In order to explain the difference,we have to understand that there are two types of solutes affecting the tonicity,they are either penetrating solutes or non-penetrating solutes. Penetrating solutes are those which can pass through the membrane of the cell affecting the osmotic pressure across the membrane,while the non-penetrating solutes cannot,therefore they only affect the tonicity.
Isotonic solution comprise non-penetrating solutes,and they are solution having the same osmotic pressure as the cell they surround.They are neither absorbed or do they absorb anything form the cell. Isoosmotic solutions on the other hand are a bit different,they comprise penetrating and non-penetrating solutes.Although they have the same osmotic pressure as the cell they surround,they comprise penetrating solutes ,which can enter the cell and increase the cell's osmotic pressure.Increasing the cell's osmotic pressure, in turn, make the cell prone to absorb water from the medium in order to equalize the osmotic pressure on both sides,and this could cause the bursting of the cell.
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