Topic 10.1 · Supplement
Cholera
Cholera is where this chapter deliberately connects back to osmosis, and Cambridge examiners know it — which is why 'the bacteria cause diarrhoea' never earns full marks here. The required answer is the actual mechanism, one step producing the next.
Cholera is a disease caused by a bacterium, and it’s a named worked example of an indirect, waterborne transmissible disease — it’s typically transmitted in contaminated water, which is exactly why clean water supply is the control measure that matters most against it, as covered in controlling the spread of disease. But the syllabus doesn’t stop at transmission — it wants you to be able to explain what the bacterium does once it’s inside the gut.
The causal chain, step by step
- Cholera bacteria reach the small intestine.
- There, they produce a toxin.
- The toxin causes chloride ions to be secreted into the intestinal lumen.
- This lowers the water potential inside the intestine.
- Water moves from the blood and surrounding tissues into the intestine by osmosis.
- The result is severe diarrhoea.
- Water and ions are lost from the body, causing dehydration and ion imbalance.
The step that separates a strong answer from a weak one is number four. It’s not enough to say chloride ions are secreted and then jump straight to water moving — the reason water moves is that the chloride ions lower the water potential of the gut contents, and water then moves down that water potential gradient, from a region of higher water potential (the blood and tissues) to the lower water potential inside the gut. That’s the same osmosis principle from cell transport, just applied to a whole organ instead of a single cell.
Why the full chain matters for unfamiliar questions
Cambridge likes to test this mechanism by dropping you in partway through — a question might start with an unfamiliar graph or description of chloride ion secretion and ask what happens next, without mentioning “cholera” at all. If you know the chain as a sequence of cause and effect rather than as a block of facts about one disease, you can pick up the reasoning at any point and follow it forward: chloride ions lower water potential, water potential differences drive osmosis, osmotic water movement into the gut produces diarrhoea, and fluid loss produces dehydration and ion loss from the blood. That transferable reasoning is worth more marks, over the course of a paper, than memorising the cholera story on its own.