Overview
What you’ll learn
- Name the main parts of the gut in order and state one function of each.
- Explain why food is digested before it is absorbed.
- State that carbohydrases, proteases and lipases digest carbohydrates, proteins and fats.
- Describe absorption in the small intestine and water absorption in the large intestine.
- Relate lifestyle to the risk of type 2 diabetes and state a role of gut bacteria.
In 30 seconds
- 1Digestion breaks large food molecules into smaller ones that can enter the blood.
- 2The path is mouth → gullet → stomach → small intestine → large intestine → rectum → anus.
- 3Carbohydrases digest carbohydrates. Proteases digest proteins. Lipases digest fats.
- 4Most digested food is absorbed in the small intestine. The large intestine absorbs water.
- 5Chewing increases the surface area for enzymes. The stomach churns food and adds acid.
- 6Regular activity and a diet that is not high in sugar are linked with a lower chance of type 2 diabetes. Many gut bacteria help break down food.
Notes · 1 of 5
The path of food
Learn the parts in order, and one job for each.
| Part | What happens |
|---|---|
| Mouth | Teeth chew food. Saliva moistens it and carbohydrases begin to digest starch. |
| Gullet | Food is pushed down to the stomach. |
| Stomach | Food is churned with acid and proteases. |
| Small intestine | Digestion finishes and digested food is absorbed. |
| Large intestine | Water is absorbed from what is left. |
| Rectum and anus | Undigested remains are stored, then egested. |
Egestion is the removal of undigested food. It is not the same as excretion, which removes waste made by the body, such as carbon dioxide.
Notes · 2 of 5
Enzyme classes
Match the class of enzyme to the food group. Do not memorise individual enzyme names.
| Enzyme class | Food group |
|---|---|
| Carbohydrases | Carbohydrates, such as starch |
| Proteases | Proteins |
| Lipases | Fats |
Enzymes are proteins that speed up digestion. They are specific: a lipase does not digest protein. Chewing helps because smaller pieces have more surface for the enzymes to work on.
Notes · 3 of 5
Absorption
Small molecules leave the gut and enter the blood.
Food must be digested because only small molecules can pass through the wall of the gut into the blood. The small intestine is long and has a large surface, so absorption is fast there.
The blood then carries the digested food to cells around the body. Cells use it in respiration and for growth. The large intestine absorbs water so the body does not lose too much.
Notes · 4 of 5
The stomach and the large intestine
Do not swap their jobs.
The stomach is a muscular bag. It churns food into a liquid and mixes it with acid. The acid kills many microbes and gives proteases a suitable condition. The stomach does not absorb most of the digested food.
The large intestine comes after the small intestine. Its main job at this level is water absorption. Fibre and other undigested material pass on to the rectum.
Notes · 5 of 5
Diabetes, diet and gut bacteria
Lifestyle does not guarantee health, but it changes the risk.
In type 2 diabetes the body does not control blood sugar well. Being active and eating a diet that is not high in sugary food, over a long time, is linked with a lower chance of developing it. One meal does not cause the condition.
The gut contains many bacteria. Some are useful: they help break down food that human enzymes do not digest, and they are part of a healthy gut. Washing hands and keeping food clean still matter, because other microbes can cause illness.
Revise
Key terms
The words examiners expect you to use precisely.
- Digestion
- The breakdown of large food molecules into smaller molecules.
- Absorption
- The movement of small food molecules from the gut into the blood.
- Egestion
- The removal of undigested food from the anus.
- Gullet
- The tube that carries food from the mouth to the stomach.
- Carbohydrase
- An enzyme that digests carbohydrates.
- Protease
- An enzyme that digests proteins.
Show 4 more key termsShow fewer key terms
- Lipase
- An enzyme that digests fats.
- Small intestine
- The part of the gut where digestion is completed and most food is absorbed.
- Large intestine
- The part of the gut where water is absorbed from undigested remains.
- Gut bacteria
- Microbes in the intestine, many of which help break down food.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying the stomach absorbs most of the digested food.
The stomach mainly churns and starts protein digestion.
Instead: Most absorption happens in the small intestine.
Mistake: Naming one enzyme such as a starch-only enzyme when the mark scheme wants the class.
A single enzyme name does not show which kind of food is digested.
Instead: Carbohydrases, proteases and lipases.
Show 4 more mistakesShow fewer mistakes
Mistake: Putting the large intestine before the small intestine.
The names describe width, not the order of the journey.
Instead: Food reaches the small intestine first. Water is absorbed later in the large intestine.
Mistake: Saying digestion and absorption are the same process.
Digestion breaks food down. Absorption moves the products into the blood.
Instead: Digestion comes first so that molecules are small enough to be absorbed.
Mistake: Calling egestion excretion.
Undigested food never entered the blood.
Instead: Egestion removes undigested food. Excretion removes waste the body has made.
Mistake: Saying all gut bacteria are harmful.
Many help digest food.
Instead: Useful bacteria are part of a healthy gut. Some other microbes cause disease.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Define digestion
Digestion is the breakdown of large food molecules into
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 5Foundation
- 5Standard
- 5Challenging
15 questions worth 35 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 6 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 6.
Mark scheme
PROChapter 6 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What is the order of the digestive system?
Mouth, gullet, stomach, small intestine, large intestine, rectum, anus.
Which enzymes do I need to name?
Use the classes: carbohydrases for carbohydrates, proteases for proteins, and lipases for fats. Individual enzyme names are not required.
Where is food absorbed?
Most digested food is absorbed in the small intestine. The large intestine absorbs water.
What is the difference between egestion and excretion?
Egestion removes undigested food. Excretion removes waste the body has produced, such as carbon dioxide.
Is the digestive system in the Sec 2 EOY?
Most schools that follow the usual G2/G3 scheme teach this chapter in Sec 2 and include it in the Sec 2 end-of-year paper. A few schools move a topic across the two years, so check your school’s assessment outline.
Up next
Chapter 7: Transport Systems in Living Things