Edexcel IGCSE Biology · Spec 2.27
Duodenum, Ileum, Colon and Rectum
Covers locate four named regions of the alimentary canal, State the main function of each region and Distinguish digestion, absorption and egestion.
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Duodenum, Ileum, Colon and Rectum
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Explained
Duodenum, ileum, colon and rectum
Four regions, four jobs, and they happen in order. Digestion continues in the duodenum, absorption happens in the ileum, water is absorbed in the colon, and faeces are stored in the rectum. Knowing which process belongs to which region is what these questions test.
The duodenum
The duodenum is the first part of the small intestine, immediately after the stomach. Two secretions arrive here: bile from the liver, stored in the gall bladder, and pancreatic juice from the pancreas.
Bile is not an enzyme. It emulsifies fats, breaking large droplets into smaller ones, which increases the surface area for lipase to work on. It also neutralises the acid arriving from the stomach, giving the enzymes the alkaline conditions they need.
The ileum
The ileum is the main region of absorption, where the soluble products of digestion pass into the blood through villi.
Villi are adapted in ways that should look familiar: there are very many of them, giving a large surface area; their walls are one cell thick, so the diffusion distance is short; and each has a rich blood supply that carries absorbed molecules away, keeping the concentration gradient steep. Microvilli on each villus increase the surface area further.
The colon
The colon absorbs water from the material that remains. What arrives is liquid; what leaves is much more solid.
That is why illnesses affecting the colon cause the symptoms they do. If material passes through too quickly, too little water is absorbed.
The rectum
The rectum stores faeces before they are egested through the anus.
Digestion, absorption and egestion are three different things
Digestion is breaking large insoluble molecules into small soluble ones. Absorption is those small molecules passing into the blood. Egestion is the removal of undigested material that was never absorbed in the first place.
Egestion is not excretion. Excretion removes waste made by the body's own reactions, such as urea and carbon dioxide. Faeces are mostly material that never entered the body's cells at all, so removing them is egestion.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology note that most candidates correctly stated that fibre is important to facilitate egestion of faeces, to move food through the gut, or to prevent constipation.
Any of those three earns the mark. What does not is the vaguer claim that fibre helps digestion, which reports elsewhere describe as too imprecise. Fibre is not digested, so saying it aids digestion describes something that does not happen. Say what it actually does: it adds bulk, which the muscles of the gut can grip.
Spec 2.27
What you need to know
- Locate four named regions of the alimentary canal
- State the main function of each region
- Distinguish digestion, absorption and egestion
Active recall
Quick check
Answer each question before opening the answer.
Where does bile enter the alimentary canal?
The duodenum
What is the main absorptive region of the small intestine?
The ileum
What is the function of the rectum?
It stores faeces before egestion
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