hypotonic

Hypotonic refers to a solution that has a lower solute concentration compared with another solution, typically the cytoplasm of a cell. When separated by a semipermeable membrane, water tends to move from the hypotonic solution into the higher-solute area by osmosis.

In biological contexts, hypotonic solutions have a lower osmolarity than the cell interior. In animal cells, exposure to a hypotonic environment causes water to enter, leading to swelling and potential lysis. In plant cells, the cell wall limits expansion, resulting in turgor pressure while avoiding bursting; a hypotonic environment generally keeps plant cells turgid.

Examples: distilled water is hypotonic relative to intracellular fluids; 0.45% saline is hypotonic compared with blood plasma, while 0.9% saline is isotonic. In intravenous therapy, hypotonic fluids are used cautiously because excessive swelling can occur. In the laboratory, hypotonic buffers can lyse cells to release contents.

Glucose solutions like 5% dextrose in water (D5W) are considered isotonic in the container but become hypotonic in the body after glucose is absorbed, potentially contributing to hyponatremia if used inappropriately.