2.5.8Structure and Functions in Living Organisms: Part 1
Peristalsis — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions
5 min read 6 parts Lesson 21 of 32
Section 1 of 6Moving food through the gut
Why food must be moved
- Why food must be moved The alimentary canal is a long tube running from the mouth to the anus, and food cannot travel along it by gravity alone.
- Muscles in the gut wall must push the food forward so that it reaches each organ in turn and can be digested and absorbed.
What peristalsis is
- What peristalsis is Peristalsis is a wave-like squeezing action produced by the muscles of the gut wall.
- The waves pass along the tube again and again, pushing the contents in one direction only.
Where it happens
- Where it happens It begins in the oesophagus, where it carries a ball of chewed food, the bolus, down towards the stomach.
- It continues in the small intestine, pushing partly digested food along towards the large intestine.
Section 2 of 6The two muscle layers
Gut wall muscle
Circular muscles
- Circular muscles These muscles form rings around the tube.
- When they contract, the ring tightens and the diameter of the lumen becomes smaller, squeezing the food.
Longitudinal muscles
- Longitudinal muscles These muscles run lengthways along the tube.
- When they contract, that section of the gut becomes shorter and wider, which helps to move the food on.
Working together
- Working together The two layers contract in a coordinated rhythm,
- so a section squeezes behind the food while the section ahead of it relaxes and opens.
- This is why the gut can move food even when you are lying down or upside down.
Section 3 of 6One wave of peristalsis
1
A bolus of food enters a section of the oesophagus and stretches the wall slightly.
2
The circular muscles behind the bolus contract, narrowing the tube and squeezing the food forward.
3
The longitudinal muscles around the food contract, shortening that section and pushing the food along.
4
The muscles ahead of the food relax, so the lumen is wide enough to receive it.
5
The wave of contraction moves on, repeating the pattern further down the tube until the food reaches the stomach.
6
In the stomach the food is churned into a thick, semi-liquid mixture called chyme, which is passed on into the small intestine, where peristalsis continues.
Section 4 of 6Helping the movement along
| Feature | What it does | Why it matters |
|---|---|---|
| Mucus | Produced continuously by cells in the gut lining and spread over the food mass | Lubricates the food and reduces friction, so it slides easily and does not damage the gut wall |
| Dietary fibre | Indigestible plant material such as cellulose, found in vegetables and wholegrain foods | Gives bulk, or roughage, so the muscles have something firm to push against |
| Rhythmic contraction | Circular and longitudinal muscles contract in turn without conscious control | Keeps food moving steadily in one direction through the whole gut |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Peristalsis | Rhythmic waves of muscle contraction that push food along the alimentary canal. |
| Bolus | A rounded mass of chewed food mixed with saliva that is swallowed. |
| Chyme | The semi-liquid mixture of partly digested food formed by churning in the stomach. |
| Lumen | The hollow space inside a tube such as the oesophagus or small intestine. |
| Circular muscle | Muscle arranged in rings around the gut that narrows the lumen when it contracts. |
| Longitudinal muscle | Muscle running along the length of the gut that shortens a section when it contracts. |
| Dietary fibre | Plant material that cannot be digested and provides bulk for the gut muscles to push against. |
Section 6 of 6Exam technique
1
Saying that food is moved by the stomach or the food itself 'pushing' — the movement is caused by muscles in the wall of the gut contracting in waves.
2
Writing that the circular muscles make the gut longer — circular muscles narrow the tube, and it is the longitudinal muscles that shorten it.
3
Confusing peristalsis with churning — churning in the stomach mixes food into chyme, whereas peristalsis moves food along in one direction.
4
Saying that fibre is digested for energy — it is not digested; its job is to add bulk so the muscles can push the food mass along.
5
Describing peristalsis as a one-off event rather than as repeated rhythmic waves.
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