Chemical DigestionCambridge IGCSE Biology: Revision notes
Section 1
What Is Chemical Digestion?
Chemical digestion is the breakdown of large, insoluble molecules into small, soluble molecules.
- Unlike physical digestion, chemical digestion changes the actual molecules involved
- It is carried out by enzymes, which speed up these breakdown reactions
- Small, soluble molecules can then be absorbed into the blood
Section 2
The Three Main Digestive Enzymes
Three main enzyme groups carry out chemical digestion:
| Enzyme | Breaks down | Into | Secreted by / acts in |
|---|---|---|---|
| Amylase | Starch | Simple reducing sugars | Salivary glands (mouth) and pancreas (small intestine) |
| Protease | Protein | Amino acids | Stomach (as pepsin) and pancreas/small intestine (as trypsin) |
| Lipase | Fats and oils | Fatty acids and glycerol | Pancreas (acts in small intestine) |
Match each enzyme to its substrate and product: amylase-starch-sugars, protease-protein-amino acids, lipase-fat-fatty acids+glycerol.
Section 3
Acid and Bile: Providing the Right Conditions
Digestive enzymes need the correct conditions to work efficiently.
- Hydrochloric acid in gastric juice kills harmful microorganisms in food and provides the acidic pH needed for the stomach's protease (pepsin) to work at its optimum
- Bile is alkaline and neutralises the acidic mixture of food and gastric juice as it enters the duodenum, giving a suitable (alkaline) pH for the small intestine's enzymes to work at their optimum
Bile is not an enzyme and does not chemically digest anything itself - it neutralises acid and emulsifies fat, creating the right conditions for enzymes.
Section 4
Digestion of Starch
Starch digestion happens in two steps:
- Amylase (from salivary glands and pancreas) breaks down starch into maltose
- Maltase, found on the cell membranes of the small intestine's epithelium lining, breaks down maltose into glucose
Section 5
Digestion of Protein
Protein is digested by two different proteases, active in different conditions:
- Pepsin breaks down protein in the acidic conditions of the stomach
- Trypsin breaks down protein in the alkaline conditions of the small intestine
Pepsin working best in acid and trypsin working best in alkali shows how enzymes have an optimum pH specific to their location.
Must Know
- Chemical digestion breaks large insoluble molecules into small soluble ones using enzymes
- Amylase - starch to sugars; protease - protein to amino acids; lipase - fats to fatty acids and glycerol
- HCl in the stomach kills microbes and gives optimum acidic pH for pepsin
- Bile neutralises stomach acid, giving optimum alkaline pH for small intestine enzymes
- Starch: amylase to maltose, then maltase to glucose
- Protein: pepsin in the acidic stomach, trypsin in the alkaline small intestine
That's the notes covered.
Carry on to the next subtopic.