You might remember the spotting tile and the iodine turning blue-black. But explaining what the colours mean, or why you needed a water bath, can feel harder once you’re looking at an exam question.
The amylase practical makes more sense when you connect each step to the biology.
Amylase breaks down starch into smaller sugars. To investigate how pH affects its activity, you test samples with iodine and measure how long starch takes to stop being detected.
This guide explains enzymes, the practical method, variables, rate calculations and common mistakes. There are practice questions at the end so you can check your understanding.
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mplementary to the enzyme’s active site. It binds there, forming an enzyme-substrate complex. The enzyme catalyses the reaction, the products leave and the enzyme can be used again.
The important word is complementary.
A key fits inside a lock because their shapes match in the right places. The key does not have the same shape as the entire lock.
For the same reason, avoid writing:
“The substrate has the same shape as the enzyme.”
A clearer answer is:
“The substrate has a complementary shape to the active site, allowing an enzyme-substrate complex to form.”
Why can amylase digest starch but not proteins?
Amylase’s active site is suited to its starch substrate. Protein substrates do not have a complementary shape, so amylase cannot catalyse their digestion.
Proteases have different active sites and break down proteins instead.
How temperature affects enzyme activity
At low temperatures, enzyme and substrate molecules have relatively little kinetic energy. Successful collisions happen less frequently, so the reaction is slow.
As temperature rises, the molecules move faster. More successful collisions happen each second, increasing the reaction rate.
The rate reaches a maximum at the enzyme’s optimum temperature.
At sufficiently high temperatures, the enzyme’s structure changes. Its active site loses the shape needed to bind the substrate effectively. The enzyme becomes denatured.
Remember these distinctions:
- Low temperature usually slows enzyme activity rather than denaturing the enzyme.
- Enzymes are molecules, so they are denatured rather than “killed”.
- Different enzymes have different optimum temperatures. Do not assume every enzyme works best at 37°C.
How pH affects enzyme activity
Each enzyme has an optimum pH.
Moving away from that pH can affect substrate binding and reduce enzyme activity. More extreme conditions can disrupt the enzyme’s structure and denature it.
In an explanation, connect the conditions to the active site and reaction rate:
“The altered conditions affect the active site, reducing successful substrate binding. Fewer enzyme-substrate complexes form each second, so the reaction rate falls.”
Avoid assuming that every small change in pH causes complete denaturation.
Human salivary amylase usually works best near neutral pH. However, amylase from a different source may behave differently. In an exam, use the data provided to identify the apparent optimum.
Digestive enzymes: amylase, protease and lipase
| Enzyme | Substrate | Products |
|---|---|---|
| Amylase | Starch | Smaller sugars, including maltose |
| Proteases | Proteins | Amino acids |
| Lipases | Lipids | Fatty acids and glycerol |
Amylase is produced by the salivary glands and pancreas. It acts in the mouth and small intestine.
Amylase does not complete every stage of carbohydrate digestion. Other enzymes break sugars such as maltose down further.
Bile is not an enzyme. It neutralises acid entering the small intestine and emulsifies fats into smaller droplets. This increases the surface area available for lipase.
What is the amylase required practical?
The investigation asks:
How does pH affect the rate at which amylase breaks down starch?
You change the pH using buffer solutions and measure the time until starch is no longer detected by iodine.
The practical is listed as:
- Required Practical 5 in AQA GCSE Biology.
- Required Practical 4 in AQA GCSE Combined Science: Trilogy.
You can check these requirements in AQA’s Biology practical guidance and Combined Science: Trilogy practical guidance.
Other boards may organise their practical work differently. Follow your course specification and your teacher’s instructions.
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Equipment for the amylase practical
You will usually need:
- Amylase solution
- Starch solution
- Buffer solutions at different pH values
- Iodine solution
- A spotting tile
- Test tubes and a rack
- Pipettes or suitable measuring equipment
- A stopwatch
- A water bath and thermometer
- Eye protection
This is a supervised laboratory practical. Follow your school’s risk assessment, avoid contact with the solutions and never taste them.
The AQA GCSE Biology practical handbook provides supporting apparatus and method guidance.
Amylase practical variables
| Variable type | What changes or is measured? |
|---|---|
| Independent variable | The pH of the reaction mixture |
| Dependent variable | Time until starch is no longer detected, used to calculate relative rate |
| Control variables | Temperature, starch and amylase concentrations, and solution volumes |
Also keep the sampling interval, sample size, iodine drops and mixing method consistent.
Why use a buffer solution?
A buffer maintains the reaction mixture at the chosen pH.
