Cell Membranes and Transport: Question 5

Worked solution

Comparing water potentials

The external solution has a water potential that is higher (less negative) than the water potential of the cytoplasm inside the red blood cells. Water always moves by osmosis from a region of higher (less negative) water potential to a region of lower (more negative) water potential. Since the cytoplasm has the lower (more negative) water potential here, water moves into the cells by osmosis.

Why the cells swell and burst

Red blood cells, like all animal cells, have no cell wall surrounding their cell-surface membrane. In a plant cell, a rigid cell wall would resist the cell’s expansion, building up a pressure potential that eventually stops further net water entry (making the cell turgid). Without a cell wall, a red blood cell has no such resistance: as water continues to enter by osmosis, the cell keeps swelling until the cell-surface membrane can no longer contain the increased volume, and it bursts. This bursting of red blood cells due to excessive water uptake is called haemolysis.

Why the other options are wrong

• B describes the opposite scenario. A hypertonic solution (more negative water potential than the cytoplasm) would cause water loss and crenation, not the situation described here.
• C wrongly applies the plant-cell concept of a rising pressure potential (from a cell wall) to an animal cell, which has no cell wall to generate this resistance.
• D wrongly describes osmosis as an active, ATP-requiring process; osmosis is a passive process driven by differences in water potential, not by active transport.

Final answer

A. Water moves into the cells by osmosis, and with no cell wall to resist the increase in volume, the cells swell and undergo haemolysis.