Explain cholera toxin and chloride secretion
Place the ion movement in the small intestine before explaining water movement.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall solute concentration and the gut lumen.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the intestinal lumen?
What you will learn
- The bacterium produces a toxin
- State the ion direction before water
WHO background: cholera cause and transmission. Original classroom questions; this model does not diagnose infection.
The bacterium produces a toxin
The cholera bacterium produces a toxin that acts on the lining of the small intestine. In the syllabus explanation, the toxin causes increased secretion of chloride ions into the intestinal lumen. The lumen is the inside space of the gut containing its contents, not the inside of the surrounding blood vessel.
Distinguish the causal organism from the chemical it produces: the bacterium is the pathogen and its toxin changes the intestinal process. The toxin is not a white blood cell and does not need to turn water into chloride ions. Detailed toxin signalling pathways are outside the required explanation.
State the ion direction before water
Chloride moves into the lumen in the secretion step. Increased dissolved solute in the lumen sets up the conditions for osmotic water movement into the gut, developed next. The original model therefore shows the chloride arrow first and a separate water arrow; it does not show water pulling chloride out as the initiating event.
A good causal explanation uses a chain rather than a list: bacterial toxin, increased chloride secretion into the small intestine, lower lumen water potential, then water moving into that space. Real intestinal contents involve several ions and transport processes, but the required school model isolates this chloride-driven sequence.
Pause and explain
What is the required initiating toxin effect?
Worked example
A diagram labels blood/tissue side and intestinal lumen. Which direction should the chloride-secretion arrow point?
Show the worked solution
- The secretion destination is the lumen.
- Read the compartments before drawing the arrow.
Answer Towards the intestinal lumen.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Complete the first three links: bacterium → ___ → ___ into the intestinal lumen.
Give me a hint
Name the produced chemical and ion process.
Compare my reasoning
- The bacterium produces toxin.
- The toxin causes chloride secretion into the lumen.
Bacterium → toxin → increased chloride secretion into the intestinal lumen.
Look for these in your work
- Name toxin
- Name chloride and destination
Correct “cholera toxin causes chloride to move from the gut into blood, pulling all water into blood”.
Give me a hint
Check the required secretion direction.
Compare my reasoning
- The initiating chloride secretion is into the lumen.
- Water subsequently moves into the gut by osmosis.
Toxin increases chloride secretion into the gut lumen; water then moves into the gut by osmosis.
Look for these in your work
- Correct ion direction
- Correct water destination
A student identifies the toxin as a bacterial cell. Explain the category error.
Give me a hint
Separate producer and product.
Compare my reasoning
- The bacterium is the organism.
- The toxin is a chemical produced by that organism.
The toxin is a chemical product of the cholera bacterium, not the bacterial cell itself.
Look for these in your work
- Name producer
- Name product
Common mistakes
- Reversing chloride into the blood as the initiating step.
- Calling the toxin itself the bacterium.
What can you explain now?
What is the required initiating toxin effect?
Compare with the explanation
Increased chloride secretion into the small intestine
Ion movement comes before the osmotic consequence.
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Make it stick
Write the toxin-to-chloride chain and draw the direction into the lumen.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.