The Role of Digestive Enzymes
Digestive enzymes are biological catalysts that break down large, insoluble food molecules into smaller, soluble molecules that can be absorbed into the bloodstream. The ESAT requires you to know the specific roles of the three main types of digestive enzymes.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Amylase enzymes catalyse the breakdown of large carbohydrates, such as starch, into simple sugars like maltose.
- Protease enzymes catalyse the breakdown of proteins into their building blocks, which are amino acids.
- Lipase enzymes catalyse the breakdown of fats (lipids) into fatty acids and glycerol.
- These digestive enzymes are extracellular, meaning they are secreted by glands and work outside of cells in the digestive tract.
- Each enzyme type is specific to its substrate: amylase only works on carbohydrates, protease on proteins, and lipase on fats.
Formulae
Carbohydrates (Starch) → Simple Sugars To represent the action of amylase.
Proteins → Amino Acids To represent the action of protease.
Fats (Lipids) → Fatty Acids + Glycerol To represent the action of lipase.
Definitions
- Amylase
- A class of enzymes that digests complex carbohydrates (polysaccharides) into simpler sugars (monosaccharides or disaccharides).
- Protease
- A class of enzymes that digests proteins into smaller polypeptides or single amino acids.
- Lipase
- A class of enzymes that digests fats (lipids) into fatty acids and glycerol.
Worked example
An experiment investigates the breakdown of different food substances. Four test tubes are set up, each containing a food substance and an enzyme solution, and incubated at an optimal temperature. The table below shows the contents of each tube. Which tube would test positive for the presence of fatty acids after incubation? | Tube | Food Substance | Enzyme Added | |---|---|---| | 1 | Starch | Amylase | | 2 | Protein | Protease | | 3 | Fat | Lipase | | 4 | Fat | Protease |
- 1
Identify the product being tested for:
fatty acids.
- 2
Recall which enzyme and substrate produce fatty acids.
Fatty acids and glycerol are the products of fat digestion by the enzyme lipase.
- 3
Examine the table to find the test tube containing both fat (the substrate) and lipase (the correct enzyme).
- 4
Tube 1 contains starch and amylase, producing simple sugars.
- 5
Tube 2 contains protein and protease, producing amino acids.
- 6
Tube 3 contains fat and lipase.
This is the correct combination to produce fatty acids.
- 7
Tube 4 contains fat but the incorrect enzyme (protease), so no significant reaction will occur.
Protease is specific to protein.
Answer: Tube 3
Common mistakes
- ×Misinterpreting data from diagrams or tables by forgetting enzyme specificity. For example, incorrectly assuming that protease in a test tube with fat will cause some digestion because it is an enzyme.
- ×Confusing the products of digestion, for example, stating that amylase breaks down starch into amino acids.
- ×Mixing up the enzyme-substrate pairs, such as thinking lipase digests protein.
No-calculator tips
- ✓In questions involving experimental setups, first match the enzyme to its substrate (Amylase-Carbohydrate, Protease-Protein, Lipase-Fat). Cross out any combinations in the diagram or table that do not match.
- ✓Focus on the names: 'Protease' breaks down 'protein'. 'Lipase' breaks down 'lipids' (fats). 'Amylase' is related to 'amylose', a component of starch.
- ✓Use elimination for multiple-choice questions. If the question is about protein digestion, immediately discard any answer options involving amylase or lipase.