Enzymes and Digestion.
Notes
What enzymes are
Enzymes are biological catalysts: protein molecules that speed up reactions without being used up. Each has an active site with a shape that fits one substrate (the lock and key model).
Effect of temperature and pH
- Temperature: rate rises as molecules move faster and collide more often with more energy. Above the optimum the active site changes shape and the enzyme is denatured. The substrate no longer fits. Denaturation is permanent.
- pH: each enzyme has an optimum pH. Extreme pH also denatures it. Pepsin works best at about pH 2 (stomach), amylase at about pH 7, and the protease in the small intestine at about pH 8 (alkaline).
Calculating rate
Rate = amount of product ÷ time, or rate = 1 ÷ time taken (when measuring how long a reaction takes). Higher-tier questions often ask you to read gradients from a graph.
Digestive enzymes
| Enzyme | Substrate | Product | Made in |
|---|---|---|---|
| Amylase | starch | simple sugars (maltose then glucose) | salivary glands, pancreas, small intestine |
| Protease | protein | amino acids | stomach, pancreas, small intestine |
| Lipase | lipids (fats) | fatty acids and glycerol | pancreas, small intestine |
Bile
Made in the liver, stored in the gall bladder. It is alkaline, neutralising stomach acid so enzymes in the small intestine work at the right pH. It emulsifies fats into small droplets, giving a larger surface area for lipase.
Required practical: enzyme rate
Starch and amylase at different temperatures. Test samples with iodine at intervals. The time for iodine to stop turning blue-black is the time taken for the starch to be broken down.
Common mistakes
- Saying enzymes are "killed" at high temperature. They are denatured, not alive.
- Saying the substrate "melts".
- Confusing bile (emulsifies, not an enzyme) with lipase.
Exam tips
- For a rate-versus-temperature graph, explain the rise (more collisions, more energy) and the fall (denaturation).
- Use the term "active site" in every enzyme explanation.
Worksheet
Total: 25 marks.
- A student measured the time for amylase to break down all the starch in a mixture at five temperatures.
| Temperature (°C) | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|
| Time for starch to disappear (s) | 240 | 120 | 60 | 45 | no change after 5 minutes |
(a) Calculate the rate of reaction at each temperature using rate = 1000 ÷ time (1 decimal place). (3)
(b) Explain the change in rate from 20 °C to 40 °C. (3)
(c) Explain why there was no change at 60 °C. (3)
(d) Suggest how the student could find the optimum temperature more precisely. (2)
- 20 cm³ of oxygen was produced in 40 seconds by catalase breaking down hydrogen peroxide. Calculate the mean rate. (2)
- Explain, using the lock and key model, why amylase breaks down starch but not protein. (3)
- Describe two functions of bile and explain why each helps digestion. (4)
- Pepsin works best at pH 2 but amylase works best at about pH 7. Explain why pepsin stops working when it passes into the small intestine, where the pH is about 8. (3)
- A student says "heating the enzyme to 80 °C killed it." Explain what is wrong with this statement. (2)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- (a) 1000 ÷ time: 20 °C 4.2, 30 °C 8.3, 40 °C 16.7, 50 °C 22.2, 60 °C 0. (B1 for method, B1 for at least three correct, B1 for all correct)
(b) Rate increases as temperature rises (B1) because particles have more kinetic energy and move faster (B1), so there are more frequent, more energetic collisions between enzyme and substrate (B1).
(c) The enzyme was denatured (B1). The high temperature broke bonds in the protein and changed the shape of the active site (B1), so the starch no longer fits and cannot be broken down (B1).
(d) Test more temperatures between 40 °C and 60 °C, for example every 2 °C (B1), and repeat each to calculate a mean (B1). - Rate = 20 ÷ 40 (M1) = 0.5 cm³/s. (A1)
- Each enzyme has an active site with a specific shape (B1). Starch fits the active site of amylase (B1), but protein has a different shape that does not fit, so no enzyme–substrate complex forms (B1).
- Bile is alkaline, neutralising stomach acid (B1) so the enzymes in the small intestine work at their optimum pH (B1). Bile emulsifies fats into small droplets (B1), increasing surface area so lipase can work faster (B1).
- The small intestine is alkaline (pH 8), far from pepsin's optimum of pH 2 (B1). The high pH changes the shape of the active site (B1), so pepsin is denatured and the protein no longer fits (B1).
- Enzymes are molecules, not living organisms, so they cannot be killed (B1). The enzyme was denatured: its shape changed permanently (B1).