What you'll learn
Enzymes are biological catalysts, and each one works best at a particular pH. This required practical investigates how pH affects the rate at which the enzyme amylase breaks down starch. For AQA GCSE Biology you need to know the method using iodine solution and a spotting tile, how to identify the end point of the reaction, how to calculate the rate, and how to explain the results in terms of enzyme structure and denaturation. This guide covers the theory of enzyme action, the full method, the control variables, the calculations, and the explanation of the results. By the end you should be able to describe the practical precisely and explain why activity falls either side of the optimum pH.
Key terms and definitions
Enzyme — A biological catalyst, made of protein, that speeds up a reaction without being used up.
Substrate — The molecule an enzyme acts on; for amylase, the substrate is starch.
Active site — The part of an enzyme where the substrate binds; its shape is complementary to the substrate.
Optimum pH — The pH at which an enzyme works fastest.
Denatured — When an enzyme's shape, including its active site, is permanently changed so the substrate no longer fits and the enzyme stops working.
Amylase — An enzyme that breaks down starch into simple sugars (maltose).
Buffer solution — A solution that keeps the pH constant at a particular value.
Independent variable — The variable you deliberately change; here, the pH.
Core concepts
How enzymes work
An enzyme has an active site with a specific shape that matches its substrate. The substrate fits into the active site, the reaction takes place, and the products are released. Because the shape must match, each enzyme works on only one type of substrate — this is called specificity.
pH affects the shape of the active site. At the optimum pH the active site fits the substrate perfectly and the reaction is fastest. If the pH becomes too high or too low, the forces holding the enzyme's shape are disrupted, the active site changes shape, and the substrate no longer fits. The enzyme is then denatured and the reaction stops. Denaturation is permanent — returning the pH to normal does not restore activity.
The principle of the investigation
Amylase breaks down starch. Iodine solution is orange-brown normally, but turns blue-black in the presence of starch. So as amylase digests the starch, the iodine will eventually stop turning blue-black. The time taken for the iodine to stop going blue-black tells you how long the reaction took, and therefore how fast the enzyme worked at that pH.
The method
- Place single drops of iodine solution in each well of a spotting tile.
- Use a syringe to add a set volume of starch solution to a test tube, and a set volume of amylase to another.
- Add a buffer solution of a known pH to the starch to fix the pH.
- Place the tubes in a water bath at a constant temperature (for example 30 °C) and allow them to reach that temperature.
- Mix the amylase with the starch and buffer, and immediately start a stopwatch.
- Every 10 seconds, use a stirring rod to transfer one drop of the mixture into a well of iodine on the spotting tile.
- Record the time when the iodine no longer turns blue-black, which shows all the starch has been broken down.
- Repeat the whole experiment with buffers of different pH values, and repeat each pH several times to check the results.
Control variables
To make this a fair test, only the pH should change. You must keep constant:
- the temperature (use a water bath)
- the volume and concentration of amylase
- the volume and concentration of starch
- the time between sampling (every 10 seconds)
- the volume of iodine in each well
Changing more than one variable would make it impossible to say the pH caused any difference.
Calculating the rate
Rate is calculated as 1000 divided by the time taken in seconds:
rate = 1000 ÷ time (s)
Using 1000 rather than 1 simply avoids very small decimal numbers. A shorter time means a faster rate. Plotting rate against pH gives a curve that peaks at the optimum pH and falls away on either side.
Explaining the results
At the optimum pH, the active site is the correct shape, so substrate molecules bind easily and the starch is digested quickly — the time is shortest and the rate highest. Above or below the optimum, the shape of the active site is increasingly distorted, fewer substrate molecules can bind, and the reaction slows. At extreme pH the enzyme becomes fully denatured and the starch is not broken down at all, so the iodine keeps turning blue-black no matter how long you wait.
