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Let’s learn about what enzymes are and how they work. Before you get started, don’t forget to print out your OnTRACK Biology Journal.

TEKS Standards and Student Expectations

B(9) The student knows the significance of various molecules involved in metabolic processes and energy conversions that occur in living organisms. The student is expected to:

B(9)(C) identify and investigate the role of enzymes

Learning Objectives

Describe the role enzymes play in biological systems.

Explain how enzymes work to speed up chemical reactions.

Explain how enzyme structure determines enzyme specificity.

Design and conduct experiments and interpret the data obtained to draw conclusions about how various factors affect enzyme activity.

Essential Questions

Why are enzymes important?

What are enzymes and what do they do?

How do enzymes work to speed up chemical reactions?

How does temperature affect the action of enzymes?

How does pH affect the action of enzymes?

How does substrate concentration affect the action of enzymes?


  • Chemical Reactions
  • Enzymes
  • Catalysts
  • Activation Energy
  • Substrates
  • Products
  • Active Site
  • Enzyme-Substrate Complex

Introduction to Enzymes

Inside your body, thousands of different chemical reactions are occurring every second. Even as you read these words, your body is breaking down nutrients, building new materials, and maintaining your body tissues. All of these critical life processes, and many more, are driven by chemical reactions. 

However, many chemical reactions occur too slowly to support life processes. For example, chemical reactions that are absolutely essential in creating the building blocks of DNA would take 78 million years in just water. In your body, this reaction happens in 10 milliseconds. How is this possible? This reaction happens so quickly in your body because of enzymes—protein catalysts that speed up the rate of chemical reactions.

Directions: Watch Learn Biology: Cells - Enzymes for an introduction to how enzymes work.

Cite Source
University Of North Carolina School Of Medicine. Without Enzyme Catalyst, Slowest Known Biological Reaction Takes 1 Trillion Years. ScienceDaily, 6 May 2003. www.sciencedaily.com/releases/2003/05/030506073321.htm
Mahalodotcom. Learn Biology: Cell-Enzymes. Retrieved from https://youtu.be/feik6wOXFA0

Enzymes are proteins that act as catalysts. Catalysts increase the rates of chemical reactions. Enzymes catalyze a reaction by lowering the activation energy, which is the amount of energy needed to begin a chemical reaction. All chemical reactions need activation energy to get started. Activation energy is needed to speed up the motion of molecules to increase the frequency and force with which they collide.

Lowering activation energy dramatically affects how quickly a chemical reaction is completed. Enzyme-catalyzed reactions occur extremely fast. Many happen millions of times faster than uncatalyzed reactions.

This graph demonstrates the effect enzymes can have on a chemical reaction.


The blue line on the graph shows the reaction taking place without the enzyme, and the red line shows the reaction with the enzyme. Notice that the activation energy needed for the reaction with the enzyme is much lower than the reaction without the enzyme. This means that the reaction can occur more rapidly.

The Enzyme-Substrate Complex

In reactions involving enzymes, the molecules at the beginning of the process are called substrates. The enzyme converts them into different molecules called the productsThe substrates bind to a site on the enzyme called the active site. The active site and the substrate have complementary shapes. The fit is so specific that the active site and substrates are often compared to a lock and key.


When the enzyme and the substrate are joined at the active site, this is called the enzyme-substrate complex.


After the enzyme has performed its work, the product or products drift away. The enzyme is then free to function in another chemical reaction. It is important to remember that enzymes are not used up during a single reaction.


Enzymes at Work

Cells use enzymes to catalyze a variety of reactions internally, including reactions involved in growth, reproduction, digestion, and nutrient absorption. Some cells also excrete enzymes outside of their cell walls to break down molecules in the environment. For example, E. coli bacteria excrete enzymes to break down food in their environment into molecules small enough to pass through the cell wall and into the cell. Some enzymes you may have heard of include the following:

  • Proteases (also called peptidases): A protease is any enzyme that can break down a long protein into smaller chains called peptides.
  • Amylases: Amylases break down starch chains into smaller sugar molecules. Your saliva contains amylase, and so does your small intestine. 
  • Cellulases: Cellulases break cellulose molecules down into simpler sugars. Bacteria in the guts of cows and termites excrete cellulases. Cellulases are the reason cows and termites are able to digest things like grass and wood.

What do you notice about all the enzymes in the list above? With few exceptions, the names of enzymes end in "-ase." The name depends on their role in the metabolic process and the substrate they interact with. For example, the enzyme that breaks down the sugar sucrose is called sucrase. Below is a chart of some more common enzymes and the substrates they act upon.

Enzyme Substrate
Maltase Breaks down maltose sugar
DNA Polymerase Builds DNA polymers
Protease Breaks down protein
Lipase Breaks down lipids


Directions: Watch Enzymes in Action: How Apples Turn Brown to see an example of how enzymes work.


Kid2Kid Videos

Directions: Watch Kid2Kid: Enzymes to learn more about the role of enzymes and how enzymes speed up chemical reactions in our bodies.

Vea Kid2Kid: Enzimas para aprender más sobre las enzimas y cómo las enzimas aceleran las reacciones químicas en nuestros cuerpos.