Without it, you cannot confidently compare reactions at controlled pH values.
Why use a water bath?
Temperature affects enzyme activity too.
A water bath helps keep temperature constant, so differences in reaction time can be linked more confidently to pH.
Amylase practical method: step by step
Use the volumes and concentrations supplied by your teacher. Keep them consistent across the pH tests.
- Place a drop of iodine solution into each well of a spotting tile.
- Measure amylase and a buffer solution into a test tube.
- Put this mixture and a separate measured starch solution into the water bath.
- Allow both solutions to reach the chosen temperature.
- Mix the starch with the amylase-buffer mixture and start the stopwatch immediately.
- Every 30 seconds, transfer a small sample to a fresh iodine well.
- Record the first sampling time at which the iodine stays orange-brown.
- Repeat with different pH buffers while keeping other conditions constant.
- Repeat each pH test, investigate unusual results and calculate an appropriate mean.
Keep iodine on the spotting tile. Do not add it to the main reaction tube or return tested samples to the mixture.
Why start the stopwatch immediately?
Amylase begins breaking down starch as soon as the solutions mix.
If you delay starting the timer, you miss part of the reaction and record a time that is too short.
Why warm the solutions before mixing?
Mixing first allows starch breakdown to begin while the solutions are still changing temperature.
Warming them separately helps each reaction begin under comparable conditions.
Understanding why each step matters also helps with other investigations. Our GCSE Biology microscopy and magnification guide explains another practical topic through its method, observations and calculations.
What do the iodine colours mean?
| Colour after adding a sample | Meaning |
|---|---|
| Blue-black | Starch is present |
| Orange-brown | Starch is not detected |
At first, starch remains in the mixture, so samples turn iodine blue-black.
As amylase breaks down the starch, less remains. Eventually, a sample no longer produces a blue-black colour.
Iodine tests for starch. It does not measure the sugar produced.
For another application of the same test, explore the Royal Society of Chemistry’s starch-testing experiment.
Does orange-brown prove every starch molecule has gone?
It tells you that starch is no longer detected by this test.
For GCSE practical work, this is used as the endpoint. It is more accurate than claiming the colour proves that absolutely no starch molecules remain.
Why does sampling every 30 seconds limit precision?
Suppose a sample turns iodine blue-black at 60 seconds, but the next stays orange-brown at 90 seconds.
The endpoint occurred somewhere between those observations.
You record 90 seconds as the first negative test. You cannot claim the exact endpoint was precisely 90 seconds.
What if the endpoint is never reached?
Record that starch was still detected at the end of the observation period.
Do not record zero seconds. That would suggest the endpoint happened immediately.
How to calculate the reaction rate
For a consistent starting amount of starch and the same endpoint, compare reactions using:
Relative rate = 1 ÷ time
If time is measured in seconds, relative rate has the unit s⁻¹, meaning “per second”.
Worked calculation
At pH 5, the endpoint is reached after 120 seconds:
Relative rate = 1 ÷ 120
Relative rate = 0.00833 s⁻¹
At pH 7, the endpoint is reached after 60 seconds:
Relative rate = 1 ÷ 60
Relative rate = 0.0167 s⁻¹
The second reaction has twice the relative rate.
Shorter time means faster reaction, when the starting conditions and endpoint are comparable.
This is a relative measure. It does not directly tell you the mass of starch digested per second.
If working with equations is the difficult part, our GCSE Maths guide to rearranging formulae explains how to identify and isolate an unknown quantity.
Example amylase practical results
These are illustrative results, not measurements from an actual experiment.
| pH | Mean time to endpoint, seconds | Relative rate, s⁻¹ |
|---|---|---|
| 5 | 180 | 0.00556 |
| 6 | 90 | 0.0111 |
| 7 | 60 | 0.0167 |
| 8 | 120 | 0.00833 |
| 9 | 240 | 0.00417 |
The fastest reaction occurred at pH 7.
A suitable conclusion is:
“Of the pH values tested, pH 7 produced the highest relative rate of starch breakdown.”
This does not establish the exact optimum. It could lie between the tested values.
To investigate further, test smaller intervals around pH 7, such as pH 6.5 and 7.5.
How to draw and interpret the graph
For a graph of relative rate against pH:
- Put pH on the horizontal x-axis.
- Put relative rate in s⁻¹ on the vertical y-axis.
- Choose a sensible scale and plot points accurately.
- Draw an appropriate line or curve of best fit, following the question’s instructions.
The highest point represents the fastest reaction.
For a graph of time against pH, the fastest reaction appears at the lowest point.
