Hypotonic solution

In the syllabus and the exam

Hypotonic solution is one half of a comparison the syllabus expects candidates to apply confidently in both directions, matched against hypertonic solution, and it is usually tested by describing a cell (plant or animal) placed in a named solution and asking candidates to predict and explain the outcome. In a plant cell, a hypotonic external solution causes water to enter by osmosis until the cell becomes turgid, with the cell wall preventing it from bursting; in an animal cell, which lacks a cell wall, the same hypotonic environment can cause the cell to swell and eventually lyse, or burst, if enough water enters. A common error is defining hypotonic purely by the words 'weak' or 'dilute' without stating the comparison is relative to the water potential of the cell itself, since a solution is only hypotonic in relation to a specific cell; the same solution could be hypotonic to one cell and hypertonic to another with a different internal concentration. Candidates are also frequently expected to distinguish turgid (a walled plant cell swollen with water, but intact) from lysed (an animal cell that has actually burst), since both describe the effect of a hypotonic solution but on different cell types with different structural limits. This term also appears in applied contexts such as why intravenous fluids given to patients must be isotonic rather than hypotonic, since a hypotonic drip would cause red blood cells to absorb water and burst, a real-world link examiners sometimes use to test understanding beyond the standard plant-and-animal-cell diagram.

Example

A red blood cell placed in a hypotonic solution, such as distilled water, absorbs water by osmosis and may eventually burst, since it has no cell wall to resist the swelling.

Related terms

Hypertonic solutionOsmosis

Source:SRC-DSKP-EN