Movement of Substances Across a Plasma Membrane and Its Application in Daily Life

• Saline solutions, normally used in medicine, are isotonic solution to the blood plasma. It contains 0.9 g sodium chloride per 100 mL (0.9% NaCl).
• Saline solution has the same osmotic pressure (solute concentration) as the fluid inside red blood cells. Hence, there’s no net movement of water in or out of the cell. This prevents red blood cells from crenation or haemolysis.

Imagine:

If the saline solution is hypertonic to blood plasma, water will move out of the cells by osmosis. Red blood cells will shrink and undergo crenation. Red blood cells shape is changed and unable to carry out normal function such as carrying oxygen to the whole bofy.

If the saline solution is hypotonic to blood plasma, water will move into the red blood cells by osmosis. Red blood cells will swell then burst (the burst of red blood cells is known as haemolysis)