Before explaining a graph, read both axis labels. A high reaction time and a high reaction rate mean different things.
For practice interpreting unfamiliar data, use questions and mark schemes from AQA’s GCSE Biology assessment resources. Choose material for your course and tier.
How to evaluate the amylase investigation
A useful evaluation explains a limitation, its effect and a relevant improvement.
| Limitation | Why it matters | Improvement |
|---|---|---|
| Sampling every 30 seconds | The endpoint falls between observations | Use shorter, consistent intervals where practical |
| Judging colour by eye | People may identify the endpoint differently | Use consistent lighting and a clear colour reference |
| Fluctuating temperature | Temperature may influence reaction rate | Monitor a thermostatically controlled water bath |
| Inconsistent sample sizes | Starch detection becomes harder to compare | Use consistent sample and iodine volumes |
| One trial at each pH | Random variation may distort the pattern | Repeat trials and calculate a mean |
| Large gaps between pH values | The optimum may fall between tested values | Test smaller intervals near the apparent optimum |
Do repeats make the experiment accurate?
Repeats help you assess variation, identify unusual results and reduce the influence of random error on a mean.
They do not automatically correct systematic errors. A thermometer that consistently reads too high will still do so across repeated trials.
Similarly, naming a colorimeter is not a complete improvement. You need to explain how a suitable measurement method would address the limitation.
Common GCSE enzyme mistakes
| Avoid writing | A more accurate explanation |
|---|---|
| “Heat kills the enzyme.” | High temperature can denature the enzyme, changing its active site. |
| “The substrate has the same shape as the enzyme.” | The substrate is complementary to the active site. |
| “The enzyme gets used up.” | The enzyme can catalyse further reactions. |
| “Iodine turns clear.” | Iodine remains orange-brown when starch is not detected. |
| “A longer time means a faster reaction.” | A shorter time to the same endpoint means a faster reaction. |
| “All enzymes work best at 37°C.” | Different enzymes have different optimum temperatures. |
GCSE enzymes practice questions
Try these before reading the answers.
- What is a biological catalyst?
- Explain why amylase cannot digest proteins.
- Why is a buffer solution used in the practical?
- Why should the starch and amylase solutions reach the chosen temperature before mixing?
- The endpoint is reached after 150 seconds. Calculate the relative rate.
- A sample is blue-black at 90 seconds and orange-brown at 120 seconds. What can you conclude?
- A student tests pH 4, 6, 8 and 10. The fastest reaction occurs at pH 6. How could they estimate the optimum more closely?
- Explain why repeating an investigation would not correct a faulty thermometer that always reads 3°C too high.
Practice answers
1. Biological catalyst
A substance involved in biological reactions that increases reaction rate without being used up.
2. Enzyme specificity
Amylase’s active site is not complementary to protein substrates, so it cannot form the enzyme-substrate complexes needed to catalyse their digestion.
3. Buffer solution
It maintains the chosen pH, allowing the effect of pH on reaction rate to be investigated.
4. Temperature control
Temperature affects enzyme activity. Bringing both solutions to the chosen temperature helps reactions start under comparable conditions.
5. The relative rate calculation
Relative rate = 1 ÷ 150
Relative rate = 0.00667 s⁻¹, to three significant figures.
6. Endpoint interpretation
The first negative starch test occurred at 120 seconds. The endpoint fell between the two observations, but its exact time is unknown.
7. Finding the optimum
Test smaller pH intervals around pH 6, while keeping other conditions constant.
8. Systematic error
The thermometer produces a consistent measurement error. Repeating measurements with the same faulty instrument does not remove that error.
FAQs
Are enzymes used up during reactions?
No. Enzymes are not used up or permanently changed, so they can catalyse further reactions.
What is the difference between an enzyme and a substrate?
An enzyme is the biological catalyst. The substrate is the molecule that binds to its active site and is changed during the reaction.
What does denatured mean?
A denatured enzyme has undergone a change in its three-dimensional structure. Its active site is no longer complementary to the substrate, so it cannot catalyse the reaction effectively.
Why does amylase break starch down?
Starch molecules are too large to absorb directly. Amylase breaks starch into smaller sugar molecules.
Does amylase work in the stomach?
Amylase works in the mouth, but the acidic conditions in the stomach stop salivary amylase from functioning effectively. Further amylase is released by the pancreas and acts in the small intestine.
What is the independent variable in the amylase practical?
The independent variable is the pH of the reaction mixture.
Why is a buffer solution used?
A buffer maintains the reaction mixture at a chosen pH, allowing the effect of pH on amylase activity to be investigated.