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18. Enzymes - Enzymes Lower Activation Energy

In this A-Level Biology Lesson "Enzymes Lower Activation Energy" we'll continue our learning of these important proteins as we begin to understand that these biological catalysts play important biological roles as intracellular and extracellular enzymes. It's good to note here that learning all about enzymes as a 'synoptic topic' is really useful since there are so many topics in your A-level biology that can be 'connected' though "enzymes" by means of a well written synoptic essay (a skill you want to start practicing as soon as possible). So, chemical reactions need energy and here we'll see how enzymes lower activation energy, note that activation energy is usually provided in the form of Heat. We go on to explain the graph showing that the peak - is the transition state. You'll need to be able to recognise and describe graphs a lot in your A-level biology, so by understanding these now will stand you in good stead for the rest of your studies! You'll need to able to compare graphs to show the amount of activation energy needed without an enzyme and graphs to show the amount of activation energy needed WITH an enzyme.

When you've watched this lesson and you're confident with how enzymes lower activation energy and can explain the graphs shown you’ll be ready to download the knowledge check PDF and test your knowledge regarding the basic properties of enzymes. When you’ve answered all the questions, compare your answers to the ones I’ve written in the back of the work booklet - and of course here you can see exactly how to write answers in a way that gains maximum marks in the exams.

3. Energy Changes (Enzymes lower activation energy).
The way enzymes work can also be shown by considering the energy changes that take place during a chemical reaction. Consider a reaction where the product has a lower energy than the substrate, so the substrate naturally turns into product.

Before it can change in the product, the substrate must overcome a "energy barrier" known as Activation Energy (EA).

The larger the Activation Energy (EA), the slower the reaction.

Why? Well, because only a few substrate molecules will by chance have sufficient energy to overcome the "energy barrier". Imagine pushing boulders over up hill before they can roll down the other side of the hill without any effort on your part... and you'll get the idea.

In reality most physiological reactions have large "energy barriers", that is they have large EA.

Enzymes dramatically lower the amount of activation energy required so that a reaction can take place - Enzymes lower activation energy so that substrate molecules can easily get over the activation energy barrier and quickly turn into product.

e.g. For the catalase reaction (2H2O2 -> 2H2O + O2) the EA is 86 kJ mol-1 with No catalyst.

62 kJ mol-1 with an inorganic catalyst (e.g. iron filings), and just

1 kJ mol-1 with the enzyme catalase.

The activation energy (EA) is actually the energy required to form the transition state, so enzymes lower the EA by stabilising the transition state, enzymes do this by changing the conditions within the active site of the enzyme.

Download the workbook with knowledge check and exam style questions with properly written answers.

Join the course to watch the video lessons in full and download the accompanying work booklets.

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