AQA Biology 4.2.2.1
The digestive system
Big molecules in, small soluble ones absorbed. Follow a meal through the gut, see which enzyme acts where, then test foods for what they contain.
Combined Science · Foundation tier
Start here
Toast at breakfast.
You eat two slices of toast at eight o’clock. By lunchtime you are hungry again, and the toast has gone.
If you had to guess, does your body use the bread as it is, or break it down into smaller bits first?
Follow the meal
Each organ has a job.
The digestive system is an organ system: organs work together to digest and absorb food. Enzymes break large, insoluble molecules into small, soluble ones that can be absorbed into the bloodstream. Tap each organ, in order.
Mouth and salivary glands
Teeth break food into smaller pieces. Salivary glands make amylase, which starts breaking starch down into sugars.
Three enzymes, three products
Where each enzyme is made, and what it makes.
Carbohydrases break carbohydrates down to simple sugars. Amylase is a carbohydrase. The products of digestion are used to build new carbohydrates, lipids and proteins; some glucose is used in respiration.
| Enzyme | Made in | Word equation |
|---|---|---|
| Amylase | salivary glands, pancreas, small intestine | starch sugars |
| Protease | stomach, pancreas, small intestine | protein amino acids |
| Lipase | pancreas, small intestine | lipid glycerol + fatty acids |
How enzymes work, and how pH and temperature affect them, is in the Enzymes lesson.
Required practical 3 · food tests
What is in this food?
Use qualitative reagents to test for a range of carbohydrates, lipids and proteins. Make a food solution, then test it. Wear eye protection. Benedict’s needs a hot water bath; ethanol is flammable, so keep it away from any flame. Pick a food and run each test.
Iodine: starch
Add iodine solution to the food.
Benedict’s: sugars
Add Benedict’s solution; heat in a hot water bath.
Biuret: protein
Add Biuret reagent.
Ethanol: lipid
Shake the food with ethanol, then pour into water.
Think again · what bile does
“Bile digests fat.”
Your call
ChooseBile is released into the small intestine. What does it do to fat?
Sort
Amylase, protease or lipase?
Place each statement with its enzyme.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- A food test: reagent, method, colour before and after.
- Explain
- How bile speeds up fat digestion.
- Name
- Enzyme, where it’s made, what it makes.
Key fact
Enzymes break large, insoluble molecules into small, soluble ones: starch to sugars, protein to amino acids, lipid to glycerol and fatty acids. They are absorbed in the small intestine.
Key note · AQA 4.2.2.1
The digestive system
- The digestive system is an organ system: organs work together to digest and absorb food.
- Amylase (salivary glands, pancreas, small intestine): starch → sugars. Amylase is a carbohydrase.
- Protease (stomach, pancreas, small intestine): protein → amino acids. Lipase (pancreas, small intestine): lipid → glycerol + fatty acids.
- Bile: made in the liver, stored in the gall bladder. Alkaline, to neutralise stomach acid; emulsifies fat, giving lipase a larger surface area.
- Products are absorbed in the small intestine (villi) and used to build new carbohydrates, lipids and proteins; some glucose is used in respiration.
- Food tests: iodine (blue-black), Benedict’s, heated (brick-red), Biuret (purple), ethanol (cloudy white emulsion).
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure which enzyme is made where?
The gut figure is schematic and not to scale. Food-test results are typical, for practice.