Edexcel IGCSE Biology · Spec 2.27-2.28
The Digestive System
The parts of the alimentary canal, what each part does, and how food is moved along by peristalsis.
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The Digestive System
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Explained
The alimentary canal, region by region
The digestive system is a long tube running from mouth to anus, called the alimentary canal, with organs attached to it that release substances into it. Food passes through the tube. It never passes through the liver or the pancreas, which is a distinction worth holding on to.
Along the tube
- Mouth, where teeth break food into smaller pieces and saliva adds amylase, which begins starch digestion.
- Oesophagus, which carries food to the stomach. No digestion or absorption happens here.
- Stomach, which produces hydrochloric acid and protease. The acid gives the protease the pH it works best at and kills many bacteria in the food.
- Small intestine, in two parts. The duodenum is where bile and pancreatic enzymes enter and most digestion is completed. The ileum is where the soluble products are absorbed.
- Large intestine, where water is absorbed from the remaining material.
- Rectum and anus, where faeces are stored and then egested.
The two organs alongside
The liver makes bile, which is stored in the gall bladder and released into the duodenum. Bile is not an enzyme. It neutralises stomach acid so that the enzymes of the small intestine work at the right pH, and it emulsifies fats into small droplets, which increases the surface area for lipase to act on.
The pancreas makes amylase, protease and lipase, and releases them into the duodenum.
Peristalsis
Food does not fall through the gut. It is pushed. Circular muscles in the gut wall contract behind the food and relax in front of it, producing a wave that squeezes the material along.
Because it is muscular rather than gravitational, you can swallow lying down, and an astronaut can eat in orbit.
Fibre matters here. It is not digested and not absorbed, but it gives the muscles something to grip, so peristalsis works properly and material moves through at a reasonable rate.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a question on the function of fibre. Credit was given for saying that fibre facilitates egestion of faeces, moves food through the gut, or prevents constipation. An answer stating that fibre helps digestion was judged too vague and gained nothing, while an answer stating that fibre helps peristalsis gained the mark. The report ends by advising candidates to use accurate, precise terminology.
On enzymes, the same report records that strong answers stated that amylase digests starch, gave a correct location such as the mouth or duodenum, and said that starch is broken down into maltose and then maltase breaks maltose into glucose. Common errors were saying that amylase breaks starch straight into glucose, placing the digestion in the stomach, and confusing the spellings of maltose and maltase.
How to answer a question about a named region
Give the region, what is released there, and what that substance does. The stomach releases hydrochloric acid, which provides the low pH that protease needs. That is one sentence carrying two marks, and it is a more reliable answer than a general description of digestion.
Spec 2.27-2.28
What you need to know
- Name the parts of the digestive system
- Describe what each part does
- Explain how food is moved along by peristalsis
Active recall
Quick check
Answer each question before opening the answer.
List the main parts of the gut in order.
Mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
What is peristalsis?
Waves of muscle contraction that push food along the gut.
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