Skip Navigation


Introduces enzyme properties.

Atoms Practice
Estimated4 minsto complete
Practice Enzymes
This indicates how strong in your memory this concept is
Estimated4 minsto complete
Practice Now
Turn In

What is a biological catalyst?

This super fast train can obviously reach great speeds. And there's a lot of technology that helps this train go fast. Speaking of helping things go fast brings us to enzymes. Life could not exist without enzymes. Essentially, enzymes are biological catalysts that speed up biochemical reactions.


Enzymes and Biochemical Reactions

Most chemical reactions within organisms would be impossible under the conditions in cells. For example, the body temperature of most organisms is too low for reactions to occur quickly enough to carry out life processes. Reactants may also be present in such low concentrations that it is unlikely they will meet and collide. Therefore, the rate of most biochemical reactions must be increased by a catalyst. A catalyst is a chemical that speeds up chemical reactions. In organisms, catalysts are called enzymes. Essentially, enzymes are biological catalysts.

How do enzymes speed up biochemical reactions so dramatically? Like all catalysts, enzymes work by lowering the activation energy of chemical reactions. Activation energy is the energy needed to start a chemical reaction. This is illustrated in Figure below. The biochemical reaction shown in the figure requires about three times as much activation energy without the enzyme as it does with the enzyme.

The reaction represented by this graph is a combustion reaction involving the reactants glucose (C6H12O6) and oxygen (O2). The products of the reaction are carbon dioxide (CO2) and water (H2O). Energy is also released during the reaction. The enzyme speeds up the reaction by lowering the activation energy needed for the reaction to start. Compare the activation energy with and without the enzyme.

Enzymes are extremely efficient in speeding up reactions. They can catalyze up to several million reactions per second. As a result, the difference in rates of biochemical reactions with and without enzymes may be enormous. A typical biochemical reaction might take hours or even days to occur under normal cellular conditions without an enzyme, but less than a second with an enzyme. 

Cellular processes consist of many chemical reactions that must occur quickly in order for the cell to function properly. Enzymes catalyze most of the chemical reactions that occur in a cell. A substrate is the molecule or molecules on which the enzyme acts. In the urease catalyzed reaction, urea is the substrate. The Figure below diagrams a typical enzymatic reaction.

License: CC BY-NC 3.0


The sequence of steps for a substrate binding to an enzyme in its active site, reacting, then being released as products.

Enzymes, an overview of these proteins, can be viewed at http://www.youtube.com/watch?v=E90D4BmaVJM&feature=related (9:43).

As you view Enzymes, focus on these concepts:

  1. the role of enzymes in nature,
  2. other uses of enzymes.

Importance of Enzymes

Enzymes are involved in most of the chemical reactions that take place in organisms. About 4,000 such reactions are known to be catalyzed by enzymes, but the number may be even higher.

In animals, an important function of enzymes is to help digest food. Digestive enzymes speed up reactions that break down large molecules of carbohydrates, proteins, and fats into smaller molecules the body can use. 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.


  • Enzymes are biological catalysts. They speed up biochemical reactions.
  • Enzymes are involved in most of the chemical reactions that take place in organisms.


Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: Enzymes as Catalysts
  1. What are enzymes?
  2. What are substrates? What is the enzyme-substrate complex?
  3. How do enzymes work?
  4. What happens to the enzyme during a reaction?


1. What are enzymes?

2. Explain why organisms need enzymes to survive.

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

  1. [1]^ License: CC BY-NC 3.0

Explore More

Sign in to explore more, including practice questions and solutions for Enzymes.
Please wait...
Please wait...