Skip to main content
K12 LibreTexts

2.25: Cellular Respiration

  • Page ID
  • f-d_c1f9bb98befa6f18d46286635aed8db43ad66f166a61788dac6557b8+IMAGE_TINY+IMAGE_TINY.jpg

    Why eat?

    Because we're hungry? Not necessarily. But biologically speaking…we eat to get energy. The food we eat is broken down, the glucose extracted, and that energy is converted into ATP.

    Cellular Respiration

    What happens to the energy stored in glucose during photosynthesis? How do living things make use of this stored energy? The answer is cellular respiration. This process releases the energy in glucose to make ATP (adenosine triphosphate), the molecule that powers all the work of cells.

    Stages of Cellular Respiration

    Cellular respiration involves many chemical reactions. The reactions can be summed up in this equation:

    C6H12O6 + 6O2 → 6CO2 + 6H2O + Chemical Energy (in ATP)

    The reactions of cellular respiration can be grouped into three stages: glycolysis (stage 1), the Krebs cycle, also called the citric acid cycle (stage 2), and electron transport (stage 3). Figure below gives an overview of these three stages, which are further discussed in the concepts that follow. Glycolysis occurs in the cytoplasm of the cell and does not require oxygen, whereas the Krebs cycle and electron transport occur in the mitochondria and do require oxygen.

    f-d_6a355510eb54114aa3c91f9f599c38b8855413a609bd9235963080b4+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.pngCellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?

    Structure of the Mitochondrion: Key to Aerobic Respiration

    The structure of the mitochondrion is key to the process of aerobic (in the presence of oxygen) cellular respiration, especially the Krebs cycle and electron transport. A diagram of a mitochondrion is shown in Figure below.

    f-d_773d01e05924ec0450657052ef0b8b5872a188bfa0f1f72fe91cc750+IMAGE_THUMB_POSTCARD_TINY+IMAGE_THUMB_POSTCARD_TINY.pngThe structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.

    As you can see from Figure above, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane.


    • Cellular respiration takes the energy stored in glucose and transfers it to ATP.
    • Cellular respiration has three stages: glycolysis, Krebs cycle, and electron transport.
    • The inner and outer membranes of the mitochondrion play an important roles in aerobic respiration.


    1. Define cellular respiration.
    2. What are the three stages of cellular respiration?
    3. Describe the structure of the mitochondrion and discuss the importance of this structure in cellular respiration.
    4. Assume that a new species of organism has been discovered. Scientists have observed its cells under a microscope and determined that they lack mitochondria. What type of cellular respiration would you predict that the new species uses? Explain your prediction.
    5. When you exhale onto a cold window pane, water vapor in your breath condenses on the glass. Where does the water vapor come from?
    Image Reference Attributions
    f-d_c1f9bb98befa6f18d46286635aed8db43ad66f166a61788dac6557b8+IMAGE_TINY+IMAGE_TINY.jpg [Figure 1] Credit: Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons)
    License: Public Domain
    f-d_6a355510eb54114aa3c91f9f599c38b8855413a609bd9235963080b4+IMAGE_THUMB_SMALL_TINY+IMAGE_THUMB_SMALL_TINY.png [Figure 2] Credit: Hana Zavadska
    Source: CK-12 Foundation
    License: CC BY-NC 3.0
    f-d_773d01e05924ec0450657052ef0b8b5872a188bfa0f1f72fe91cc750+IMAGE_THUMB_SMALL_TINY+IMAGE_THUMB_SMALL_TINY.png [Figure 3] Credit: Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons)
    License: Public Domain
    • Was this article helpful?