Enzyme activity and the food tests
Temperature, pH and denaturing
Warming an enzyme and its substrate makes the molecules move faster, so they collide more often and the rate rises.
Above the optimum the bonds holding the enzyme's shape break, the active site changes shape and the substrate no longer fits: the enzyme is denatured, and that is permanent.
pH works the same way: far from its optimum pH the active site changes shape. Most human enzymes work best near 37 °C and pH 7; pepsin in the stomach works best near pH 2.
The rate falls above the optimum because the enzyme is denatured, not because it is killed or used up.
Required practical: amylase and pH
- Put a drop of iodine in each well of a spotting tile.
- Warm starch, amylase and a buffer of one pH separately in a water bath (same temperature throughout).
- Mix them and start the timer. Every 10 seconds, put a drop of the mixture onto fresh iodine.
- Stop when the iodine stays orange-brown: the starch has gone. Repeat for each pH and three times each.
Fast reactions take short times, so we turn time into a rate:
rate = 1000 ÷ time
- Starch gone after 1 minute 20 seconds
- time = 60 + 20 = 80 s
- rate = 1000 ÷ 80 = 12.5 s⁻¹
Independent variable: pH. Dependent: time for the starch to disappear. Control: temperature, volumes and concentrations. Leave out anomalous repeats before taking a mean.
Change minutes and seconds all into seconds first: 1 min 20 s is 80 s, not 1.20 s.
Required practical: food tests
Grind the food with distilled water, then test the liquid.
| Nutrient | Test | Positive result |
|---|---|---|
| starch | iodine solution | orange-brown to blue-black |
| reducing sugar | Benedict's, heat at about 75 °C | blue to green, yellow, orange or brick red |
| protein | Biuret reagent | blue to purple |
| lipid | shake with ethanol, add water | cloudy white emulsion |
Wear eye protection (Biuret contains sodium hydroxide) and keep ethanol away from flames.
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