Being able to visualise a biological process can greatly improve understanding. I use sponges as a visual demonstration of enzymes and substrates for my A-level students. It helps them to become confident with biological vocabulary and to discuss misconceptions.

Enzymes are biological molecules that speed up chemical reactions. The substrate is the molecule at the start of the process. The enzyme reacts with it to form a different molecule (the product). Only a small part of the enzyme is involved in the process, and this is called the active site.

The enzyme is unchanged by the process and can be used repeatedly, so after the product is formed and leaves the active site, the enzyme can bind with other substrates.

Various factors can affect the activity of an enzyme, including the temperature, pH value, concentration and inhibitors, which are molecules that slow down or stop an enzyme reaction.

What happens in the lesson?

Students cut a sponge to represent an enzyme and substrate. Using the sponges as a visual aid, they then see how enzyme-substrate complexes form by fitting the complementary shapes together. They can also see how each enzyme will only fit its substrate molecule.

To demonstrate how high temperatures and changes in pH alter enzyme reactions, students can change the shape of the active site by squeezing the sponge, showing that the active site changes shape.

The enzyme can bind with many substrates, so to simulate this I have a stockpile of pre-prepared sponges (enzyme and substrates) that the students use. They move the enzymes around, counting the number of times they can combine with different substrates in a given period of time, say, 30 seconds.

Then we cut sponges into the shape of competitive and non-competitive inhibitors to observe the difference. Competitive inhibitors block the active site so that the substrate can’t bind to it at that moment, thus slowing down the reaction. It is a similar shape to the substrate but not exactly the same shape, which is something students can get wrong.

The non-competitive inhibitor, on the other hand, is a different shape from the substrate and doesn’t block the active site - instead it goes to another part of the enzyme, and halts or slows the reaction by changing the shape of the whole structure. Students now cut out a representation of a non-competitive inhibitor using a sponge, demonstrating that when it combines with the enzyme, the shape of the active site is altered so that the substrate can no longer bind.

Rebecca Smith teaches at Altrincham Grammar School for Girls in Cheshire.