Hypertonic solution

In the syllabus and the exam

Hypertonic solution completes the comparison the syllabus builds around cells and osmosis, and candidates need to describe what happens when a cell is surrounded by a solution with lower water potential (more concentrated) than the cell itself, so that water leaves the cell down its water potential gradient. In a plant cell, this water loss causes the cytoplasm and cell membrane to pull away from the rigid cell wall, a visible phenomenon called plasmolysis that is often shown in microscope diagrams for candidates to identify; in an animal cell, the same hypertonic environment causes the cell to shrink and become crenated, since there is no cell wall to give the cell a fixed outer shape. A common mistake is describing plasmolysis and crenation as the same process, when plasmolysis specifically refers to the membrane detaching from a cell wall in walled cells, while crenation describes the shrivelling of the entire outer surface of a wall-less cell. Another frequent error is assuming a hypertonic solution is always concentrated in an absolute sense; as with hypotonic solutions, 'hypertonic' only has meaning relative to the water potential inside a specific cell, so the same external solution could be hypertonic to one cell type and isotonic to another. This term also explains why adding salt or sugar preserves food: a hypertonic environment draws water out of any bacterial or fungal cells present by osmosis, dehydrating and killing them, a practical application examiners sometimes link to questions on food preservation and spoilage.

Example

A plant cell placed in a hypertonic sugar solution loses water by osmosis, causing the cell membrane to shrink away from the cell wall in a process called plasmolysis.

Related terms

Hypotonic solutionOsmosis

Source:SRC-DSKP-EN