What you need to know
- Name carbohydrases, proteases and lipases and state their substrates and products.
- Identify where important digestive enzymes are made and act.
- Explain how temperature and pH affect enzyme activity.
- Plan, analyse and evaluate an enzyme investigation.
- Evaluate models of digestion and describe selected enzyme applications.
The main digestive enzymes
| Enzyme group | Substrate | Products |
|---|---|---|
| Carbohydrases | Carbohydrates | Simple sugars |
| Amylase | Starch | Maltose and other sugars |
| Proteases | Proteins | Amino acids |
| Lipases | Lipids | Fatty acids and glycerol |
Enzymes are biological catalysts with specifically shaped active sites. Digestion is needed because starch, proteins and many lipids are too large or insoluble to cross the intestinal epithelium efficiently.
Misconception AlertCarbohydrase and amylase are two names for exactly the same enzyme.Select to reveal the correctionSelect to hide the correction
Carbohydrase describes a group of carbohydrate-digesting enzymes. Amylase is one carbohydrase and acts on starch.
Where digestive enzymes are made and act
| Source | Enzymes released | Main site of action |
|---|---|---|
| Salivary glands | Amylase | Mouth |
| Stomach | Protease | Stomach |
| Pancreas | Amylase, proteases and lipase | Small intestine |
| Small-intestine wall | Carbohydrases and proteases | Small intestine |
The pancreas releases enzymes into the small intestine through a duct. Bile and pancreatic alkaline fluid help create conditions suitable for intestinal enzymes.
Effects of temperature and pH
As temperature rises towards the optimum, particles move faster and successful collisions occur more often. Above the optimum, bonds maintaining the enzyme's shape break. The active site changes and the enzyme is denatured.
Each enzyme has an optimum pH. Stomach protease works in acidic conditions; many small-intestine enzymes work best near neutral or alkaline pH. Moving far from the optimum changes charges and bonds that maintain active-site shape.
At low temperature the enzyme is usually not denatured; the reaction is slow because particles have less kinetic energy.
Investigating enzyme activity
Amylase and pH
- Add iodine drops to a spotting tile.
- Place starch, amylase and a chosen pH buffer in a tube at constant temperature.
- Start the timer when enzyme and substrate mix.
- At fixed intervals, transfer a drop to iodine.
- Record when iodine remains orange-brown, showing that starch is no longer detected.
- Repeat and calculate a mean for each pH.
Control enzyme concentration, substrate concentration, volumes, temperature and sampling interval. The colour endpoint is subjective, so use frequent sampling, repeated measurements and, if possible, objective colour measurement.
Lipase model
Lipase releases fatty acids, causing pH to fall. An indicator can show the change, but the endpoint must be defined clearly. Bile can be added to investigate emulsification while other variables remain controlled.
Models and applications
A dialysis-tubing model can represent the gut wall: small soluble products pass through, whereas larger molecules are retained. Its limitations include the absence of living epithelial cells, active transport, villi, blood flow and peristalsis.
Enzymes are also used outside the body. Biological washing powders contain proteases and lipases that digest stains. Lactase can produce lactose-reduced milk. Enzyme preparations may replace enzymes that a patient cannot produce sufficiently.
State what the model represents accurately, then identify a specific biological feature that it leaves out and how that changes the comparison.
Quick retrieval check
1. What products does lipase form?
2. Where is pancreatic amylase released?
3. Why does enzyme activity fall above the optimum temperature?
4. Why is 1 ÷ time used as an estimate of rate?
Exam connection
Starch disappears in 120 s at pH 5 and 45 s at pH 7. Calculate the rates and compare them.
Show the answer
pH 5: 1 ÷ 120 = 0.0083 s−1. pH 7: 1 ÷ 45 = 0.022 s−1. The estimated rate at pH 7 is about 2.7 times greater.