IGCSE Biology 0610: Haemolysis of Red Blood Cells in Distilled Water

A student compares two samples of red blood cells. Sample P is mixed with distilled water, which contains no dissolved solutes. Sample Q is mixed with a sodium chloride solution that is much more concentrated than the cytoplasm inside a red blood cell.

(a) State whether the water potential of the distilled water is higher or lower than the water potential of the cytoplasm inside a red blood cell, and explain your answer. [2]

(b) Predict what happens to the red blood cells in sample P, using the term haemolysis in your answer, and explain your prediction in terms of water potential. [3]

Part (a): Comparing water potential

Distilled water contains no dissolved solutes, so it is the most dilute solution possible. A more dilute solution (or pure water) always has a higher water potential than a more concentrated one, such as the cytoplasm inside a red blood cell, which contains many dissolved substances. So the water potential of the distilled water is higher than the water potential of the red blood cell’s cytoplasm.

Part (b): Predicting the outcome in sample P. Haemolysis

Because the distilled water has a higher water potential than the cytoplasm inside the red blood cells, water moves by osmosis from the distilled water, through the partially permeable cell membranes, into the cells. Red blood cells, like other animal cells, have no cell wall to resist this inward movement of water. As a result, the cells keep swelling as more and more water enters, until eventually the cell membrane can no longer hold together and it bursts, releasing the cell contents. This bursting of a red blood cell caused by taking in too much water is called haemolysis.

Part (c): The effect of the concentrated salt solution in sample Q

The sodium chloride solution in sample Q is much more concentrated than the cytoplasm inside a red blood cell, so it has a lower water potential than the cytoplasm. Water therefore moves by osmosis out of the red blood cells, through their partially permeable cell membranes, into the surrounding solution. As the cells lose water, they shrink and become smaller and more shrivelled in appearance.

Final answers

(a) The distilled water has a higher water potential than the cytoplasm, because it contains no dissolved solutes.
(b) The red blood cells in sample P take in water by osmosis and swell until they burst, haemolysis, because the distilled water has a higher water potential than their cytoplasm.
(c) The red blood cells in sample Q lose water by osmosis and shrink, because the concentrated salt solution has a lower water potential than their cytoplasm.