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B3.7 Making Digestion Efficient

GCSE Combined Science · AQA · Chapter B3: Organisation and the Digestive System

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: bile: neutralising stomach acid and emulsifying fats; villi and absorption; modelling the gut. 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, and explain its two jobs
  • Explain how emulsification speeds up fat digestion, using surface area
  • Describe how villi are adapted for efficient absorption
  • Use a Visking tubing model of the gut and evaluate it

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 and its two jobs

Bile is made in the liver, stored in the gall bladder and released into the small intestine when food arrives. It is not an enzyme – it does not break any chemical bonds.

  • Job 1 – neutralising acid: bile is alkaline. Food leaving the stomach is very acidic, but the enzymes in the small intestine work best in slightly alkaline conditions. Bile neutralises the acid 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, giving a much larger surface area, so lipase digests the fat faster.

Maths skill: surface area of fat droplets

Worked example: surface area of a fat “cube”

Model a drop of fat as a cube of side 3 mm. Surface area = 6 × 3 × 3 = 54 mm².
Split it into 27 cubes of side 1 mm. Each has area 6 × 1 × 1 = 6 mm², so the total = 27 × 6 = 162 mm².
The volume of fat is the same (27 mm³) but the surface area is 3 times bigger.

Answer: surface area increases from 54 mm² to 162 mm² (× 3)

Exam tip: Bile has two jobs: it is alkaline (neutralises stomach acid) and it emulsifies fat (bigger surface area for lipase). Learn both.

Absorbing the products: villi

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

  • Large surface area – the villi, and microvilli on their cells.
  • Thin wall – only one cell thick, so a short diffusion distance.
  • Good blood supply – capillaries carry 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 lower than in the blood).

Modelling the gut with Visking tubing

Visking tubing is partially permeable: small molecules such as glucose and water can pass through it, but large molecules such as starch cannot. It can be used as a model of the small intestine, with the water outside standing for the blood.

Explaining the results: amylase in X digests starch to sugar, and the small sugar molecules diffuse out through the tubing. Starch is too large to pass through, so it stays inside in both tubes. Limits of the model: the water does not flow like blood, so no steep concentration gradient is kept; there are no villi, no active transport and no peristalsis.

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 stored? (Choose from: Gall bladder, Liver, Pancreas, Stomach.)

Answer: Gall bladder

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 fat.

Answer: False

Working: False. Bile emulsifies fat and neutralises acid; lipase digests the fat.

Question 3. The diagram shows a large drop of fat before and after bile is added. What is bile doing? (Choose from: Emulsifying the fat, giving a larger surface area for lipase, Digesting the fat into fatty acids and glycerol, Neutralising the fat, Making more fat.)

Answer: Emulsifying the fat, giving a larger surface area for lipase

Working: Bile breaks large drops into many small droplets (emulsification). The total surface area is much larger, so lipase digests the fat faster.

Related lessons

Continue in the full course

This lesson is section B3.7 (AGCSC0307) 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.

 

 

 

 

 

 

 

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