Revision Notes

7.2 Alimentary Canal

The Stages of Food Breakdown

  • Food taken into the body goes through 5 different stages during its passage through the alimentary canal (the gut):
    • Ingestionthe taking of substances, e.g. food and drink, into the body through the mouth
    • Mechanical digestion – the breakdown of food into smaller pieces without chemical change to the food molecules
    • Chemical digestion – the breakdown of large, insoluble molecules into small, soluble molecules
    • Absorption – the movement of small food molecules and ions through the wall of the intestine into the blood
    • Assimilation – the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
    • Egestion – the passing out of food that has not been digested or absorbed, as faeces, through the anus

Structure & Function of the Alimentary Canal

The human digestive system, IGCSE & GCSE Biology revision notesThe human digestive system


The Alimentary Canal table, IGCSE & GCSE Biology revision notes

Diarrhoea Causes & Treatment

  • Diarrhoea is the loss of watery faeces from the anus
  • If it is severe and continues for a long time, it can lead to death
  • Severe diarrhoea can cause the loss of significant amounts of water and ions from the body, causing the tissues and organs to stop working properly
  • It can be effectively treated by oral rehydration therapy
  • This is a drink with a small amount of salt and sugar dissolved in it
  • There are many causes of diarrhoea, one of which is infection with Vibrio cholerae bacteria, which causes the disease cholera
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How Does Vibrio Cholerae Cause Diarrhoea?

How cholera leads to diarrhoea, IGCSE & GCSE Biology revision notesHow cholera leads to diarrhoea


  • Ingested via infected water or food, if it enters the small intestine it can cause illness in the following way:
    1. Bacteria attach to the wall of the small intestine
    2. They produce a toxin
    3. The toxin stimulates the cells lining the intestine to release chloride ions from inside the cells into the lumen of the intestine
    4. The chloride ions accumulate in the lumen of the small intestine and lower the water potential there
    5. Once the water potential is lower than that of the cells lining the intestine, water starts to move out of the cells into the intestine (by osmosis)
    6. Large quantities of water are lost from the body in watery faeces
    7. The blood contains too little chloride ions and water

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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