Osmosis in Animal Cells

Animal cells are surrounded only by a cell surface membrane. They have no cell wall, so nothing resists the cell swelling if water enters. Red blood cells show clearly what happens when animal cells are placed in solutions of different water potential.Isotonic solution (same water potential as the cell): water enters and leaves at the same rate, so there is no net movement of water. The cell keeps its normal biconcave shape.Hypotonic solution (higher water potential than the cell, e.g. pure water): water enters the cell by osmosis. The cell swells and, because there is no wall, the membrane may burst. Bursting of red blood cells is called haemolysis.Hypertonic solution (lower water potential than the cell, e.g. concentrated salt solution): water leaves the cell by osmosis. The cell shrinks and its membrane becomes wrinkled; it is described as crenated.Both swelling and shrinking stop cells working properly, so the water potential of blood plasma and tissue fluid must be kept close to that of the cells. In mammals the kidneys control this (osmoregulation).This is also why fluids given through an intravenous drip are isotonic with blood, for example 0.9% sodium chloride solution. Giving pure water would cause haemolysis.Worked example: a red blood cell with a water potential of −700 kPa is placed in a solution of −300 kPa. The solution has the higher water potential, so water enters the cell and it swells, and may burst.