Sources of error and how to reduce them
Several things can make the results less reliable. Judging the exact moment the iodine stops turning blue-black is difficult, because the colour change is gradual, so different people may record slightly different times. Taking samples only every 10 seconds also means the true end point lies somewhere between two samples. The reaction may also start before the stopwatch is started, if the enzyme and substrate are mixed a moment too early. You can reduce these errors by sampling more frequently, by having the same person judge the colour each time, and by mixing and starting the timer at exactly the same moment. Repeating each pH several times and taking a mean reduces the effect of random errors and lets you spot anomalous results that should be left out of the mean.
Why a control is useful
A control experiment helps show that the enzyme is responsible for the change. If you set up a tube of starch and buffer with no amylase added, the iodine should keep turning blue-black throughout, because there is nothing to break down the starch. This confirms that any disappearance of the blue-black colour in the other tubes is caused by the amylase digesting the starch, not by anything else.
Worked examples
Example 1: Calculating a rate
At pH 6, the iodine stopped turning blue-black after 40 seconds. Calculate the rate. Rate = 1000 ÷ 40 = 25 (arbitrary units). A shorter time gives a larger rate value.
Example 2: Comparing two pH values
At pH 7 the reaction took 50 s and at pH 6 it took 40 s. Which pH is closer to the optimum, and why? pH 6 is closer to the optimum, because the reaction was completed in a shorter time, meaning the enzyme worked faster. The active site shape must be closer to its ideal form at pH 6.
Example 3: Explaining a result at extreme pH
At pH 2, the iodine still turned blue-black after ten minutes. Explain this result. At pH 2 the enzyme has been denatured: the active site has permanently changed shape, so starch molecules can no longer bind. The starch is therefore not broken down, and iodine continues to turn blue-black.
Example 4: Improving the investigation
Suggest two ways to improve the reliability of this investigation. First, repeat each pH at least three times and calculate a mean, which reduces the effect of random error. Second, take samples more frequently, for example every 5 seconds instead of every 10, so the end point is identified more precisely.
Common mistakes and how to avoid them
A very common error is saying the enzyme is "killed" at extreme pH. Enzymes are not alive — they are denatured. Using the correct term is essential.
Students often forget to mention the buffer solution. The buffer is what keeps the pH constant at the value being tested; without it, the pH could drift and the results would be invalid.
Another mistake is describing the colour change the wrong way round. Iodine turns blue-black when starch is present; the end point is when it stops turning blue-black because the starch has gone.
When calculating rate, remember that a shorter time means a faster rate. Students sometimes wrongly conclude that the longest time shows the fastest reaction.
Finally, do not say the active site "melts" or "breaks". It changes shape, so the substrate no longer fits — that is the phrasing that earns the mark.
Exam technique for "Required practical: effect of pH on enzyme activity"
Method questions expect precise detail: name the spotting tile and iodine, state the buffer, state the constant temperature via a water bath, and say that samples are taken at regular intervals until the iodine no longer turns blue-black. Vague answers lose marks even when the idea is right.
Explanation questions almost always want the chain: pH changes → active site changes shape → substrate no longer fits → rate falls → at extremes, enzyme denatured. Write it as a sequence.
Expect a calculation using rate = 1000 ÷ time, and be ready to plot or interpret a curve, identifying the optimum as the peak. When asked about improvements, distinguish reliability (repeats and means) from accuracy (more frequent sampling, more precise measuring).
Quick revision summary
- Enzymes have an active site with a specific shape; pH affects that shape.
- Amylase breaks down starch; iodine turns blue-black with starch and stays orange-brown without it.
- Method: buffer to set pH, constant temperature water bath, sample onto iodine on a spotting tile every 10 s, record when iodine no longer turns blue-black.
- Control the temperature, and the volumes and concentrations of amylase and starch.
- rate = 1000 ÷ time (s); shorter time means faster rate; the optimum pH gives the peak.
- Away from the optimum the active site changes shape; at extremes the enzyme is denatured permanently.