Digestive System and EnzymesAQA GCSE Biology: Revision notes
Section 1
How do enzymes break down large food molecules?
Digestive enzymes convert large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream.
| Enzyme type | Breaks down | Products |
|---|---|---|
| Carbohydrases (e.g. amylase) | Carbohydrates | Simple sugars |
| Proteases | Proteins | Amino acids |
| Lipases | Lipids (fats) | Glycerol and fatty acids |
Section 2
Where are digestive enzymes produced?
- Amylase is produced in the salivary glands, pancreas and small intestine
- Proteases are produced in the stomach (pepsin), pancreas and small intestine
- Lipases are produced in the pancreas and small intestine
The products of digestion are absorbed and used to build new carbohydrates, lipids and proteins in the body; some glucose is used directly in respiration.
If asked where an enzyme acts, name a specific organ rather than just saying 'the digestive system'.
Section 3
What is the role of bile?
Bile is produced in the liver and stored in the gall bladder before being released into the small intestine. It has two roles:
- It is alkaline, neutralising hydrochloric acid that arrives from the stomach so enzymes in the small intestine can work at their optimum pH
- It emulsifies fat, breaking it into small droplets; this increases the surface area of fat, speeding up its breakdown by lipase
Bile is not an enzyme — it does not break down fat chemically, it physically increases fat's surface area so lipase works faster.
Section 4
How do we test for food molecules?
Qualitative reagent tests identify food groups (Required Practical 4):
- Benedict's test for sugars: add Benedict's reagent, heat in a water bath; a colour change from blue to green/orange/brick-red indicates sugar
- Iodine test for starch: add iodine solution; a colour change from orange-brown to blue-black indicates starch
- Biuret test for protein: add Biuret reagent; a colour change from blue to purple/lilac indicates protein
A food sample turning brick-red with Benedict's reagent after heating contains reducing sugar.
Section 5
How is the effect of pH on amylase investigated?
In Required Practical 5, the rate of the amylase-catalysed breakdown of starch is investigated at different pH values using continuous sampling with iodine reagent:
- Mix amylase, starch solution and a buffer of known pH; keep temperature constant using a water bath
- At regular time intervals, sample the mixture and add a drop to iodine solution in a spotting tile
- Iodine stays orange-brown once all starch has been digested; note the time this happens for each pH
- Repeat at different pH values and compare
Rate can be calculated as .
Controlling temperature with a water bath in this practical ensures pH is the only variable affecting rate.
Must Know
- Carbohydrases (e.g. amylase) → simple sugars; proteases → amino acids; lipases → glycerol + fatty acids
- Bile (made in liver, stored in gall bladder) neutralises stomach acid and emulsifies fat to speed up lipase action
- Benedict's test (sugars, brick-red), iodine test (starch, blue-black), Biuret test (protein, purple)
- Amylase + starch + iodine reagent is used to test the effect of pH on enzyme rate (RP5)
- Rate of reaction can be calculated as time taken
- Digestion products are absorbed and rebuilt into new carbohydrates, lipids and proteins, or used in respiration
That's the notes covered.
Carry on to the next subtopic.