Lesson focus: The digestive system is a coordinated route in which mechanical and chemical digestionBreaking large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. produce small soluble molecules for absorptionWhen small digested food molecules pass from the gut into the blood..
Sequence digestive organs and explain the distinct roles of ingestionTaking food into the body through the mouth., digestion, absorption and egestionGetting rid of undigested food from the body as faeces (poo)..
Build on earlier learning
NutrientsNutrients have biological roles only when suitable molecules become available to cells. This lesson follows how the digestive system processes food.
You will be successful when you can…
- Sequence the main digestive organs.
- Distinguish mechanical from chemical digestion.
- Explain the roles of liver, gall bladder and pancreas.
- Distinguish egestion from excretion.
Words that carry the science
- ingestion
- Taking food into the mouth.
- digestion
- Breaking large insoluble food molecules into smaller soluble molecules.
- alimentary canal
- The continuous tube from mouth to anus through which food passes.
- peristalsis
- Coordinated waves of circular and longitudinal muscle contraction that move contents along the gut.
- absorption
- Movement of digested molecules across the gut wall into blood or lymph.
- egestion
- Removal of undigested material as faeces.
Follow food and secretions through the alimentary canal
01The canal moves and mechanically processes food
Teeth break food into smaller pieces and increase surface area. Saliva moistens food and contains amylase. Swallowing passes the bolus into the oesophagus, where peristalsisWaves of muscle squeezing that push food along the gut, like squeezing toothpaste along a tube. moves it to the stomach.
The stomach stores and churns food. Acid provides a low pH that helps stomach protease action and kills many swallowed microorganisms.
02Accessory organs add secretions
The liverA large organ that makes bile, which emulsifies fats, and breaks down harmful substances in the blood. makes bile, which is stored in the gall bladder and released into the small intestine. Bile neutralises acidic stomach contents and emulsifies lipid into droplets, increasing surface area for lipase.
The pancreasAn organ that makes digestive enzymes and sends them into the small intestine. releases digestive enzymes and alkaline fluid into the small intestine. The small intestine completes much digestionBreaking large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. and absorbs products.
03The large intestine recovers water
The large intestine absorbs much remaining water and ions. Gut microorganisms act on some material, and faeces are stored in the rectum before leaving through the anus.
egestionGetting rid of undigested food from the body as faeces (poo). removes undigested food from the gut. excretionGetting rid of waste made by chemical reactions inside cells, such as carbon dioxide or urea. removes metabolic waste made by cells, such as urea or carbon dioxide, so the processes are different.
The journey through the digestive system
Follow a mouthful of food through the gut. Enzymes break large molecules into small ones that can be absorbed into the blood.
Stage 1 of 6
The liver and pancreas are not part of the tube: food never passes through them. They add bile and enzymes to the small intestine.
Trace a starch-rich mouthful
Describe the route and major processing of a mouthful of bread.
- Teeth break it up and salivary amylase begins starch digestion in the mouth.
- peristalsisWaves of muscle squeezing that push food along the gut, like squeezing toothpaste along a tube. carries the bolus down the oesophagus to the stomach.
- In the small intestine, pancreatic and intestinal enzymes continue carbohydrate digestion to absorbable sugars.
- Products cross the small-intestine wall; remaining material enters the large intestine, where water is absorbed.
How to make the reasoning visible: The journey combines physical movement, mechanical processing, chemical digestion and selective absorptionWhen small digested food molecules pass from the gut into the blood..
Build an organ–process route
Accessory organs supply secretions even though food does not pass through them.
| Structure | Does food pass through it? | Main role | Key distinction |
|---|---|---|---|
| Mouth | Yes | Ingestion, chewing and salivary digestion | Mechanical and chemical processes begin |
| Oesophagus | Yes | Peristalsis | Transport, not major digestion |
| Stomach | Yes | Churning, acid and protease | Acidic compartment |
| Liver/gall bladder | No | Make/store and release bile | Bile is not an enzyme |
| Pancreas | No | Releases enzymes and alkaline fluid | Secretion into small intestine |
| Small intestine | Yes | Completes digestion and absorbs products | Large exchange surface |
| Large intestine | Yes | Absorbs remaining water and ions | Forms faecal material |
Why is the pancreas part of digestion if food never enters it?
Explain one way mechanical digestion supports chemical digestion.
Check your understanding
Answer from memory, then check against the suggested answer.
Which organ makes bile?
What is peristalsis?
Where does most nutrient absorption occur?
A blockage prevents bile entering the small intestine. Predict an effect on lipid digestion.
Explain why bile helps lipase but is not itself an enzyme.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Food travels through the digestive system from the mouth to the large intestine.
Describe what happens to food in the mouth and stomach.
(2)Explain the roles of the liver and pancreas.
(3)What happens in the small intestine and large intestine?
(1)Total for this question: 6 marks
Command word: For describe and explainA command word pair: say what happens, then give the reasons why it happens., keep the route in order and connect each organ to a specific process.