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Biology LibreTexts

3.10: Chemical Reactions in Living Things

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  • Assembly Line

    We stay alive because millions of different chemical reactions are taking place inside our bodies all the time. Each of our cells is like the busy auto assembly line pictured here. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The "workers" on the cellular assembly line are mainly enzymes. These are the proteins that make biochemical reactions happen.

    Car assembly line
    Figure \(\PageIndex{1}\): (CC BY 2.0; Brian Snelson via Wikimedia Commons).

    What Are Biochemical Reactions?

    Chemical reactions that take place inside living things are called biochemical reactions. The sum of all the biochemical reactions in an organism is referred to as metabolism. Metabolism includes both exothermic (heat-releasing) chemical reactions and endothermic (heat-absorbing) chemical reactions.

    Catabolic Reactions

    Exergonic reactions in organisms are called catabolic reactions. These reactions break down molecules into smaller units and release energy. An example of a catabolic reaction is the breakdown of glucose during cellular respiration, which releases energy that cells need to carry out life processes.

    Anabolic Reactions

    Endergonic reactions in organisms are called anabolic reactions. These reactions absorb energy and build bigger molecules from smaller ones. An example of an anabolic reaction is the joining of amino acids to form a protein. Which type of reactions — catabolic or anabolic — do you think occur when your body digests food?


    Most biochemical reactions in organisms need help in order to take place. Why is this the case? For one thing, temperatures are usually too low inside living things for biochemical reactions to occur quickly enough to maintain life. The concentrations of reactants may also be too low for them to come together and react. Where do the biochemical reactions get the help they need to proceed? The help comes from enzymes.

    An enzyme is a protein that speeds up a biochemical reaction. It is a biological catalyst. An enzyme generally works by reducing the amount of activation energy needed to start the reaction. Figure \(\PageIndex{2}\) shows the activation energy needed for glucose to combine with oxygen. Less activation energy is needed when the correct enzyme is present than when it is not present.

    Activation graph with enzyme
    Figure \(\PageIndex{2}\): Enzyme Action. This graph shows what happens when glucose combines with oxygen. An enzyme speeds up the reaction by lowering the activation energy. Compare the activation energy needed with and without the enzyme. (CC BY-SA 3.0; Matthias M. via

    How Well Enzymes Work

    Enzymes are involved in most biochemical reactions, and they do their jobs extremely well. A typical biochemical reaction that would take several days or even several centuries to occur without an enzyme is likely to occur in just a split second with the proper enzyme! Without enzymes to speed up biochemical reactions, most organisms could not survive. Enzymes are substrate-specific. The substrate of an enzyme is the specific substance it affects (Figure \(\PageIndex{3}\)). Each enzyme works only with a particular substrate, which explains why there are so many different enzymes. In addition, for an enzyme to work, it requires specific conditions, such as just the right temperature and pH. Some enzymes work best under acidic conditions, for example, while others work best in neutral environments.

    Enzyme-Deficiency Disorders

    There are hundreds of known inherited metabolic disorders in humans. In most of them, a single enzyme is either not produced by the body at all or is produced in a form that doesn't work. The missing or defective enzyme is like an absentee worker on the cell's assembly line. The absence of the normal enzyme means that toxic chemicals build-up or an essential product isn't made. Generally, the normal enzyme is missing because the individual with the disorder inherited two copies of a gene mutation, which may have occurred originally many generations in the past.

    Any given inherited metabolic disorder is generally quite rare in the general population. However, there are so many different metabolic disorders that a total of 1 in 1,000 to 2,500 newborns can be expected to have one. In certain ethnic populations, such as Ashkenazi Jews (Jews of central and eastern European ancestry), the rate of certain inherited metabolic disorders is much higher.

    enzyme catalyzing reaction
    Figure \(\PageIndex{3}\): The figure shows how enzymes convert substrates into products. the substrate (A) binds at the activation site of the enzyme(D) and makes a substrate-enzyme complex (B). Then, the enzyme converts the substrate into the product (C) by breaking or making bonds between the atoms of the substrate. (CC BY-NC SA 2.0; SweetChickaD via flickr)

    Feature: Reliable Sources

    The most common of all known enzyme-deficiency disorders is glucose-6-phosphate-dehydrogenase, or G6PD, deficiency. In the U.S., the disorder occurs most often in African-American males. The enzyme G6PD is needed to prevent the abnormal breakdown of red blood cells. Without the enzyme, red blood cells break down prematurely and anemia results.

    Choose one of the following topics about G6PD deficiency:

    • genetic basis
    • signs and symptoms
    • diagnosis and treatment
    • worldwide distribution

    For the topic, you chose, go online to learn more about it. Find at least three sources of additional information that you think are reliable. Compare the information provided by the different sources, and identify any discrepancies among them. Do additional online research as needed to try to find a reliable consensus view of the discrepant issue.


    • Biochemical reactions are chemical reactions that take place inside living things. The sum of all the biochemical reactions in an organism is referred to as the metabolism.
    • Metabolism includes catabolic reactions, which are energy-releasing, or exergonic, reactions; and anabolic reactions, which are energy-absorbing, or endergonic, reactions.
    • Most biochemical reactions need a biological catalyst called an enzyme to speed up the reaction by reducing the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, called the enzyme's substrate.
    • There are many inherited metabolic disorders in humans. Most of them are caused by a single defective or missing enzyme.


    1. What are biochemical reactions?
    2. Define metabolism.
    3. Compare and contrast catabolic and anabolic reactions.
    4. Explain the role of enzymes in biochemical reactions.
    5. What are enzyme-deficiency disorders?
    6. True or False. Metabolism is one specific type of catabolism.
    7. True or False. Biochemical reactions include catabolic and anabolic reactions.
    8. Explain why the relatively low temperature of living things, as well as the low concentration of reactants, would cause biochemical reactions to occur very slowly in the body without enzymes.
    9. Answer the following questions about what happens after you eat a sandwich.

      a. Pieces of the sandwich go into your stomach, where there are digestive enzymes that break down the food. Which type of metabolic reaction is this? Explain your answer.

      b. Through the process of digestion, some of the sandwich is broken down to glucose, which is then further broken down to release energy that your cells can use. Is this an exergonic or endergonic reaction? Explain your answer.

      c. The proteins in the cheese, meat, and bread in the sandwich are broken down into their component amino acids. Then your body uses those amino acids to build new proteins. Which kind of metabolic reaction is represented by the building of these new proteins? Explain your answer.

    10. Explain why your body doesn’t just use one or two enzymes for all of its biochemical reactions.

    11. A ________ is the specific substance that an enzyme affects in a biochemical reaction.

    12. An enzyme is a biological _____________ .

      A. catabolism

      B. form of activation energy

      C. catalyst

      D. reactant

    Explore More

    Watch the video below to learn more about catabolic and anabolic reactions.

    The video below discusses the function of enzymes.