7.4 Chemical digestion
- Syllabus
- 0610–2026–2027
- Topic
- 7.4
- Level
- —
Learning objectives
7.4.1Chemical digestion as the breakdown• Describe chemical digestion as the breakdown of large insoluble molecules into small soluble molecules7.4.2Role of chemical digestion• State the role of chemical digestion in producing small soluble molecules that can be absorbed7.4.3Functions of enzymes as follows: (a)• Describe the functions of enzymes as follows: (a) amylase breaks down starch to simple reducing sugars (b) proteases break down protein to amino acids (c) lipase breaks down fats and oils to fatty acids and glycerol7.4.4Where, in the digestive system• State where, in the digestive system, amylase, protease and lipase are secreted and where they act7.4.5Functions of hydrochloric acid• Describe the functions of hydrochloric acid in gastric juice, limited to killing harmful microorganisms in food and providing an acidic pH for optimum enzyme activity7.4.6Digestion of starch in the digestive• Describe the digestion of starch in the digestive system: (a) amylase breaks down starch to maltose (b) maltase breaks down maltose to glucose on the membranes of the epithelium lining the small intestine7.4.7Digestion of protein by proteases• Describe the digestion of protein by proteases in the digestive system: (a) pepsin breaks down protein in the acidic conditions of the stomach (b) trypsin breaks down protein in the alkaline conditions of the small intestine7.4.8Bile is an alkaline mixture that• Explain that bile is an alkaline mixture that neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach, to provide a suitable pH for enzyme action
Define chemical digestion
Chemical digestion is the breakdown of large, insoluble food molecules into small, soluble molecules.
Chemical bonds are broken, so the products are different molecules from the original food molecules. Digestive enzymes catalyse these reactions.
| Before chemical digestion | After chemical digestion |
|---|---|
| large molecules | small molecules |
| insoluble | soluble |
| cannot be absorbed intact | can be prepared for absorption |
Chemical digestion changes molecules. Physical digestion only makes food pieces smaller without changing their molecules.
Explain why chemical digestion enables absorption
Large, insoluble food molecules cannot dissolve in the intestinal contents or pass through the intestinal wall into the blood.
Chemical digestion converts them into small, soluble molecules. These can dissolve and move across the epithelium of the small intestine, so they can be absorbed into the blood.
large and insoluble → enzymatic breakdown → small and soluble → crosses the intestinal wall → absorbed into blood
Digestion produces absorbable molecules; absorption is the later movement of those molecules from the intestine into the blood.
Match digestive enzymes to substrates and products
Each digestive enzyme acts on a particular substrate and produces particular smaller molecules.
| Enzyme | Substrate | Product(s) |
|---|---|---|
| amylase | starch | simple reducing sugars |
| proteases | protein | amino acids |
| lipase | fats and oils | fatty acids and glycerol |
The enzyme name is not the product: amylase acts on starch, proteases act on protein, and lipase acts on lipids (fats and oils).
Do not swap glycerol and glucose. Lipase produces fatty acids and glycerol; carbohydrate digestion produces sugars.
Locate where digestive enzymes are secreted and act
The site of secretion is the organ that releases an enzyme; the site of action is where the enzyme meets its substrate and catalyses digestion.
| Enzyme group | Main secretion site(s) | Site(s) of action |
|---|---|---|
| amylase | salivary glands and pancreas | mouth and small intestine |
| proteases | stomach and pancreas | stomach and small intestine |
| lipase | pancreas and small-intestinal wall | small intestine |
Pancreatic amylase, protease and lipase are secreted through the pancreatic duct into the duodenum, so the pancreas is a secretion site while the small intestine is their action site.
An enzyme can be produced in one organ and act in another. The pancreas is not part of the alimentary canal, but its enzymes act in the small intestine.
Explain the two roles of stomach acid
Hydrochloric acid in gastric juice makes the stomach contents strongly acidic.
| Role | Why it matters |
|---|---|
| kills harmful microorganisms in food | reduces the chance that pathogens survive passage through the stomach |
| provides an acidic pH | gives stomach protease, especially pepsin, suitable conditions for high enzyme activity |
These are the two syllabus functions: antimicrobial action and an optimum acidic pH for enzyme activity.
Hydrochloric acid does not digest protein as an enzyme. It provides the conditions in which pepsin catalyses protein digestion.
Trace starch digestion from starch to glucose
Starch digestion occurs in two enzyme-controlled stages.
| Stage | Enzyme and reaction | Location |
|---|---|---|
| 1 | amylase: starch → maltose | mouth and small intestine |
| 2 | maltase: maltose → glucose | membranes of the epithelium lining the small intestine |
Salivary amylase begins starch digestion in the mouth. Pancreatic amylase continues it in the small intestine; membrane-bound maltase then produces glucose at the absorptive surface.
Amylase does not complete the route to glucose in this Supplement pathway: it produces maltose, and maltase produces glucose.
Compare pepsin and trypsin in protein digestion
Proteases digest protein, but pepsin and trypsin work in different parts of the alimentary canal and at different pH conditions.
| Protease | Site of action | Suitable condition | Role |
|---|---|---|---|
| pepsin | stomach | acidic | begins protein digestion |
| trypsin | small intestine | alkaline | continues protein digestion |
Hydrochloric acid provides the acidic conditions for pepsin in the stomach. After the mixture enters the duodenum, bile helps create alkaline conditions suitable for trypsin.
Pepsin is not the protease of the alkaline small intestine, and trypsin is not the protease of the acidic stomach.
Explain how bile creates a suitable pH
Bile is an alkaline mixture released into the duodenum.
Food leaving the stomach is mixed with acidic gastric juice. Bile neutralises this acidic mixture, raising its pH in the duodenum.
The resulting alkaline conditions provide a suitable pH for digestive enzymes acting in the small intestine, including trypsin and lipase.
Neutralisation and emulsification are different roles of bile. This objective concerns pH: bile is alkaline, but it is not an enzyme and does not chemically digest food.