Each digestive enzyme acts on one type of substrate and makes specific products. The link you need is: enzyme, substrate, products, and where it works. Exam questions ask for any one of these when they give you the other three.
This skill belongs to human nutrition and digestion and builds on the enzyme ideas in biological molecules and enzymes.
What are the main enzyme links?
An enzyme’s active site fits one substrate, like a key fits one lock. That is why the matching is fixed.
| Enzyme | Substrate | Products | Where it works |
|---|---|---|---|
| Amylase | Starch | Maltose | Mouth (saliva), small intestine (from pancreas) |
| Maltase | Maltose | Glucose | Lining of the small intestine |
| Protease (pepsin in the stomach) | Protein | Amino acids (shorter chains first) | Stomach, small intestine |
| Lipase | Fats and oils | Fatty acids and glycerol | Small intestine |
Bile is not an enzyme. It is made in the liver and emulsifies fat into small droplets, which gives lipase a larger surface area to work on. Bile also helps neutralise the acid that arrives from the stomach.
Worked example
The data are invented. A class mixes starch solution with amylase in three tubes and tests samples with iodine solution every minute. Iodine stays blue-black while starch is present and turns orange-brown when it has gone.
| Tube | Temperature | Time for starch to disappear |
|---|---|---|
| 1 | 37 °C | 4 min |
| 2 | 20 °C | 12 min |
| 3 | 80 °C | not gone after 30 min |
Step 1, name the link: substrate starch, enzyme amylase, product maltose.
Step 2, compare rates: rate = 1 ÷ time. Tube 1: 1 ÷ 4 = 0.25 per minute. Tube 2: 1 ÷ 12 ≈ 0.083 per minute.
Step 3, compare: 0.25 ÷ 0.083 = 3, so tube 1 is three times faster. Check by times: 12 ÷ 4 = 3.
Step 4, explain tube 3: 80 °C denatures amylase. The active site changes shape and the substrate no longer fits, so starch remains.
The mistake to watch for
A common slip is to name the product as the substrate’s “smaller version” without saying what it is.
Mistaken answer: “Protease breaks protein into smaller pieces.”
“Smaller pieces” does not name the products. A mark scheme wants amino acids.
The correction is to finish every sentence with a named product. Write “protease breaks protein into amino acids” and “lipase breaks fat into fatty acids and glycerol”.
A second slip is to say that heat “kills” the enzyme. Enzymes are not alive; say they are denatured.
Check yourself
Try these without notes, then open each answer.
1. Name the substrate and products for lipase.
Show answer
Substrate: fats (lipids). Products: fatty acids and glycerol.
2. In a similar invented experiment, starch disappears in 6 minutes at 30 °C and in 3 minutes at 37 °C. How many times faster is the reaction at 37 °C?
Show answer
Rate at 30 °C = 1 ÷ 6 ≈ 0.167 per minute. Rate at 37 °C = 1 ÷ 3 ≈ 0.333 per minute. The ratio is 0.333 ÷ 0.167 = 2, so it is 2 times faster. Check: 6 ÷ 3 = 2.
3. Why does bile not count as a digestive enzyme, even though it helps fat digestion?
Show answer
Bile does not catalyse a chemical reaction. It emulsifies fat, breaking large droplets into small ones and giving a larger surface area for lipase.
Where this leads next
Once the matching feels automatic, see how the products leave the gut in explaining absorption using villus structure. The human nutrition and digestion practice set then mixes all the links.
If a question still feels like guessing which enzyme to choose, a teacher can go through your answers in online one-to-one Biology tuition. The probability tree and counting board and inheritance model board show a related habit: state the model and its assumptions first, then work out the result.