Why do animal cells burst when placed in pure water?
To answer this question adequately a brief explanation of a biological process known as osmosis will be provided.
Osmosis is a process by which water passes from an area of high water concentration to an area of low water concentration through a semi-permeable membrane (osmotic flow). Pure water i.e. solely H2O and no other solutes present, would be completely comprised of water molecules and therefore possess the highest potential water concentration. While the water or cytoplasm present in animal cells has a low concentration of water in comparison, due to the fact it contains a large number of solutes.
When placing an animal cell in pure water, the pure water through the process of osmosis diffuses into the animal cell. Certain animal cells such as erythrocytes (red blood cells) have difficulty in controlling their cell volume in response to osmotic flow. This in turn can often cause them to swell to levels that apply too much pressure to their cells walls, which as a consequence may eventually ‘burst’ (osmotic lysis).
The reason this specifically happens to animal cells, is due to plant cells often having much more turgid cell walls which are difficult to burst via undue osmotic pressure.