AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Required practical activity 5: investigate the effect of pH on the rate of…
Full specification wording
The human digestive system: Required practical activity 5: investigate the effect of pH on the rate of reaction of amylase enzyme.
In this practical you investigate how pH affects the rate at which amylase breaks down starch.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 21 of 25
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In this practical you investigate how pH affects the rate at which amylase breaks down starch. You set up a series of test tubes containing amylase solution and buffer solutions of different pH values, then add starch solution and start timing. At intervals, you remove a sample and test it with iodine solution on a spotting tile. While starch remains, iodine turns blue-black; when starch has been fully digested, iodine stays yellow-brown. The time taken for starch to disappear is a measure of the rate of reaction, so a shorter time means a faster rate. You should control variables such as temperature, volumes and concentrations, and repeat the investigation to improve reliability. The results typically show an optimum pH at which amylase works fastest, with the rate decreasing at pH values further from the optimum.
What earns the marks
- Describes setting up test tubes containing amylase and buffer solutions of different pH values, then adding starch solution to start the reaction.
- Uses iodine solution on a spotting tile at regular time intervals to test for the presence of starch.
- Records the time taken for starch to disappear, recognising that a shorter time indicates a faster rate of reaction.
- Controls key variables such as temperature, volume and concentration of amylase and starch solutions, and the pH buffer used.
- Calculates a rate of reaction, for example using rate = 1 ÷ time, and uses this to compare the effect of different pH values.
- Plots a graph of rate against pH and describes the pattern, including the optimum pH at which the rate is highest.
- Explains the results in terms of enzyme activity, including that pH affects the shape of the enzyme’s active site and that extreme pH values denature the enzyme.
Common mistakes and how to put them right
Assuming that a longer time means a faster reaction.
Put it right: A shorter time for starch to disappear means a faster rate, because the enzyme has broken down the starch more quickly.
Failing to control temperature when comparing different pH values.
Put it right: Keep the temperature constant, for example using a water bath, so that pH is the only independent variable.
Testing the mixture only at the end of the experiment.
Put it right: Take samples at regular intervals so that the time at which starch disappears can be identified.
Exam tips
- State clearly what is being measured and how it indicates rate, such as time for starch to disappear or rate calculated as 1 ÷ time.
- Identify the independent variable, dependent variable and at least two control variables before describing the method.
- When explaining the shape of the graph, link the optimum pH to enzyme activity and explain that extreme pH changes the active site so the enzyme is denatured.
Try the lesson
The start of the The human digestive system: Required practical activity 5: investigate the effect of pH on the rate of reaction of amylase enzyme. lesson, made from checked teaching notes for this specification point.
Have a go first
A student investigates the effect of pH on the rate of reaction of amylase. Which substance is broken down by amylase in this investigation?
Listen
In this practical you investigate how pH affects the rate at which amylase breaks down starch. You set up a series of test tubes containing amylase solution and buffer solutions of different pH values, then add starch solution and start timing. At intervals, you remove a sample and test it with iodine solution on a spotting tile. While starch remains, iodine turns blue-black; when starch has been fully digested, iodine stays yellow-brown. The time taken for starch to disappear is a measure of the rate of reaction, so a shorter time means a faster rate. You should control variables such as temperature, volumes and concentrations, and repeat the investigation to improve reliability. The results typically show an optimum pH at which amylase works fastest, with the rate decreasing at pH values further from the optimum. A strong answer earns marks for these points. Describes setting up test tubes containing amylase and buffer solutions of different pH values, then adding starch solution to start the reaction. Uses iodine solution on a spotting tile at regular time intervals to test for the presence of starch. Records the time taken for starch to disappear, recognising that a shorter time indicates a faster rate of reaction. Controls key variables such as temperature, volume and concentration of amylase and starch solutions, and the pH buffer used. A common slip: Assuming that a longer time means a faster reaction. Instead, a shorter time for starch to disappear means a faster rate, because the enzyme has broken down the starch more quickly. Exam tip: State clearly what is being measured and how it indicates rate, such as time for starch to disappear or rate calculated as 1 ÷ time.
Watch it explained
What is the one idea the video explains? Say it in a sentence.
Read the described scene instead
In this practical, you investigate how pH affects the rate at which amylase breaks down starch. You use amylase solution and buffer solutions of different pH values. The pH is the independent variable, and the time for starch to disappear is the dependent variable. Set up test tubes containing amylase solution and a buffer. Then add starch solution and start timing. At regular intervals, remove a sample and test it with iodine solution on a spotting tile. While starch remains, iodine turns blue-black; when starch has been fully digested, iodine stays yellow-brown. To make the comparison fair, control key variables: temperature, the volumes and concentrations of amylase and starch, and the pH buffer used. Keep temperature constant, for example with a water bath. Repeat the investigation to improve reliability. To compare different pH values, use the time taken for starch to disappear. A shorter time means a faster rate, because the enzyme has broken down the starch more quickly. You can calculate rate as one divided by time. For example, this makes a shorter time give a larger rate. A common mistake is assuming a longer time means a faster reaction. In fact, a shorter time for starch to disappear means a faster rate. Another mistake is failing to control temperature, so pH is no longer the only independent variable. Also, do not test only at the end; take samples at regular intervals. Results typically show an optimum pH at which amylase works fastest. The rate decreases at pH values further from the optimum. Plot rate against pH to see this pattern. pH affects the shape of the enzyme’s active site, and extreme pH values denature the enzyme.
Key words
Say what each word means before you turn the card.
Rate of reaction
Try answering aloud or in your head before turning the card.
Control variables
Try answering aloud or in your head before turning the card.
Optimum pH
Try answering aloud or in your head before turning the card.
Denaturation
Try answering aloud or in your head before turning the card.
See every definition
- Rate of reaction
- A measure of how quickly starch is broken down, shown by the time taken for starch to disappear, so a shorter time means a faster rate.
- Control variables
- Variables such as temperature, volumes and concentrations that should be kept constant in the investigation.
- Optimum pH
- The pH at which amylase works fastest, with the rate decreasing at pH values further from it.
- Denaturation
- What happens to the enzyme at extreme pH values, which affect the shape of the enzyme's active site.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of The human digestive system: Required practical activity 5: investigate the effect of pH on the rate of reaction of amylase enzyme.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge