Food arrives as large molecules: starch, proteins and fats (see Monomers and Polymers). They are too big to cross the gut wall. Enzymes break them into small units by hydrolysis, and the small intestine absorbs them.
Key idea
Each digestive enzyme fits its own substrate. Amylase splits starch into sugars. Proteases (such as pepsin and trypsin) split proteins into amino acids. Lipase splits fats into fatty acids and glycerol. Villi, finger-like folds lining the small intestine, absorb the products into the blood, and fats into the lymph.
| Enzyme | Made in | Works in | Breaks down | Into |
|---|---|---|---|---|
| Salivary amylase | salivary glands | mouth (about pH 7) | starch | sugars |
| Pepsin | stomach lining | stomach (about pH 2) | proteins | shorter chains |
| Pancreatic amylase | pancreas | small intestine (about pH 8) | starch | sugars |
| Trypsin | pancreas | small intestine | proteins | shorter chains, then amino acids |
| Lipase | pancreas | small intestine | fats | fatty acids and glycerol |
Why it works
Why so many enzymes? An enzyme works because its active site fits one kind of substrate (the lock-and-key idea). The bond that holds sugars together in starch is not the bond that holds amino acids together in a protein, so one enzyme cannot cut both. And each part of the gut has its own pH, so each enzyme is built to work at the pH where it acts.
Worked example: Following a cracker
Where does the starch in a cracker start being digested, and where is it finished and absorbed?
- Salivary amylase starts splitting starch in the mouth.
- Stomach acid stops salivary amylase.
- Pancreatic amylase and enzymes on the intestine wall finish the job in the small intestine, where the sugars are absorbed by the villi.
Answer: It starts in the mouth and is finished and absorbed in the small intestine.
Bile, made by the liver, is not an enzyme. It breaks fat into tiny droplets (emulsifies it), giving lipase much more surface to work on. The large intestine absorbs water, and bacteria living there make some vitamins.
Common mistake
It is easy to think one enzyme digests all foods. Each enzyme fits particular substrates, and each part of the gut has its own pH and its own enzymes. Pepsin, for example, works in the stomach's acid and stops working in the small intestine.