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AQA GCSE Biology (9-1): Getting the most out of Required Practicals - Enzyme Activity

Posted by James Simms on November 25, 2019

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Specification location: AQA GCSE Biology Paper 1, Section 4.2 - Organisation

Summary of method: A volume of starch, a volume of amylase and various pH buffer solutions are heated to a uniform temperature (usually 35^oC). Drops of iodine solution are placed in a spotting tile and a drop of starch placed in the first well (as a reference). The starch, amylase and one of the pH buffer solutions are mixed together and a drop is removed every 10 seconds and added to one of the iodine wells. If the amylase has not digested all of the starch, the well will turn black; if the starch has been entirely digested, the well will remain orange-brown. This is repeated until the well no longer changes colour. The entire method is then repeated for the other pH buffer solutions (e.g. pH5, pH6, pH7, pH8).

For more detail on the set-up of this experiment, watch our high-quality required practical video on YouTube.

Enzyme activity required practical variables:

Independent variable: pH of the amylase’s environment

Dependent variable: Time to completely digest starch

Control variables: Volume of amylase and starch solutions, concentration of starch and amylase solutions, temperature, concentration of iodine solution

Common issues	Potential solutions
Time	I have generally found this experiment to be difficult to explain, complete and analyse within an hour. There are several possible solutions. Students could measure out the volumes of the different solutions and placing them in a water bath for heating before the enzymes content is recapped and the method is explained. Alternatively, the solutions could be provided premeasured and prewarmed. I’m not really a fan of either of these methods, as the first removes context for the first half of the activity and the second removes opportunities to develop practical skills and discuss the subject-specific terminology around them (e.g. random errors). By far, the best solution I have found is for pupils to preload the investigation’s methodology through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on https://www.theeverlearner.com/). This minimises lag time at the start through misunderstanding and also the time needed to be spent on covering content on enzymes again. A pre-prepared method sheet or access to a high-quality tutorial video can then be used to access the practical aspect much more quickly.
Confusion	In my experience, students often find this method to be complicated, primarily because of the number of different reagents, the different volumes of those reagents that they need to accurately measure and when it is acceptable to reuse measuring equipment (e.g. measuring out multiple starch volumes) and when it is not (e.g. using a measuring cylinder previously used for amylase to measure out starch). As a solution, extensive teacher modelling will probably be required, along with a crystal-clear method sheet and a tutorial video. This should remove most, but probably not all, issues with this.
End points	It is sometimes difficult for students to recognise when the endpoint of the experiment is. They will want to continually refer to the ‘positive’ reaction in the first well and to the normal colour of iodine solution in order to make that determination.

Sample enzyme activity required practical results:

pH of solution	Time for amylase to completely break down the starch in seconds (at 35^oC)
5	30
6	50
7	160
8	>300

Source: AQA GCSE Biology (9-1) required practical handbook

AQA GCSE Science subject-specific terminology:

This is an ideal opportunity to discuss different types of error with students. For example, the ‘drop’ of starch solution placed in each spotting tile well will vary in size, which will affect the digestion time (a random error). For more help with this, try this video.
As the drops of solution will be removed every ten seconds, it is highly likely that the real value will lie between these times: it’s an ideal opportunity to discuss the ideas of both intervals and range.