The digestive systemIB MYP Sciences: Revision notes
Section 1
The alimentary canal in order
The alimentary canal is the long tube that food passes through, from mouth to anus. In order:
- Mouth: teeth chew the food and saliva is added
- Oesophagus: carries food to the stomach
- Stomach: churns the food and mixes it with acid and enzymes
- Small intestine: digestion is completed and digested food is absorbed
- Large intestine: absorbs water
- Rectum: stores faeces
- Anus: faeces are passed out of the body
Section 2
Mechanical and chemical digestion
Digestion breaks large insoluble food molecules into small soluble ones that can be absorbed.
- Mechanical digestion breaks food into smaller pieces without changing the molecules, e.g. chewing and the churning of the stomach. This gives a larger surface area for enzymes.
- Chemical digestion uses enzymes to break molecules into smaller ones, e.g. amylase breaking starch into sugars.
Section 3
Peristalsis
The gut wall has muscles. Waves of muscle contraction squeeze behind the food and push it along the gut. This is peristalsis. It moves food through the oesophagus, stomach and intestines, and works even if you are upside down.
Section 4
Organs that help digestion
Some organs add substances to the food but food does not pass through them.
- Salivary glands: make saliva containing amylase
- Liver: makes bile
- Gall bladder: stores bile and releases it into the small intestine
- Pancreas: makes enzymes (amylase, protease and lipase) that are released into the small intestine
Bile is alkaline. It neutralises the acid from the stomach and emulsifies fats, breaking them into small droplets with a larger surface area for lipase.
The gall bladder stores bile, but the liver makes it.
Section 5
Absorption in the small intestine
Digested food molecules pass through the wall of the small intestine into the blood by diffusion and active uptake. The inner wall is covered with villi, which are adapted for absorption:
- finger-like projections give a large surface area
- the wall is one cell thick, so there is a short distance to travel
- a rich blood supply (capillaries) carries absorbed food away and keeps a steep concentration gradient
The blood then carries the digested food to the cells of the body.
Section 6
Large intestine and assimilation
What is left after the small intestine is mostly water, fibre and dead cells. The large intestine absorbs most of the water, leaving solid faeces. These are stored in the rectum and removed through the anus.
Assimilation is when the absorbed food molecules become part of the cells, for example amino acids are used to build new proteins and glucose is used in respiration.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The digestive system
- A boy in Istanbul eats a lunch of bread, cheese and olives. Over the next few hours the food travels through his alimentary canal, and digestive juices are added to it by several organs.Explain the functions of bile in digestion.2 marks
- A student in Singapore swallows a mouthful of bread. In the mouth the bread is chewed and mixed with saliva, and it is then swallowed as a soft ball of food.Explain why chewing food helps chemical digestion.2 marks
- The small intestine is about 6 m long and its inner lining is covered with millions of tiny finger-like projections called villi. A patient with coeliac disease has villi that have become flattened, and she absorbs much less digested food into her blood than a healthy person.Explain how villi are adapted to absorb digested food efficiently.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).