3.7 Making Digestion Efficient

A free GCSE Biology lesson for AQA (8461) from OBA School: bile: neutralising stomach acid and emulsifying fats; surface area and the villi. 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

• State where bile is made, stored and released
• Explain how bile neutralises stomach acid and why this matters
• Explain how emulsification speeds up fat digestion
• Calculate how breaking up fat changes its surface area
• Describe how villi make absorption efficient

Key words

• Bile: A green, alkaline liquid made by the liver and stored in the gall bladder. It is not an enzyme.
• Neutralise: To react an acid with an alkali so the pH moves towards 7.
• Emulsify: To break large drops of fat into many tiny droplets.
• Surface area: The total area of the outside of an object; for fat, the area lipase can reach.
• Villi: Tiny finger-like projections of the small intestine lining (one is a villus).

Bile: made, stored, released

Bile is made in the liver and stored in the gall bladder. When food enters the small intestine, bile is released through the bile duct. Bile does two jobs. It is not an enzyme – it does not break any chemical bonds.

Key fact: Bile has two jobs: (1) it is alkaline, so it neutralises hydrochloric acid from the stomach; (2) it emulsifies fats.

Job 1: neutralising stomach acid

Food leaving the stomach is very acidic. The enzymes in the small intestine (from the pancreas and the intestine wall) work best in slightly alkaline conditions. Alkaline bile neutralises the acid, giving the right pH so these enzymes work at a fast rate.

Job 2: emulsifying fat

Fats do not mix with water and form large drops. Lipase can only reach the outside of each drop. Bile breaks the drops into many tiny droplets. This gives a much larger surface area, so more fat molecules are exposed to lipase and fat is digested faster.

Worked example: surface area of a fat “cube”

Model a drop of fat as a cube of side 2 mm. Surface area = 6 × 2 × 2 = 24 mm².
Split it into 8 cubes of side 1 mm. Each has area 6 × 1 × 1 = 6 mm², so the total = 8 × 6 = 48 mm².
The volume of fat is the same (8 mm³) but the surface area has doubled.

Answer: surface area increases from 24 mm² to 48 mm² (× 2)

Absorbing the products: villi

The small intestine is long and its lining is folded and covered with millions of villi. Each villus is adapted for efficient absorption:

• Large surface area – villi and the microvilli on their cells.
• Thin wall – only one cell thick, so a short diffusion distance.
• Good blood supply – a network of capillaries carries absorbed molecules away, keeping a steep concentration gradient.
• Products are absorbed by diffusion and by active transport (e.g. glucose when its concentration in the gut is low).

Common mistakes

• Calling bile an enzyme or saying bile “digests” fat. Bile emulsifies fat; lipase digests it.
• Saying bile is made in the gall bladder. It is made in the liver and stored in the gall bladder.
• Saying bile is acidic. It is alkaline and neutralises stomach acid.
• Saying emulsifying increases the amount of fat. The volume is the same; only the surface area increases.

Revision questions

Try these, then check your answers below each one.

Question 1. Where is bile made? (Choose from: Gall bladder, Liver, Pancreas, Stomach.)

Answer: Liver

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

Question 2. True or false? Bile is an enzyme that breaks down fats.

Answer: False

Working: False. Bile emulsifies fats (a physical change); lipase digests them.

Question 3. Why does bile neutralise the acid from the stomach? (Choose from: Enzymes in the small intestine work best in slightly alkaline conditions, Acid would dissolve the small intestine, Bile needs acid to work, To kill bacteria.)

Answer: Enzymes in the small intestine work best in slightly alkaline conditions

Working: Alkaline bile raises the pH so that pancreatic and intestinal enzymes work at a fast rate.

Related lessons

• Previous lesson: 3.6 How the Digestive System Works
• Next lesson: 3.8 Required Practicals: Organisation and the Digestive System
• 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 3.7 (AGCBH0307) 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.