Video Transcript
The graph provided shows the rate of an enzyme-controlled reaction compared to the concentration of substrate. True or false: After a while, the rate of reaction stops increasing because all the enzymes have been broken down.
To answer this question, let’s recall what happens during an enzyme-controlled reaction. Enzymes are biological catalysts. They increase the rate of a reaction without being used up themselves. The substrate molecule binds to an area on the surface of the enzyme, called its active site, forming an enzyme–substrate complex. The reaction can then take place, following which the product, or products, are released. The enzyme remains unchanged so it can then bind with another substrate molecule and catalyze further reactions.
The rate of an enzyme-controlled reaction can be affected by a range of different factors. Our question asks us about the impact of substrate concentration on the rate of reaction. As shown in the graph provided, initially, as substrate concentration increases, the rate of reaction increases. This is because when the substrate concentration is low, many of the enzyme’s active sites remain empty, as there are not many substrate molecules available to bind to them. So, the rate of reaction is low. However, as the substrate concentration increases, more of the active sites have substrate bound at any moment in time. So, more enzymes catalyze the conversion of their substrate into the products.
As soon as the product is released from the active site, more substrate molecules are available to bind to it. So, the rate of reaction increases as substrate concentration increases. As you can see in the graph, this is only true up to a certain point. The rate of reaction increases up to its maximum, also called 𝑉 max. At this point, all of the enzyme’s active sites are being used at any moment in time. We can say the enzyme is saturated with substrate. So, increasing the substrate concentration further will not increase the rate of reaction, as all the enzyme’s active sites are already occupied; the enzyme is working at its maximum capacity.
This explains why the graph plateaus. The substrate concentration continues to increase, but the rate of reaction cannot increase further because there are no more enzyme active sites available for the substrate to bind to.
The question asks us to decide whether it is true or false that the rate of reaction stops increasing because all the enzymes have been broken down. We have just seen how the rate of reaction stops increasing because there are no available enzymes to bind to more substrate molecules. Also, recall that enzymes catalyze reactions but themselves remain unchanged, so are not broken down.
Therefore, the statement “After a while, the rate of reaction stops increasing because all the enzymes have been broken down.” is false.
