Worksheets and lesson ideas to challenge students aged 11 to 16 to think hard about enzymes (GCSE and Key Stage 3)
Overview: enzymes are protein molecules that act as catalysts, speeding up chemical reactions without themselves getting used up. Each enzyme will only speed up a specific reaction, for example, catalase will speed up the decomposition of hydrogen peroxide into water and oxygen but it will not speed up the breakdown of starch into glucose. Enzymes (e.g. catalase) have active sites with specific shapes that bind to the substrate molecule (e.g. hydrogen peroxide) forming an enzyme-substrate complex. The enzyme-substrate complex then breaks down into the enzyme and product, allowing the enzyme to go on and react with another substrate molecule. Temperature and pH affect enzyme function because they can change the shape of the enzyme’s active site, preventing it from binding to the substrate, just as a broken lock will no longer fit the key. When the shape of an enzyme changes we call this denaturation. Any factor that increases the frequency of collisions between enzymes and substrates (increasing concentration, surface area or temperature) will increase the rate of reaction.
Key concept: enzymes are protein molecules with specific shapes that speed up chemical reactions. Factors such as concentration, temperature and pH affect enzyme action.
Misconception [scientific idea]: enzymes die when they are heated [enzymes cannot be killed as they are not living, instead enzymes denature when they are heated above a certain temperature]; enzymes denature when it’s cold [frequency of collisions decreases between substrate and enzyme]
Where to start?
The decomposition of hydrogen peroxide is a great place to begin thinking about enzymes. The decomposition of hydrogen peroxide into water and oxygen will happen spontaneously, but the addition of a catalyst e.g. catalase will speed this reaction up. Don’t believe me?! Then have a go at the decomposition of hydrogen peroxide demonstration using MnO2 (a chemical catalyst) and a piece of liver (containing catalase, a biological catalyst). Heat the liver and it no longer works. This will then begin an exploration of denaturation.
Modelling enzyme action
One of the best ways to help students understand enzyme action is to build Plasticine models of enzymes breaking down (or building up) substrate molecules. Students can modify the models to show denaturation and the effects of temperature, inhibitors and pH. Make sure you stress the different effects of temperature – denaturation versus collision theory. If possible, ask students to film their models and add annotations to help them consider the dynamic nature of enzyme action.
Factors that affect enzyme action
GCSE activity for students to apply their knowledge of enzymes. Students work in pairs to apply their understanding of factors that affect enzymes. They will need to consider pH, temperature and enzyme specificity. This activity assesses and consolidates learning by asking students to apply their knowledge to novel situations. (PDF)
- What do enzymes look like? The Protein Data Bank provides some beautiful structures of enzymes.
- What does lemon juice, snake venom and cyanide have in common?
- When we cool an enzyme reaction the rate of reaction decreases. Do enzymes denature at low temperatures?
- How does decreasing the pH cause denaturation?
- Why do you die of heat stroke?