5.1.2B2. Biological Molecules and Enzymes
Enzyme Investigations — Combined Science 0653 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 6 Unit quiz
Section 1 of 6The key test: iodine and starch
What is being measured
- What is being measured Amylase is an enzyme that breaks down starch into sugar.
- Iodine solution is orange-brown, but turns blue-black when starch is present.
Reading the result
- Reading the result When a drop of the mixture no longer turns the iodine blue-black, all the starch has been digested.
- The shorter the time taken, the faster the enzyme is working under those conditions.
Section 2 of 6Investigating temperature
1
Place drops of iodine solution in the wells of a spotting tile.
2
Heat separate tubes of starch solution and amylase solution in water baths set to the temperature being tested, for example 10, 20, 30, 40 and 50 °C.
3
Mix the amylase into the starch solution, start a stopwatch and keep the tube in the water bath.
4
Every minute, use a dropper to remove a drop of the mixture and add it to a fresh well of iodine.
5
Continue until the iodine stays orange-brown, and record the time taken for the starch to disappear.
6
Repeat the whole experiment at each temperature.
Section 3 of 6Investigating pH
1
Put single drops of iodine in rows on the tile and label a test tube with the pH to be tested.
2
Use a syringe to add a measured volume of amylase, then a measured volume of buffer solution of that pH.
3
Add a measured volume of starch solution using a second syringe and start the stopwatch as you mix.
4
After 10 seconds, transfer a drop of the mixture to the first iodine drop, which should turn blue-black.
5
Repeat every 10 seconds until the iodine stays orange-brown, and record that time.
6
Repeat with buffers of different pH values.
Section 4 of 6Variables and explaining the results
| Aspect | Temperature experiment | pH experiment |
|---|---|---|
| Independent variable | Temperature of the water bath | pH of the buffer solution |
| Dependent variable | Time for starch to disappear | Time for starch to disappear |
| Control variables | Volume and concentration of enzyme and starch, pH | Volume and concentration of enzyme and starch, temperature |
| Low value | Slow: molecules have little kinetic energy, so fewer successful collisions | Slow if far from the optimum |
| Very high or extreme value | Enzyme denatured: the active site changes shape, so the substrate no longer fits | Enzyme denatured by too acidic or too alkaline a pH |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Enzyme | A protein that acts as a biological catalyst to speed up a reaction. |
| Amylase | An enzyme that breaks down starch into sugar. |
| Buffer solution | A solution that keeps the pH at a fixed value. |
| Denatured | Describes an enzyme whose active site has changed shape so that it no longer works. |
| Optimum | The temperature or pH at which an enzyme works fastest. |
| Substrate | The substance that an enzyme acts upon. |
Section 6 of 6Exam technique
1
Saying that low temperature denatures enzymes — it only slows them; high temperature denatures.
2
Reusing the same iodine well for several samples, so colours become impossible to read.
3
Mixing the enzyme and starch before they have reached the required temperature.
4
Not keeping volumes and concentrations equal, which makes the comparison unfair.
5
Describing the end point as 'the iodine goes clear' — it stays orange-brown, showing there is no starch left.
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