The Sequencing of A Level Enzyme Practical Lessons

I teach OCR A Biology in a small, non-selective, 11-18 girls school in Jersey. This year the school has moved to a 6 hour per week per subject timetable to aid this difficult transition based upon the end of the students’ Year 11 experience. Early on in the term, it was clear that the lack of forced revision for terminal examinations meant that learning from the early topics of GSCEs, eg Cell Structure, Digestion, etc, was not solid. This term has been a juggle reviewing and consolidating their learning from the last 3 years, whilst raising it to A level standard. The need for concrete observations for abstract concepts has never been greater at my school.

That being said, what I am going to outline is our usual plan for enzyme practicals for Y12. Some cohorts get a little bit more, some get a little bit less in terms of practical opportunities depending on the department finances, lost time due to special assemblies, and timetabling (we have some students taking A levels at other collaborative schools which may mean that they’d be predictably 10 minutes late each week).

In terms of the specification, 2.1.4 d (ii) practical investigations into the effects of pH, temperature, enzyme concentration and substrate concentration on enzyme activity is it, but to meet the requirements for the Practical Endorsement students would only need to do a single enzyme practical. So why bother doing anymore?

For me the benefits or practical work are clear. Each observation offers a tangible link to the concept for higher level thinking, building schema. By no means I am suggesting that practical work replaces the explicit instruction, but rather forms an essential part of the scheme. As students become confident applying their knowledge to the observed trends, the application of mathematical skills becomes embedded rather than a bolt-on, and their grasp of those practical skills becomes a fluency.

At GCSE, most students will only have done a starch and amylase spotting tile practical. Although it is a perceived “rite of passage” I am not a fan. How ridiculous is it that the only enzyme practical they end up doing relies on the interpretation of negative results. Cognitive Load?! Then there’s the quality of the data relying on the frequency of samples, which then requires students to be “on it” from the off. So the Y12 plan takes a backward step:

Practical 1 – How does the source of catalase affect the rate of hydrogen peroxide decomposition?

Most students have seen the elephant’s toothpaste demo in Chemistry, with manganese (IV) oxide but I do it again as the foundation of catalysts, exothermic reactions and measuring change. In terms of what I want them to develop, it’s the competence in collecting oxygen by the displacement of water and what standard deviation actually shows. Using bacterial catalase, baker’s yeast, celery, potato and bovine liver as the sources gives the opportunity to think about the conservation of the gene across kingdoms and why it’s there (Spec point 2.1.4 b), why different versions of the enzyme may exist (I’ve had a couple of students in the past delve into the phylogenetic analysis of the gene), but also what else may be happening with the idea of eliciting confounding variables such as surface area, concentration, etc.

Practical 2 – How does the concentration of catalase affect the rate of hydrogen peroxide decomposition?

At this point students have had a little experience of serial dilutions, but are not typically competent in this yet. So this practical is about developing the skill, and using ICT to support data analysis. By pulling the class data together it gives the opportunity to see the spread of data achieved despite using the same method, and same source of catalase. Which leads them to recalling the other factors which they have come across at GCSE.

Practical 3 – How does temperature affect the rate of hydrogen peroxide decomposition?

So now is the first introduction of thermostatically controlled waterbaths, and one of those life skills where what it says on the front, may not be what’s inside. This is the first point that resolution of analogue instruments is dropped in, when using a thermometer as the example. The catalase is at an agreed concentration from the previous practical based upon manageable data collection, typically they choose 1% catalase solution. At this point they are pretty confident compiling their class data, calculating the standard deviation and plotting the results. Typically the students calculate a Q10 between 1.5 and 1.85 which I think is good enough, at least they get to calculate it from their lines rather than textbook data. In terms of the theory, I like modelling the impact of temperature using hair curling foam things to show what denaturing an enzyme means in terms of shape change. Within the practical itself, the enzyme and substrate are preheated which brings in the application of activation energy as the hotter waterbath will show that hydrogen peroxide will decompose without the enzyme present giving the rationale for control experiments.

Practical 4 – How does pH affect the rate of hydrogen peroxide decomposition?

The waterbaths are at their optimum temperature from the last practical, but the catalase solutions has been made with different pH buffers. Depending on the time and resources available students record the pH of these solutions with pH probes, or just note what is down on the outside of the beaker. At this point no students typically require any assistance beyond the title on the board.

Practical 5 – How does the concentration of hydrogen peroxide affect the rate of oxygen production by catalase?

Serial dilutions again, this time using pH buffer instead of distilled water. Usually they are competent by this point and need no assistance. They get good data, the standard deviation is pretty sound too. I should mention that we don’t need to reference Michaelis-Menton at all, but vMAX is the limit of what they need. The students are pretty good at this point when it comes to explaining their data because it is only one thing to consider each time so they communicate effectively using the appropriate terminology.

Practical 6 – How does the concentration of copper sulfate solution affect the rate of oxygen production by catalase?

They do the dilutions for both the copper sulfate – catalase solution and the hydrogen peroxide. The data that comes out is good, they see what non-competitive, irreversible inhibition looks like on the graph. I would like to slot one more in with competitive inhibition but I think that limits us to cyanide ions which would take a battle to allow.

Practical 7 – How do chloride ions affect the rate of amylose digestion by amylase?

So it’s not catalase, but it is a specified enzyme, with the specified co-factor, so how to make it not a spotting tile mess? Answer: realtime colourimetry using the data harvest protocol. The nice thing about this is that the students can analyse the initial rates of reactions and make quantitative judgements regarding the effect of co-factors.

So that’s it. Undoubtedly there is a myriad of enzyme practicals that could slot in here, but I prefer to put the others in the topic context rather than the enzyme concept. Enzymes are central to so many of our phenomena, it is only right that students are experts before they get to named processes like photosynthesis.

If you would like any help with the technical support for any of the practicals, data sets or Google Form templates for class data, just let me know.

J. Hale

Twitter: @BeaulieuBio

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