3.2 The Human Digestive System

A free GCSE Biology lesson for AQA (8461) from OBA School: the organs of the digestive system, the path of food, and digestion, absorption and egestion. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why large food molecules must be digested before they can be used
• Name the organs of the digestive system in order and describe what each does
• Describe the roles of the salivary glands, liver, gall bladder and pancreas
• Describe where food is absorbed and what happens to undigested food

Key words

• Digestion: Breaking large, insoluble food molecules into small, soluble molecules.
• Mechanical digestion: Physically breaking food into smaller pieces, e.g. chewing and churning.
• Chemical digestion: Enzymes break the bonds in large food molecules.
• Absorption: Small soluble molecules pass from the gut into the blood.
• Egestion: Removing undigested food from the body as faeces, through the anus.
• Gut: The muscular tube from mouth to anus (also called the alimentary canal).

Why we digest food

The food we eat contains large molecules such as starch, proteins and lipids. These are too big and are insoluble, so they cannot pass through the gut wall into the blood. Digestion breaks them into small, soluble molecules that can be absorbed and carried to cells.

• Mechanical digestion: teeth chew food; the muscular stomach wall churns it. This gives a bigger surface area for enzymes to work on.
• Chemical digestion: enzymes break the large molecules down.

Key fact: Large insoluble molecules → (digestion) → small soluble molecules → (absorption) → blood.

The path of food through the gut

• Mouth; What happens there Teeth chew food. Saliva from the salivary glands contains amylase, which starts digesting starch.
• Oesophagus; What happens there Muscular tube (gullet) that carries food to the stomach by waves of muscle contraction.
• Stomach; What happens there Churns food. Makes protease and hydrochloric acid. The acid kills most bacteria in food.
• Small intestine; What happens there Enzymes from the pancreas and the small intestine wall finish digestion. Small soluble molecules are absorbed into the blood.
• Large intestine; What happens there Water is absorbed from the undigested food, forming faeces.
• Rectum and anus; What happens there Faeces are stored in the rectum and leave the body through the anus (egestion).

Glands and organs that help

Food never passes through these organs, but they add substances to the gut.

• Salivary glands; What it makes or does Make saliva, which contains amylase and moistens food.
• Liver; What it makes or does Makes bile, which neutralises stomach acid and emulsifies fats (see 3.7).
• Gall bladder; What it makes or does Stores bile and releases it into the small intestine.
• Pancreas; What it makes or does Makes amylase, protease and lipase and releases them into the small intestine.

Exam tip: The liver makes bile; the gall bladder only stores it. This is a favourite exam question.

Absorption and egestion

Most absorption happens in the small intestine. Its lining is covered with millions of tiny finger-like villi, which give a huge surface area and have a good blood supply. Glucose, amino acids, fatty acids and glycerol move into the blood here. The large intestine absorbs water. What is left (fibre, bacteria, dead cells) is egested as faeces.

Worked example: time spent in each part of the gut

A meal takes 40 hours to pass through the gut. It spends 4 hours in the stomach, 5 hours in the small intestine and 30 hours in the large intestine. The rest of the time is in the mouth and oesophagus.
Time in mouth and oesophagus = 40 − (4 + 5 + 30) = 1 hour.
Percentage of time in the large intestine = 30 ÷ 40 × 100 = 75%.

Answer: 1 hour; 75%

Common mistakes

• Saying digestion happens so food can be “broken down” without explaining why: the molecules must be small and soluble to be absorbed.
• Thinking food passes through the liver or pancreas. They only add substances to the gut.
• Saying the gall bladder makes bile. The liver makes it; the gall bladder stores it.
• Saying the large intestine absorbs food. Most food is absorbed in the small intestine; the large intestine absorbs water.
• Confusing egestion (removing undigested food) with excretion (removing waste made by cells).

Revision questions

Try these, then check your answers below each one.

Question 1. In which order does food pass through these parts of the gut? (Choose from: mouth, oesophagus, stomach, small intestine, large intestine, mouth, stomach, oesophagus, small intestine, large intestine, mouth, oesophagus, stomach, large intestine, small intestine, mouth, oesophagus, liver, stomach, small intestine.)

Answer: mouth, oesophagus, stomach, small intestine, large intestine

Working: Mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.

Question 2. Name the part of the gut where most digested food is absorbed into the blood.

Answer: small intestine

Working: Most absorption happens in the small intestine.

Question 3. True or false? Food passes through the pancreas on its way to the small intestine.

Answer: False

Working: False. The pancreas releases enzymes into the small intestine, but food never passes through it.

Related lessons

• Previous lesson: 3.1 Tissues and Organs
• Next lesson: 3.3 The Chemistry of Food
• In this chapter: 3.4 Catalysts and Enzymes
• In this chapter: 3.5 Factors Affecting Enzyme Action
• In this chapter: 3.6 How the Digestive System Works

Continue in the full course

This lesson is section 3.2 (AGCBH0302) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.