Hi everyone, welcome to IGCSE Study Buddy where you can revise biology topics from the Cambridge IGCSE syllabus. This video summarizes topic 5, Enzymes. So what is an enzyme? First we must understand that metabolic reactions are constantly taking place in organisms in order to maintain their living state. A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction itself.
So it's something that makes a reaction faster but it does not get changed itself. An enzyme is a biological catalyst, that is they are made in living cells. Enzymes are proteins that speed up the rate of all metabolic reactions.
Enzymes, like catalysts, can be used over and over again because they are not used up during the reaction. Enzymes are crucial to living organisms as they speed up the metabolic reactions which would take too long to occur without them. It maintains the reaction rates of all metabolic reactions necessary to sustain life.
For example, without digestive enzymes, animals would not be able to break down food molecules quickly enough to provide the energy and nutrients they need to survive. A substrate is the substance on which the enzyme acts. Products are the molecules produced after the reaction.
The enzyme has a shape called the active site, which exactly fits the substances on which it acts. According to this example, the active site is shown by the red line. An enzyme-substrate complex is formed temporarily when the enzyme combines with the substrate. Here is an illustration. of an enzyme-substrate complex.
Enzymes are specific to their substrate molecules. The shape of the active site of the enzyme molecule and the substrate molecule are complementary. So, as demonstrated in the diagram, the yellow substrate has a shape that pairs perfectly with the shape of the enzyme 1's active site. The enzymes and substrate molecules have complementary shapes like two pieces of a jigsaw puzzle so they fit together. This is also known as the lock and key mechanism.
However, the yellow substrate will not be able to perfectly fit into the active site of enzyme 2 because the shapes of the substrate and the active site are not matching. So enzyme 2 will not work in this reaction. Therefore, enzymes are specific.
This means simply that an enzyme which normally acts on one substance will not act on a different one. This diagram illustrates how enzymes work. As we know, the shape of the substrate must be complementary or matching to that of the active site of the enzyme.
As the enzyme binds with the substrate, an enzyme-substrate complex is formed. The reaction then occurs on the enzyme and the enzyme-product complex is formed. The products eventually leave the enzyme. The enzyme itself is unchanged at the end of the reaction.
Now let's look at the factors that affect enzyme activity. They are temperature and pH. The rate of an enzyme-catalyzed reaction increases as the temperature increases. As the temperature is increased, the molecules gain more kinetic energy, so they move faster and there is a greater chance of successful molecular collisions happening.
Therefore, The rate of reaction increases. All enzymes have an optimum temperature. These are certain temperatures in which a particular enzyme works best in, and it can vary between different enzymes.
So different enzymes may work best in different temperatures. For any enzyme, if the temperature gets too high, it will get denatured. This means that the bonds in the enzymes will break causing the shape of the enzyme's active site to change. Remember, the active site has a very special shape and it fits only one specific type and shape of substrate. A change in the active site will therefore cause the enzyme to stop working.
This picture is an example of what will happen to the active site of an enzyme if the temperature is too high. As you may notice, the shape of the active site has changed and the substrate's shape does not match with it anymore and therefore the enzyme will not work in this reaction. This graph explains how temperature affects enzyme activity. So, as you may see, initially the rate of the reaction is low when the temperatures are low. Molecules need to collide with one another and have enough energy for a reaction to occur.
However, in low temperatures, molecules are travelling at lower speeds or less kinetic energy and therefore the rate of successful collisions are lower. Enzyme activity activity is therefore low in low temperatures. It is important to note however that low temperatures do not denature enzymes. Then you may notice the rate of the reaction increases as the temperature increases.
And in this graph, for this enzyme, the rate of reaction reaches its highest at an optimum temperature of 40 degrees Celsius. However, Beyond this temperature, you may notice that the rate of reaction decreases again because the enzyme becomes denatured and can no longer function as a biological catalyst. Most enzymes have an optimum temperature of approximately 37 degrees Celsius in the human body and start getting denatured at above 50 degrees Celsius. pH is the other factor.
that affects enzyme activity. Enzymes also have an optimum pH. This is the pH in which an enzyme works best and different enzymes may have a different optimum pH. If the pH deviates too much from their optimum, it will cause the bonds to break in the enzymes, thereby changing the shape of their active site, resulting in the enzyme getting denatured. The substrate will not be able to fit perfectly in the active site. This means the enzyme cannot function anymore.
This graph shows how pH affects the rate of a reaction with enzymes. The rate of reaction in this graph is the highest when the pH is 8. That means that this enzyme's optimum pH is 8. When the pH strays too far from the optimum on either side of the graph, the rate of reaction decreases because the enzyme gets denatured. Examples of two enzymes with a different optimum pH are pepsin, and amylase. Pepsin is an enzyme found in the stomach's acidic conditions and therefore made to work best in a pH of approximately 2. Amylase, on the other hand, is found in saliva and therefore has an optimum pH of 7. So although pepsin will work best at a pH of 2, it will not function at a pH of 7, unlike amylase.
Similarly, amylase will speed up the reaction rate at a pH of 7 but will not work at a pH of 2, unlike pepsin. So that sums up the main things to be learnt in chapter 5, enzymes. Hope you found it useful.
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