IB DP Biology- D2.3 Water potential - IB Style Questions For SL Paper 2 - FA 2025

(a)
Deduction: The salt concentration increased from A to B, so solution B is more concentrated (hypertonic) than the cytoplasm of the red blood cells.
Water therefore moves out of the cells by osmosis, causing them to shrink and become crenated.

Reasoning steps:
• In micrograph B, red blood cells appear shrivelled compared with those in A.
• Osmosis is the net movement of water from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential).
• When the surrounding solution becomes hypertonic, water leaves the cells, so their volume decreases and they shrink.

(b)
Answer: In distilled water, red blood cells take up water by osmosis, swell, and burst (hemolyse), so no intact cells are visible.

Reasoning steps:
• Distilled water is hypotonic relative to the cytoplasm (very low solute concentration, very high water potential).
• Water enters the red blood cells by osmosis down the water potential gradient.
• The plasma membrane cannot withstand the increased internal pressure, so the cells lyse, releasing hemoglobin into the surrounding solution.
• Once lysed, the individual cells are no longer visible as distinct structures under the microscope.