7.1.7Human Nutrition
Bile and Emulsification — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 8
Part 1 of 4Making and releasing bile (Extended tier only)
1Made in the liver
- Made in the liver Liver cells continuously produce bile, a greenish fluid.
2Stored in the gall bladder
- Stored in the gall bladder Bile is held and concentrated here until it is needed.
3Released after a meal
- Released after a meal When food leaves the stomach, bile travels down the bile duct.
4Enters the duodenum
- Enters the duodenum It mixes with the food at the start of the small intestine, where it carries out two jobs.
Part 2 of 4The two functions of bile
Bile
Neutralises stomach acid
- Neutralises stomach acid Bile is alkaline.
- It neutralises the hydrochloric acid in the food arriving from the stomach.
Gives the right pH for enzymes
- Gives the right pH for enzymes Enzymes in the small intestine have an optimum pH that is higher (more alkaline) than that of stomach enzymes,
- so the contents must be made less acidic.
Emulsifies fats
- Emulsifies fats Bile breaks large droplets of fat into many tiny droplets.
- This process is called emulsification.
Part 3 of 4How emulsification speeds up fat digestion
1
Fats in food clump together into large globules, which have a small surface area compared with their volume.
2
Bile salts break each large globule into many tiny droplets without changing the fat chemically.
3
The total surface area of fat exposed to the digestive juices is now much greater.
4
The enzyme lipase can attach to many more points on the surface, so it works faster.
5
Lipase breaks the lipids into the smaller molecules glycerol and fatty acids, which are then absorbed.
Part 4 of 4Mechanical or chemical digestion?
| Feature | Mechanical digestion | Chemical digestion |
|---|---|---|
| What happens | Food is broken into smaller pieces | Large molecules are broken into small ones by breaking bonds |
| Is the molecule changed? | No, the molecules stay the same | Yes, new substances are formed |
| Example | Chewing; emulsification of fat by bile | Lipase turning fat into glycerol and fatty acids |
| Role of enzymes | None needed | Enzymes catalyse the reaction |
Key terms
| Term | Meaning |
|---|---|
| Bile | An alkaline fluid made in the liver and stored in the gall bladder that helps digest fats. |
| Emulsification | Breaking large fat droplets into smaller ones, increasing the surface area. |
| Lipase | An enzyme that digests lipids into glycerol and fatty acids. |
| Duodenum | The first section of the small intestine, where bile and pancreatic juice enter. |
| Surface area | The total area of a substance exposed to its surroundings; greater area means faster reactions. |
Avoid these mistakes
1
Saying that bile contains enzymes or digests fat chemically. Bile only emulsifies fat; lipase does the chemical digestion.
2
Stating that bile is made in the gall bladder. It is made in the liver and only stored in the gall bladder.
3
Calling emulsification chemical digestion. It is a physical process that increases surface area and does not break bonds.
4
Forgetting why neutralising acid mattersintestinal enzymes would work slowly or be denatured in acidic conditions.
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