.9% NaCl) is commonly used intravenously due to its isotonicity with human blood.
• A hypertonic solution has a higher solute concentration than the cell. Water flows out of the cell, causing it to shrink and become crenated. Placing plant cells in a highly concentrated sugar solution will cause plasmolysis.
• The kidney's ability to regulate urine concentration involves controlling tonicity. By varying solute reabsorption and secretion, the kidney ensures proper fluid balance and blood pressure regulation within the body to keep an isotonic state as much as possible.