How I teach enzyme function (without PowerPoint)

Here's how I taught about enzymes recently with students 14–16 using just diagrams and dialogue. You can read about the benefits of teaching without PowerPoint here.

What are enzymes? For students that's a strange word. So, firstly, we'd better have a look. To do so, my favourite animation is one of lactase by biointeractive. Not only is the animation nice, but it's a context that is familiar to students.

Key distinctions

Yet, a distinction must be drawn before we can explore that context. After watching the animation, we watched it a second time and discussed what's going on. I told them that enzymes are proteins, and then I asked a question: What is bigger, a cell or an enzyme? I had them vote by raising their hands. As anticipated, many thought enzymes were larger.

This was a common confusion; some students took a while to discern between the different "invisible entities of the body". To help them, I returned to the organic molecules. I drew a cell, as shown below, and asked students what they were made of. They replied that they were made of organic molecules.

I asked students for examples of those organic molecules and wrote some down. I then reminded them that enzymes were a type of protein encoded in genes.

"So, which is bigger, cells or enzymes?" I asked again. This time, they voted for cells. But we weren't finished yet. I asked, "How many enzymes do you think fit into a typical cell?" The students offered small numbers, like 100, or 1000, maybe 1,000,000. I told them there could be billions. This gave them a better sense of scale.

There was another key distinction that students typically confuse. This time I asked: "Are enzymes alive?" And some students thought they might be. After watching the animation, I suppose, they may have perceived causative action and related it to life. I then returned to the idea of cells being the smallest living unit. With this rule, enzymes can't be alive, they're just molecules.

Putting Enzymes in Context

We were ready to return to the context that gives purpose to the concept of enzymes. The reason this context is nice is because it involves a natural contrast—a difference, variation—that students are familiar with. As a class, we'd discussed lactase's function, but what if it didn't function?

I taught the students about lactose tolerance, why it likely evolved (see this video) in two human populations and why the rest of the mammals are lactose intolerant after weaning. "What if you were lactose intolerant and drank milk, what would happen?" I asked.

The key is that the enzyme isn't present, so lactose isn't broken down, and therefore it can't be absorbed by the cells of the intestine. And if it isn't absorbed, it stays in the lumen and the bacteria there have a feast. Ultimately, the person would experience gases and diarrhea, (depending on the quantity of lactose).

"What about lactose-free milk... how could we make that?" I listened to some of the students' ideas, often more complicated than the answer, and then I told them. You get the milk, and you add lactase to it. So the only difference is that one contains lactose, and the other contains glucose and galactose (plus, maybe, some lactase).

The Mechanism of Enzymes

Now, my students were ready to tackle the mechanism of enzymatic reactions. I drew a diagram in three stages with my students, which I discuss next.

1. Enzyme and substrate

I drew the first diagram while the students drew with me. I then distinguished the example from the concept by writing "lactase" and "lactose" and telling the students that they were the "examples". We needed a name for the concept: lactase is a type of enzyme but what is lactose? An example of a substrate. I told the students that the part of the enzyme that was active—the part that carried out the reaction—was called the "active site".

2. Enzyme-substrate complex

As they'd seen in the animation, I drew the substrate binding to the enzyme and named it. I then discussed the lock and key analogy. (I noted a crucial difference: locks don't tend to modify keys).

3. Enzyme and product

Finally, I drew the third step of the diagram. As with the first step, I discussed glucose and galactose as examples of "products". Enzymes don't always break down single substrates into different products, they can do the opposite. I gave the students an example: glycogen synthase (produces glycogen from glucose monomers).

A Stock and Flow Model

I wanted the students to think deeper about what happens during enzyme-catalysed reactions and relate this to their central importance to the life (the autopoiesis) of cells.

To do this, I drew the model below and asked students to draw their own. I asked them to name the stocks and flows using the information in the diagram of the enzymes we'd already drawn (above).

After hearing some of my students' answers, I filled in my model and asked them to add in the answers (below).

Key to this model is a fundamental distinction: the substrate changes (into the product) but the enzyme stays the same. I wanted the students to perceive this. So I asked them if the model made sense.

"Let's say that I drank a drop of milk that had 100 lactose molecules, according to this model, how many enzymes would I need to process them?" The students answered that you'd need 100.

"Imagine a full meal, how many enzymes would you need? The number would be huge. How much protein would we need to consume to be able to make so many enzymes? And what about all the other enzymes doing work in all the cells of the body".

Life would be impossible without catalysts. The model doesn't make sense.

As always with these models, I asked students to trace the ideas with their fingertips. Firstly, I asked them to trace the substrate through the reaction. "What happens to the substrate?" It's converted into products.

Then I asked them to look at the previous diagram; the enzyme isn't consumed in the reaction it stays the same. Therefore, to make the model coherent, we added another flow.

According to the variation theory of learning, it's vital to see how concepts can vary.

I then taught them the term "biological catalyst" and explained why it was central to life and autopoiesis. And, therefore, why enzymes are encoded in genes: so the memory—of how to construct these essential parts of the system—is retained through time. Learn the principles behind teaching this way in Difference Maker.