B3.6 How the Digestive System Works

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: digestive enzymes: carbohydrases, proteases and lipases, where they are made, and how the products are used. 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

• Name the three groups of digestive enzymes and the reactions they catalyse
• State where carbohydrases, proteases and lipases are produced
• Explain the role of hydrochloric acid in the stomach
• Describe how the products of digestion are used in the body
• Analyse simple data from digestion experiments

Key words

• Carbohydrase: An enzyme that breaks down carbohydrates into simple sugars, e.g. amylase.
• Amylase: A carbohydrase that breaks down starch into sugars.
• Protease: An enzyme that breaks down proteins into amino acids.
• Lipase: An enzyme that breaks down lipids into fatty acids and glycerol.
• Duodenum: The first part of the small intestine, where pancreatic juice and bile are added.

Digestive enzymes and their reactions

Digestive enzymes are made by glands and released into the gut, so they work outside cells. They break large insoluble molecules into small soluble molecules that can be absorbed into the bloodstream.

• Carbohydrases (e.g. amylase); Substrate Carbohydrates (e.g. starch); Products Simple sugars
• Proteases; Substrate Proteins; Products Amino acids
• Lipases; Substrate Lipids (fats and oils); Products Fatty acids + glycerol

Where the enzymes are made

Amylase acts in the mouth and small intestine; protease acts in the stomach and small intestine; lipase acts in the small intestine. No digestive enzymes are made in the oesophagus, liver, gall bladder or large intestine.

Exam tip: The pancreas and small intestine make all three groups of enzyme. If you are unsure in an exam, these are safe answers.

Hydrochloric acid in the stomach

Glandular tissue in the stomach wall makes hydrochloric acid. It does not digest food. It gives a pH of about 2, the optimum pH for the stomach protease, and it kills most of the pathogens that we swallow with food and drink.

When food leaves the stomach, the acid must be neutralised because enzymes in the small intestine work best in slightly alkaline conditions. Bile does this (see B3.7).

Using the products of digestion

After absorption, the blood carries the small molecules to cells, which use them to build new large molecules or to release energy:

• Glucose: used in respiration to release energy; extra glucose is built into glycogen (in the liver and muscles) or fat.
• Amino acids: joined in a new order to make the body’s own proteins, such as enzymes, antibodies and muscle proteins.
• Fatty acids and glycerol: built into new lipids, e.g. for cell membranes or fat stores.

Worked example: analysing digestion data

A student mixed cooked egg white with protease. 90 mg of protein were present at the start; after 15 minutes 63 mg remained.
Mass digested = 90 − 63 = 27 mg. Percentage digested = 27 ÷ 90 × 100 = 30%.
Mean rate = 27 ÷ 15 = 1.8 mg per minute.

Answer: 30% digested; 1.8 mg/min

Common mistakes

• Saying lipase makes “fatty acids” only. The products are fatty acids and glycerol.
• Saying the stomach acid digests food. The acid gives the right pH and kills pathogens; enzymes digest.
• Saying the liver makes digestive enzymes. The liver makes bile, which is not an enzyme.
• Forgetting the pancreas makes all three groups of enzyme.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows the digestive system with some organs replaced by letters. Which letter shows the organ that makes carbohydrase, protease and lipase? (Choose from: P, Q, R, S.)

Answer: R

Working: R is the pancreas, which makes all three groups of enzyme. P is the salivary glands, Q the stomach and S the liver (which makes bile, not enzymes).

Question 2. Which is a function of hydrochloric acid in the stomach? (Choose from: It kills most pathogens in food, It digests fat, It neutralises bile, It breaks starch into sugars.)

Answer: It kills most pathogens in food

Working: Hydrochloric acid kills most pathogens and gives the low pH that the stomach protease needs.

Question 3. True or false? The liver makes lipase.

Answer: False

Working: False. The liver makes bile, which is not an enzyme. Lipase is made by the pancreas and the small intestine.

Related lessons

• Previous lesson: B3.5 Factors Affecting Enzyme Action
• Next lesson: B3.7 Making Digestion Efficient
• In this chapter: B3.1 Tissues and Organs
• In this chapter: B3.2 The Human Digestive System
• In this chapter: B3.3 The Chemistry of Food

Continue in the full course

This lesson is section B3.6 (AGCSC0306) 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.