Video Transcript
Why will most enzyme-controlled reactions occur slowly at a low temperature? A lower temperature means the molecules have less elastic energy, so they do not bind together as frequently. A lower temperature means the molecules have more kinetic energy, so they collide frequently. This is incorrect; most enzyme-controlled reactions will occur quickly at low temperatures. Or a lower temperature means the molecules have less kinetic energy, so they do not collide as frequently.
The question is asking us, why reactions that involve enzymes are particularly slow at low temperatures? Enzymes are biological catalysts, which speed up the rate of reaction without being used up themselves.
Let’s start by reviewing how enzymes can catalyze or speed up the rate of chemical reactions. In enzyme-controlled reactions, a substrate or substrates are converted into a product or products. The blue structure represents an enzyme. The substrate, the pink structure above it, is going to collide with it and bind with it in a region on its surface called the enzyme’s active site. An enzyme’s active site has a complementary shape to a specific substrate molecule. So only this molecule can bind to it. When a substrate binds to an enzyme’s active site, it forms an enzyme substrate complex. The products will then be released from the enzyme’s active site. And the enzyme is ready to catalyze another reaction.
At low temperatures, the enzyme and the substrate molecules are moving, but they’re not moving very fast, indicated by the small arrows from each molecule. This is because at low temperatures, the molecules have low kinetic energy. The low speeds of the molecules means that the rate of substrates colliding with the enzyme’s active sites is also quite low. You can see in this example only one of the substrates has collided with an enzyme’s active site to form an enzyme substrate complex. This means that over this one second period, only one set of products has formed.
As temperature increases, substrate and enzyme molecules have more kinetic energy. So they move a lot faster, indicated by the longer arrows from each molecule. This means that the substrate successfully collide with the enzyme’s active site more frequently. This results in more enzyme substrate complexes forming and more products being released over this one second period compared to in lower temperatures.
As more products are being formed, the rate of enzyme-controlled reaction increases. Let’s see what this would look like on a graph. You can see from the graph that when the temperature is low, the rate of reaction is also low. As the temperature increases to about 40 degrees Celsius, however, the rate of reaction also increases. This is because the enzyme and substrate molecules have more kinetic energy.
Let’s look through our options and eliminate those which are incorrect. The form of energy which controls the speed of the substrate and enzyme molecules and therefore the rate of the enzyme-controlled reaction is kinetic energy. Therefore, the option referring to elastic energy is incorrect. As we’ve indicated on the graph here, at higher temperatures, the enzyme and substrate molecules have more kinetic energy. Therefore, the option stating that lower temperatures result in more kinetic energy is incorrect.
We can see from the graph that when the temperature is low, the rate of enzyme-controlled reactions is also low. This means that the option stating that reactions occur quickly at low temperatures is also incorrect as we know that at low temperatures, the reactions occur slowly. We can deduce, therefore, that the reason why most enzyme-controlled reactions occur slowly at a low temperature is that a lower temperature means the molecules have less kinetic energy. So they do not collide as frequently.
