OxfordAQA International A Level Biology
OxfordAQA International A Level Biology 3.2.3 Cholera Exam Questions
Free OxfordAQA International A Level Biology revision for 3.2.3 Cholera: what you need to know, common mistakes and exam-style questions with answers. Covers the cholera toxin, chloride ion secretion, diarrhoea, oral rehydration solutions and the ethics of trials.
Practise 3.2.3 Cholera with past papers
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About OxfordAQA International A Level Biology 3.2.3 Cholera
Cholera is a distinctive OxfordAQA Unit 2 topic. You need to explain how toxins produced by the cholera bacterium increase the secretion of chloride ions into the lumen of the intestine, how this changes the water potential gradient and causes severe diarrhoea, and how oral rehydration solutions treat cholera and other diarrhoeal diseases. You also need to discuss the science behind improved oral rehydration solutions and the ethical issues involved in trialling them on humans. This topic applies your knowledge of membranes, osmosis, co-transport and absorption from earlier topics, so examiners often set it as an applied explanation question.
What you need to know
- The cholera toxin Cholera bacteria release a toxin in the small intestine. The toxin increases the secretion of chloride ions from epithelial cells into the lumen of the intestine.
- Diarrhoea Chloride ions in the lumen lower the water potential there, so water moves out of the epithelial cells and tissues into the lumen by osmosis. This produces severe, watery diarrhoea and dehydration.
- Oral rehydration Oral rehydration solutions (ORS) contain water, sodium ions and glucose. Sodium and glucose are absorbed by co-transport, which lowers the water potential of the epithelial cells, so water is reabsorbed by osmosis.
- Improved ORS Improved solutions adjust the concentrations of salts and glucose, or use sources such as starch, so that more water is absorbed without drawing extra water into the gut. Potassium can be included to replace lost ions.
- Ethics of trials Trials on humans raise ethical issues such as informed consent, testing on vulnerable patients including children, and whether it is fair to give some patients an existing solution as a control.
Common mistakes in 3.2.3 Cholera
- Saying the toxin damages the epithelial cells so water leaks out. The toxin increases chloride ion secretion; water then follows by osmosis.
- Saying ORS works because water is absorbed directly. Water follows the absorption of sodium ions and glucose by osmosis.
- Describing the water movement using concentration of water. Use water potential: the lumen has a lower water potential, so water moves into it.
- Forgetting glucose in ORS. Glucose is needed for sodium-linked co-transport.
Try three 3.2.3 Cholera exam-style questions
1. Explain how the cholera toxin causes diarrhoea. (3 marks)
Show the mark scheme answer
Toxin increases secretion of chloride ions into the lumen of the intestine; lowers the water potential of the lumen; water moves out of cells into the lumen by osmosis.
2. Explain why an oral rehydration solution contains both sodium ions and glucose. (3 marks)
Show the mark scheme answer
Sodium ions and glucose are absorbed together by co-transport; this lowers the water potential of the epithelial cells; water is absorbed by osmosis.
3. Suggest one ethical issue in testing a new oral rehydration solution on patients with cholera. (1 mark)
Show the mark scheme answer
Patients, often young children, may not be able to give informed consent; or control group may receive a less effective treatment.
3.2.3 Cholera FAQs
What does OxfordAQA International A Level Biology 3.2.3 Cholera cover?
It covers the cholera toxin, chloride ion secretion, diarrhoea, oral rehydration solutions and the ethics of trials.
Which unit is 3.2.3 Cholera examined in?
It is part of Unit 2, Biological systems and disease, which is examined at International AS. The Unit 5 synoptic paper at A2 can also draw on this content.
Which earlier topics do I need for cholera questions?
You need water potential and osmosis, active transport and co-transport, and the structure of the ileum. Cholera questions test whether you can apply these ideas to a new situation.
