3.6 How the Digestive System Works

A free GCSE Biology lesson for AQA (8461) from OBA School: digestive enzymes: carbohydrases, proteases and lipases, where they are made and what the products are used for. 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 main groups of digestive enzymes and the reactions they catalyse
• State where amylase, protease and lipase 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 (mainly maltose).
• Protease: An enzyme that breaks down proteins into amino acids, e.g. pepsin.
• Lipase: An enzyme that breaks down lipids into fatty acids and glycerol.
• Hydrochloric acid: Made in the stomach; gives the low pH that pepsin needs and kills most bacteria in food.

Digestive enzymes and their reactions

Digestive enzymes work outside cells, in the gut. They break large insoluble molecules into small soluble molecules that can be absorbed into the bloodstream.

• Carbohydrases (e.g. amylase); Reaction starch → simple sugars (maltose, then glucose)
• Proteases (e.g. pepsin); Reaction proteins → amino acids
• Lipases; Reaction lipids → fatty acids + glycerol

Where the enzymes are made

• Amylase; Made in Salivary glands, pancreas, small intestine; Works in Mouth and small intestine
• Protease; Made in Stomach (pepsin), pancreas, small intestine; Works in Stomach and small intestine
• Lipase; Made in Pancreas, small intestine; Works in Small intestine

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

Hydrochloric acid in the stomach

Glands in the stomach wall make hydrochloric acid. It gives a pH of about 2, which is the optimum pH for the stomach protease (pepsin). It also kills most of the bacteria that we swallow with our food.

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 3.7).

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; also built into glycogen (stored in liver and muscles).
• Amino acids: built into new proteins such as enzymes, hormones and muscle proteins. Excess amino acids are broken down in the liver to form urea.
• Fatty acids and glycerol: built into new lipids, e.g. for cell membranes or fat stores.

Worked example: analysing digestion data

A student mixed egg-white protein with protease. 80 mg of protein were present at the start; after 20 minutes 52 mg remained.
Mass digested = 80 − 52 = 28 mg. Percentage digested = 28 ÷ 80 × 100 = 35%.
Mean rate = 28 ÷ 20 = 1.4 mg per minute.

Answer: 35% digested; 1.4 mg/min

Common mistakes

• Saying amylase turns starch “into glucose” only. Amylase mainly produces maltose; other carbohydrases then make glucose. “Sugars” is safe.
• 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 bacteria; enzymes digest.
• Forgetting the pancreas makes all three groups of enzymes.

Revision questions

Try these, then check your answers below each one.

Question 1. Which organ produces amylase, protease and lipase? (Choose from: Stomach, Liver, Pancreas, Gall bladder.)

Answer: Pancreas

Working: The pancreas (and the small intestine wall) make all three groups of enzymes.

Question 2. Which is a function of hydrochloric acid in the stomach? (Choose from: Kills most bacteria in food, Emulsifies fats, Digests starch, Neutralises bile.)

Answer: Kills most bacteria in food

Working: The acid kills bacteria and gives the low pH that the stomach protease needs.

Question 3. True or false? Salivary glands produce protease.

Answer: False

Working: False. Salivary glands produce amylase, which starts the digestion of starch.

Related lessons

• Previous lesson: 3.5 Factors Affecting Enzyme Action
• Next lesson: 3.7 Making Digestion Efficient
• In this chapter: 3.1 Tissues and Organs
• In this chapter: 3.2 The Human Digestive System
• In this chapter: 3.3 The Chemistry of Food

Continue in the full course

This lesson is section 3.6 (AGCBH0306) 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.