B3.2 The Human Digestive System

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the organs of the digestive system, the path of food, peristalsis, 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
• Explain how peristalsis moves food along the gut
• Describe where food is absorbed and what happens to undigested food

Key words

• Digestion: Breaking large, insoluble food molecules into small, soluble molecules.
• Absorption: Small soluble molecules pass from the gut into the blood.
• Peristalsis: Waves of muscle contraction in the gut wall that push food along.
• Bolus: A soft ball of chewed food that is swallowed.
• Egestion: Removing undigested food from the body as faeces, through the anus (also called defecation).
• 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, where they are used in respiration and to build new molecules.

• 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 bonds in the large molecules.

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 moistens food so it is easy to swallow and contains amylase, which starts digesting starch.
• Oesophagus; What happens there Muscular tube (gullet) that carries food to the stomach by peristalsis. No digestion happens here.
• Stomach; What happens there Churns food. Makes protease and hydrochloric acid (about pH 2). The acid kills most pathogens in food.
• Small intestine; What happens there Enzymes from the pancreas and the intestine wall finish digestion. Small soluble molecules are absorbed into the blood through the villi. The first part is called the duodenum.
• Large intestine; What happens there Water (and some salts) is absorbed from the undigested material, forming faeces.
• Rectum and anus; What happens there Faeces are stored in the rectum and leave through the anus (egestion).

Peristalsis

The gut wall contains rings of muscle. The muscle just behind a bolus contracts and squeezes it forward, while the muscle in front relaxes. These waves of contraction, called peristalsis, happen in the oesophagus and all along the intestines. They work even if you are upside down.

Exam tip: Peristalsis is a good example of muscular tissue in the gut wall doing its job.

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 lubricates food.
• Liver; What it makes or does Makes bile, which neutralises stomach acid and emulsifies fats (see B3.7).
• Gall bladder; What it makes or does Stores bile and releases it into the small intestine.
• Pancreas; What it makes or does Makes carbohydrase (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, which is very long and has a folded lining covered in millions of tiny villi. These give a huge surface area and have a rich blood supply. The large intestine is wider but much shorter; it absorbs water. What is left (mainly fibre, bacteria and dead cells) is egested as faeces.

Worked example: time spent in each part of the gut

A meal takes 32 hours to pass through the gut. It spends 3 hours in the stomach, 4 hours in the small intestine and 24 hours in the large intestine and rectum. The rest of the time is in the mouth and oesophagus.
Time in mouth and oesophagus = 32 − (3 + 4 + 24) = 1 hour.
Percentage of time in the large intestine and rectum = 24 ÷ 32 × 100 = 75%.

Answer: 1 hour; 75%

Common mistakes

• Saying food is digested just so it is “broken down”. Say 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 digested 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 part of the gut are most of the products of digestion absorbed into the blood?

Answer: small intestine

Working: Most absorption happens in the small intestine; the large intestine mainly absorbs water.

Question 2. True or false? Bile is made in the gall bladder.

Answer: False

Working: False. Bile is made in the liver and stored in the gall bladder.

Question 3. Why must large food molecules such as protein be digested? (Choose from: They are too large and insoluble to be absorbed into the blood, Digestion makes them taste better, Large molecules are poisonous, Digestion turns them into water.)

Answer: They are too large and insoluble to be absorbed into the blood

Working: Large insoluble molecules cannot pass through the gut wall. Digestion turns them into small soluble molecules that can be absorbed.

Related lessons

• Previous lesson: B3.1 Tissues and Organs
• Next lesson: B3.3 The Chemistry of Food
• In this chapter: B3.4 Catalysts and Enzymes
• In this chapter: B3.5 Factors Affecting Enzyme Action
• In this chapter: B3.6 How the Digestive System Works

Continue in the full course

This lesson is section B3.2 (AGCSC0302) of our free GCSE Combined Science 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 Combined Science course.