1The digestive system
The digestive system is a long tube from your mouth to your anus, with the liver and pancreas helping. It breaks food into small molecules that can pass into your blood.
digestive systemliverpancreas
Lesson 4.3 notes
The digestive system
Follow food from your mouth to the end of your gut, and see how teeth, muscles, acid and enzymes break it down.
2In the mouth
Teeth chew food into small pieces, which gives enzymes more surface to work on. Saliva wets the food, and an enzyme in it starts breaking starch into sugar.
teethsaliva
3Down the oesophagus
When you swallow, waves of muscle squeeze behind the food and push it down the oesophagus to your stomach. This works even if you're upside down.
oesophagus
4In the stomach
The stomach's muscle wall churns food with acid and enzymes. The acid kills most of the bacteria in food, and a protease enzyme starts breaking down proteins.
stomachacidprotease
5Enzymes
Enzymes are biological catalysts: they speed up reactions without being used up. Each one works on one kind of food: carbohydrases break down starch, proteases proteins and lipases fats.
enzymesbiological catalystscarbohydrasesproteaseslipases
6In the small intestine
In the small intestine, enzymes from the pancreas finish digesting the food. Bile, made in the liver, breaks fat into tiny droplets so that lipase can digest it faster.
small intestinebile
7The end of the gut
The large intestine takes water back into the blood. What's left, including fibre that can't be digested, forms faeces. They're stored in the rectum and leave through the anus.
large intestinefaecesrectumanus
Explore: Digestion journey
Send a meal through the gut. Stop at each organ to see what happens to the starch, protein and fat in it.
Common mistake: Digestion needs enzymes, not just chewing
Chewing and churning only break food into smaller lumps. Enzymes break the big molecules in food into small ones, small enough to pass into the blood.
Remember
- Food goes: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
- Enzymes are biological catalysts. They speed up the breakdown of big food molecules into small ones, without being used up.
- The liver makes bile, the pancreas makes enzymes, and stomach acid kills most of the bacteria in food.
Going further at GCSE