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2.25: Cellular Respiration

Difficulty Level: At Grade Created by: CK-12
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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.

An introduction to cellular respiration can be viewed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/19/2f7YwCtHcgk (14:19).

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 cytosol of the cell and does not require oxygen, whereas the Krebs cycle and electron transport occur in the mitochondria and do require oxygen.

Cellular 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.

The 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: the Krebs cycle and electron transport.
  • The inner and outer membranes of the mitochondrion play a important roles in aerobic respiration.

Practice I

Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology\begin{align*}\rightarrow\end{align*} Non-Majors Biology \begin{align*}\rightarrow\end{align*} Search: Cellular Respiration
  1. What is the goal of cellular respiration?
  2. What are the two stages of cellular respiration?
  3. Which organisms are able to perform glycolysis?
  4. What is the main product of glycolysis? What happens to this product?

Practice II


1. Describe cellular respiration.

2. Using the chemical equation of cellular respiration and the above figure as a guide, describe what happens to each of the atoms of carbon during this process.

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?




In the presence of oxygen.
citric acid cycle

citric acid cycle

Second stage of aerobic respiration in which two pyruvate (pyruvic acid) molecules from the first stage react to form ATP, NADH, and FADH2; also known as the Krebs cycle.
electron transport

electron transport

Third stage of aerobic respiration in which the majority of ATP is produced.


First stage of cellular respiration in which glucose is split to form two molecules of pyruvate (pyruvic acid) and two (net) molecules of ATP.
Krebs cycle

Krebs cycle

Second stage of aerobic respiration in which two pyruvate (pyruvic acid) molecules from the first stage react to form ATP, NADH, and FADH2; also known as the citric acid cycle.

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Difficulty Level:

At Grade


Date Created:

Feb 24, 2012

Last Modified:

Jan 30, 2016
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