By the end of this lesson
- Match digestive enzymes to substrates and products.
- State principal sites of enzyme action.
- Explain why chemical digestion is required for absorption.
BIG QUESTION
Why is chewing food into tiny pieces still not enough for absorption?
Mechanical digestion increases area, but enzymes must hydrolyse large molecules into small soluble products.
Carbohydrases release glucose in stages
amylaseAn enzyme that breaks down starch into maltose; it is made in the salivary glands and the pancreas. hydrolyses starch to maltose. It is secreted by salivary glands and the pancreas and acts in the mouth and small intestine.
maltaseAn enzyme that breaks down maltose into glucose. at the small-intestine surface hydrolyses maltose to glucose, which can be absorbed.
Proteases release amino acids
proteaseAn enzyme that hydrolyses (breaks down) proteins into peptides and then amino acids, such as pepsin in the stomach. hydrolyse proteins into smaller peptides and ultimately amino acids. Stomach protease works in acidic conditions; pancreatic and intestinal proteases continue digestion in the small intestine.
Amino acids are small and soluble enough for absorption and later protein synthesis.
Lipases release fatty acids and glycerol
lipaseAn enzyme that breaks down lipids (fats and oils) into fatty acids and glycerol. hydrolyse lipids to fatty acids and glycerol. Pancreatic lipase acts in the small intestine, aided by bile emulsification.
Emulsification is physical dispersion; lipase performs the chemical hydrolysis.
Digestion makes absorption possible
Starch, proteins and many lipids are too large or insoluble to cross the intestinal epithelium efficiently. Digestion produces smaller molecules that can be transported across membranes.
Hydrolysis uses water to split bonds; synthesis in cells can later rebuild different molecules.
Digestive enzyme map
Keep enzyme, substrate and product in one complete statement.
| Enzyme | Substrate | Product and main site |
|---|---|---|
| Amylase | Starch | Maltose; mouth and small intestine. |
| Maltase | Maltose | Glucose; small-intestine surface. |
| Proteases | Proteins/peptides | Amino acids; stomach and small intestine. |
| Lipases | Lipids | Fatty acids and glycerol; mainly small intestine. |
Language that earns precision
- chemical digestion
- Enzyme-catalysed hydrolysis of large molecules into smaller molecules.
- amylase
- A carbohydrase that hydrolyses starch to maltose.
- maltase
- An enzyme that hydrolyses maltose to glucose.
- protease
- An enzyme that hydrolyses proteins or peptides.
- lipase
- An enzyme that hydrolyses lipids to fatty acids and glycerol.
Common misconceptions
Claim: Amylase turns starch directly into glucose.
Correction: At this level, amylase produces maltose and maltase then produces glucose.
Claim: Protease digests protein into sugar.
Correction: Proteases ultimately produce amino acids.
Claim: Bile is a lipase.
Correction: Bile contains no digestive enzyme; it emulsifies lipid and affects pH.
Claim: Enzymes are used up in digestion.
Correction: They catalyse reactions and can act repeatedly.
Match the answer to the command word
Write substrate → enzyme → product precisely.
Use hydrolysis and link small soluble products to absorption.
Do not merge amylase and maltase into one step.
Recall, apply, then justify
Describe the complete digestion of starch to glucose.
Hint
Two enzymes and one intermediate.
Suggested marking points
- Amylase hydrolyses starch.
- Maltose is produced.
- Maltase hydrolyses maltose.
- Glucose is produced.
State the products of protein and lipid digestion.
Hint
One product class for protein; two for lipid.
Suggested marking points
- Protein digestion produces amino acids.
- Lipid digestion produces fatty acids.
- Lipid digestion also produces glycerol.
Explain why chemical digestion is required before absorption.
Hint
Compare molecule size and solubility.
Suggested marking points
- Many food molecules are large and/or insoluble.
- Enzymes hydrolyse them into smaller soluble molecules.
- These products can cross the intestinal epithelium into blood or lymph.
Predict the effects of a drug that selectively inhibits maltase but not amylase.
Planning hint
Track substrate, intermediate and product.
What a strong design includes
- Starch could still be hydrolysed to maltose.
- Maltose would accumulate in the small intestine.
- Less glucose would be produced from that starch pathway.
- Less glucose would be absorbed from the affected carbohydrate.
- Unabsorbed solute could affect water movement or microbial fermentation in the lower gut.