Why do red blood cells swell/burst in an iso-osmotic solution?
The cell membrane is made of a lipid bi-layer with protein molecules embedded inside this bi-layer. It is selectively permeable and based on concentrations of substances inside and outside the cell, and particle size, there can be a net movement of molecules into or out of the cell from high to low concentration. If an erythrocyte(red blood cell) is in solution and on both sides of the cell membrane, concentrations are the same, or isotonic, no movement of molecules across the membrane will occur. However, if you place a red blood cell in water, there will be more water outside the cell than inside. This will cause inward osmosis, and water will enter the cell through the membrane. This will cause the cell to swell and eventually to burst.