1. Define enzymes as proteins that function as biological catalysts

This learning objective is pretty easy 😊

Just memorise the definition:

Enzymes are proteins that function as biological catalysts


  1. Explain enzyme action with reference to the complementary shape of the active site of an enzyme and its substrate and the formation of a product

In a reaction, you generally have two types of chemicals: the reactants and the products.

The reactants react together to form the products.

In an enzymatic reaction (i.e. a reaction catalysed by an enzyme), the reactants are known as ‘substrates’.

Enzymes work on substrates to form products.

Enzymes have an ‘active site’ – this is the part of the enzyme that binds to the substrate. Every enzyme’s active site is ‘specific’. This means that one particular active site can only bind to one type of substrate.

There are a lot of theories that explain how enzymes work. One of the most important ones is the lock and key mechanism. This is the mechanism you need to learn for your syllabus:


The shape of the active site is ‘complementary’ to its substrates – this means that the substrate(s) fits into the enzyme in the same way a key fits into a lock. This complementary nature is what makes the enzyme specific to a substrate.

So, in a reaction, the substrate will be randomly moving around. As a result of this random motion, the substrate will collide with and bind to an enzyme that it is specific to. This results in the formation of an enzyme-substrate complex. The enzyme then catalyses the reaction – either breaking up a substrate (a catabolic reaction) or joining two substrates together (an anabolic reaction).

It then releases the products, to make space for more substrates, so that the enzyme can catalyse more reactions.


  1. Investigate and describe the effect of changes in temperature and pH on enzyme activity

The temperature an enzyme works best at is its ‘optimum temperature’.

The pH an enzyme works best at is its ‘optimum pH’.

Most enzymes in our body have an optimum pH of 7, and an optimum temperature of 37oC, because those are the conditions in most parts of our body, and our enzymes are well adapted to function inside us.

One exception is pepsin. This enzyme is present in our stomach, and functions best in our stomach’s acidic (HCl) conditions – pH2.

The general rule goes: the lower the temperature (when lower than optimum temperature), the slower the enzyme works; the higher the temperature (when higher than optimum temperature), the less the enzyme works.

The lower the pH (when lower than optimum), the less the enzyme functions; the higher the pH (when higher than optimum), the less the enzyme works.



  1. Explain the effect of changes in temperature on enzyme activity, in terms of kinetic energy, shape and fit, frequency of effective collisions and denaturation

The kinetic energy of an object is that energy that it possesses due to its motion. The faster it moves, the higher its kinetic energy.

Now, I’ve already explained that enzymes bind to their substrates due to the random motion of the substrates. What I haven’t mentioned, though, is that everything in a reaction has kinetic energy – the enzymes, the substrates, the products. So you have a lot of moving things. Because there are so many things moving around in random directions, there will be a lot of random collisions.

Generally, objects with kinetic energy move in a straight line, until they collide with something else. Then, their kinetic energy changes so that they either change direction, speed, or stop moving altogether.

This random motion of particles in a fluid resulting from their collision with other moving particles in the fluid, is called ‘Brownian motion’.

Due to Brownian motion, enzymes and substrates will collide a lot. Sometimes, even when an enzyme and substrate collide, it won’t bind (maybe because the substrate didn’t collide with the active site of the enzyme, or the part of the substrate that is supposed to bind to the active site didn’t collide to the active site, maybe they just bounced off of each other instead of binding, etc.) A collision that does not result in the completion of the reaction is unsuccessful.

To create an enzyme-substrate complex, we need an effective collision.

Now for the main part of the explanation (refer to the enzyme activity-temperature graph above as you read this explanation):

As you increase the temperature of a reaction, you are supplying the reaction with more thermal energy.

As the particles absorb more thermal energy, they move faster, because this thermal energy becomes converted into kinetic energy.

As the particles move faster, there are more collisions in a given amount of time.

As there are more collisions, there are a higher number of effective collisions in a given amount of time (there is a higher frequency of effective collisions).

As there is a higher frequency of effective collisions, the enzymes catalyse more reactions in a given period of time (the rate of reaction increases).

As the temperature rises, the enzymes continue to work faster in this way, up until the enzymes are working as fast as they possibly can. Once enzymes achieve their maximum activity, they are working at their optimum temperature.

If you increase the temperature even more than this, the atoms in the enzymes start to gain too much kinetic energy to maintain the shape of the enzyme. They vibrate so vigorously that they break the bonds holding them together, causing the active site of the enzyme to lose its shape – the enzyme becomes denatured. So, what you need to put on your exam paper is, when you increase the temperature to a value greater than the optimum temperature of the enzyme, the enzyme becomes denatured. This causes the active site to lose its shape, so it can no longer bind to its substrate. If it can’t bind to its substrate, the enzyme can’t function anymore.

As the temperature increases beyond the optimum temperature, more enzymes become denatured.

As more enzymes become denatured, less effective collisions can take place.

As less effective collisions take place, enzyme activity falls back down to 0.


  1. Explain the effect of changes in pH on enzyme activity in terms of shape and fit and denaturation.

Refer to the enzyme activity-pH graph above.

The lower the pH, the more acidic an environment is.

The higher the pH, the more alkaline an environment is.

Enzymes function best at their optimum pH.

The further away the pH of the environment is from the enzyme’s optimum pH, the more enzymes slow down and denature. This causes the active site to lose its shape, so the substrate(s) can no longer fit into the active site.

This causes enzyme activity to fall.




Notes submitted by Sarah.

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