Video Transcript
The graph provided shows the rate of an enzyme-controlled reaction compared to
temperature. Which of the following best describes what has happened at point 2? (A) The elastic energy of the enzyme and substrate molecules has increased with an
increase in temperature. (B) The concentration of the enzyme and substrates has reached its peak, and the rate
of reaction has reached a plateau. (C) The enzyme’s optimum temperature has been reached, where the rate of reaction is
at its fastest. Or (D) the enzyme has been denatured, and the reaction has stopped.
Let’s get rid of the options for now so we can review the key facts about how enzymes
work and how their activity is affected by changes in temperature. We should then be able to come up with the correct answer to this question.
Enzymes are biological catalysts that increase the rate of a reaction by lowering the
activation energy required for a reaction to occur. They may act on just one substrate when breaking a molecule apart or more than one
substrate when joining them together. We will just refer to one substrate for ease. Enzymes are made of polypeptides, and they are not used up in the reaction
itself. They form enzyme–substrate complexes with the particular molecule that they work on
and release the products once the reaction is complete. Enzymes have an active site that is complementary to the shape of a substrate
molecule so that only one particular molecule can fit into it.
For an enzyme to work at its best, a number of conditions must be met, which depends
on the enzyme in question. For example, an enzyme will work optimally at a specific temperature or pH. If we look at the graph, we can see how the rate at which an enzyme works can be
influenced by temperature. The graph shows that as the temperature increases, the rate of the reaction also
increases until a certain point where the reaction rate starts to decline. At low temperatures, the reaction rate is very low because the molecules have very
little kinetic energy, and so they move around very slowly. This means the enzyme and substrate are less likely to bump into each other to form
an enzyme–substrate complex.
As the temperature increases, the kinetic energy contained within the substrate and
enzyme molecules also increases. As a result, there are more successful collisions. This increases the rate of the reaction; more products are formed per unit of
time. The temperature at which the enzyme works at its fastest or at which the rate of
reaction is greatest is known as the optimum temperature.
Beyond the optimum temperature, the enzymes do not work as efficiently. This is because at higher temperatures, bonds holding the enzyme structure together
break, which causes the shape of the active site to change. When this happens, the substrate can no longer bind to the enzyme and the rate of
reaction decreases. When there is a permanent change in the structure of the enzyme like this, the enzyme
is said to be denatured.
Let’s now return to the question. We can see that at point 1, the rate is increasing as the molecules gain kinetic
energy. At point 2, the rate of reaction has reached its peak. And at point 3, the rate of reaction is decreasing due to the change in structure of
the enzyme. The correct answer to this question is therefore (C). At point 2, the enzyme’s optimum temperature has been reached, where the rate of
reaction is at its fastest.
