2.5.9Structure and Functions in Living Organisms: Part 1
Role of Digestive Enzymes — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 22 of 32
Part 1 of 5Why digestion needs enzymes
Breaking bonds
- Breaking bonds Large food molecules are held together by chemical bonds.
- Digestion breaks these bonds to make small, soluble molecules that can be absorbed.
Biological catalysts
- Biological catalysts Enzymes are biological catalysts: they speed up reactions but are not used up or changed themselves.
- Without them digestion would be far too slow to supply the body's needs at body temperature.
Mechanical help
- Mechanical help Chewing, churning and emulsification cut food into smaller pieces first, giving enzymes a larger surface area to work on.
Part 2 of 5The three groups of digestive enzymes
Digestive enzymes
Carbohydrases
- Carbohydrases Break carbohydrates down into simple sugars.
- Amylase turns starch into maltose; maltase then turns maltose into glucose.
Proteases
- Proteases Break proteins down into amino acids.
- Pepsin from the stomach cuts proteins into shorter polypeptide chains, and other proteases in the small intestine finish the job.
Lipases
- Lipases Break lipids (fats and oils) into glycerol and fatty acids.
- They are made by the pancreas and act in the small intestine.
Part 3 of 5Where each enzyme is made and where it works
| Enzyme | Made in | Acts in | Reaction |
|---|---|---|---|
| Amylase | Salivary glands, pancreas, small intestine | Mouth and small intestine | Starch to maltose |
| Maltase | Small intestine | Small intestine | Maltose to glucose |
| Pepsin (a protease) | Stomach | Stomach | Protein to polypeptides |
| Other proteases | Pancreas, small intestine | Small intestine | Polypeptides to amino acids |
| Lipase | Pancreas | Small intestine | Lipid to glycerol and fatty acids |
Part 4 of 5Following starch all the way
1
In the mouth, salivary amylase begins to break starch into maltose while the food is chewed.
2
In the stomach, acid denatures the salivary amylase, so starch digestion pauses.
3
Pancreatic amylase enters the small intestine and carries on converting starch to maltose.
4
Maltase in the lining of the small intestine converts maltose to glucose.
5
Glucose is small and soluble, so it is absorbed into the blood and carried to cells for respiration.
Part 5 of 5Enzyme conditions
Each enzyme has an optimum pH
- Each enzyme has an optimum pH Stomach pepsin works best in strong acid,
- whereas enzymes in the small intestine work best in slightly alkaline conditions.
- This is why bile neutralises stomach acid when food leaves the stomach.
Specific action
- Specific action Each enzyme has an active site that fits only one type of substrate, so amylase cannot digest protein.
Key terms
| Term | Meaning |
|---|---|
| Enzyme | A protein that acts as a biological catalyst, speeding up a specific reaction. |
| Carbohydrase | An enzyme that breaks carbohydrates down into simple sugars. |
| Protease | An enzyme that breaks proteins down into amino acids. |
| Lipase | An enzyme that breaks lipids down into glycerol and fatty acids. |
| Substrate | The molecule an enzyme acts on, for example starch for amylase. |
| Denatured | Having an active site permanently changed in shape, for example by strong acid, so the enzyme no longer works. |
Avoid these mistakes
1
Saying enzymes are 'used up' in digestion. They are catalysts and can be used repeatedly.
2
Forgetting that starch needs two enzymes, amylase then maltase, to reach glucose.
3
Claiming that the pancreas is part of the alimentary canal. It is an accessory organ that releases enzymes into the small intestine.
4
Writing that enzymes are killed by heat or acid. Enzymes are not alive; they are denatured.
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