Chapter 3, Section 4 (Cellular Respiration)

  • Learning Goal:
    The student will be able to…
    …describe the ingredients required to provide living cells with the energy they need.
    …describe how the process of cellular respiration works.

    First, write down the vocab words. Then, click below to practice learning more about cell respiration. 

    Cellular Respiration


    cellular respiration
    the process by which cells get energy from glucose

    the sugar that the mitochondria uses to power cell respiration, added with CO2

    the organelle in the cell that does all the work in cell respiration

    Products of Cell Respiration
    when cell respiration is complete, it produces water, oxygen (O2) and energy

    What Is Respiration?

    Food supplies your body with glucose, an energy-rich sugar. Respiration is the process by which cells obtain energy from glucose. During respiration, cells break down simple food molecules such as sugar and release the energy they contain.

    Storing and Releasing Energy

    Energy stored in cells is something like money in a savings account. During photosynthesis, plants capture energy from sunlight and “save” it in the form of carbohydrates, including sugars and starches. Similarly, when you eat, you add to your body’s energy savings account. When cells need energy, they “withdraw” it by breaking down the carbohydrates in the process of respiration.

    Breathing and Respiration

    The term respiration has two meanings. You have probably used it to mean “breathing,” that is, moving air in and out of your lungs. To avoid confusion, the respiration process that takes place inside cells is sometimes called cellular respiration. Breathing brings oxygen, which is usually necessary for cellular respiration, into your lungs.

    The Two Stages of Respiration

    Like photosynthesis, respiration is a two-stage process. The first stage takes place in the cytoplasm of the organism’s cells. There, molecules of glucose are broken down into smaller molecules. Oxygen is not involved, and only a small amount of energy is released.

    The second stage of respiration takes place in the mitochondria. There, the small molecules are broken down into even smaller molecules. These chemical reactions require oxygen, and they release a great deal of energy. This is why the mitochondria are sometimes called the “powerhouses” of the cell.

    Trace the steps in the breakdown of glucose in Figure 16. Note that energy is released in both stages. Two other products of respiration are carbon dioxide and water. These products diffuse out of the cell. In most animals, the carbon dioxide and some water leave the body during exhalation, or breathing out. Thus, when you breathe in, you take in oxygen—a raw material for respiration. When you breathe out, you release carbon dioxide and water—products of respiration.

    The Respiration Equation

    Although respiration occurs in a series of complex steps, the overall process can be summarized in the following equation:

    Notice that the raw materials for respiration are sugar and oxygen. Plants and other organisms that undergo photosynthesis make their own sugar. The glucose in the cells of animals and other organisms comes from the food they consume. The oxygen used in respiration comes from the air or water surrounding the organism.

    Comparing Photosynthesis and Respiration

    Can you notice anything familiar about the equation for respiration? You are quite right if you said it is the opposite of the equation for photosynthesis. This is an important point. During photosynthesis, carbon dioxide and water are used to produce sugars and oxygen. During respiration, the sugar glucose and oxygen are used to produce carbon dioxide and water. Photosynthesis and respiration can be thought of as opposite processes.

    Together, these two processes form a cycle that keeps the levels of oxygen and carbon dioxide fairly constant in Earth’s atmosphere. As you can see in Figure 17, living things use both gases over and over again.

    Click below to watch a an animation about cellular respiration.

    Cell Respiration