2.5.10Structure and Functions in Living Organisms: Part 1
Bile — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions
5 min read 6 parts Lesson 23 of 32
Section 1 of 6Making and storing bile
1Made in the liver
- Made in the liver Liver cells continuously produce bile, a green, alkaline fluid.
- Bile contains bile salts but no digestive enzymes.
2Stored in the gall bladder
- Stored in the gall bladder Bile is carried along a duct to the gall bladder, where it is stored and slightly concentrated until needed.
3Released into the duodenum
- Released into the duodenum When food leaves the stomach, bile flows along the bile duct into the first part of the small intestine and mixes with the food.
Section 2 of 6The two jobs of bile
Bile
1. Neutralises stomach acid
- 1. Neutralises stomach acid Food arriving from the stomach is mixed with hydrochloric acid and is strongly acidic.
- Bile is alkaline, so it neutralises the acid and makes the contents of the small intestine slightly alkaline.
- This matters because enzymes in the small intestine have a higher optimum pH than stomach enzymes and would work badly in acid.
2. Emulsifies lipids
- 2. Emulsifies lipids Bile salts break large drops of fat into many tiny droplets.
- This is called emulsification.
- The droplets have a much larger total surface area, so lipase enzymes can reach more of the fat at once.
Section 3 of 6What emulsification really is
A physical change
- A physical change Emulsification is mechanical digestion.
- Splitting a droplet into smaller droplets does not break chemical bonds or change the molecules.
- A useful comparison is tearing a sheet of paper into small pieces: you have more pieces but the paper itself is unchanged.
Chemical digestion follows
- Chemical digestion follows Once the surface area is increased, lipase breaks each lipid molecule into glycerol and three fatty acids, which are the chemical changes.
- Bile therefore makes lipid digestion faster, but it does not carry out the chemical breakdown itself.
Exam wording
- Exam wording Strictly it is the bile salts that emulsify fat, but saying 'bile emulsifies lipids' is accepted.
Section 4 of 6Putting it together in an answer
1
State that bile is made in the liver and stored in the gall bladder.
2
Say that it is released into the duodenum, where it neutralises acid from the stomach.
3
Explain that the alkaline conditions suit the enzymes of the small intestine.
4
Explain that bile salts emulsify lipids into small droplets with a larger surface area.
5
Conclude that lipase can then digest the lipids into fatty acids and glycerol faster.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Bile | An alkaline fluid made in the liver that neutralises acid and emulsifies lipids. |
| Gall bladder | A small organ that stores bile before it is released into the small intestine. |
| Emulsification | The breaking of large lipid droplets into smaller ones, increasing surface area. |
| Bile salts | The substances in bile that actually cause emulsification. |
| Neutralise | To react with an acid so that the pH moves towards 7. |
Section 6 of 6Exam technique
1
Saying bile is made in the gall bladder. It is made in the liver and only stored in the gall bladder.
2
Stating that bile digests fat chemically. It emulsifies fat; the lipase enzyme does the chemical digestion.
3
Describing emulsification as a chemical change. It is mechanical because no bonds are broken.
4
Forgetting to link the alkaline pH of bile to the higher optimum pH of enzymes in the small intestine.
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