Stages of Cellular Respiration Cellular respiration captures the energy from food in three main stages—glycolysis, the Krebs cycle, and the electron transport chain. Although cells can use just about any food molecule for energy, we will concentrate on just one as an example—the simple sugar glucose. Glucose first enters a chemical pathway known as glycolysis (gly KAHL ih sis). Only a small amount of energy is captured to produce ATP during this stage. In fact, at the end of glycolysis, about 90 percent of the chemical energy that was available in glucose is still unused, locked in chemical bonds of a molecule called pyruvic (py ROO vik) acid.

How does the cell extract the rest of that energy? First, pyruvic acid enters the second stage of cellular respiration, the Krebs cycle, where a little more energy is generated. The bulk of the energy, however, comes from the final stage of cellular respiration, the electron transport chain. This stage requires reactants from the other two stages of the process, as shown by dashed lines in Figure 9–2. How does the electron transport chain extract so much energy from these reactants? It uses one of the world's most powerful electron acceptors—oxygen.

FIGURE 9–2 The Stages of Cellular Respiration There are three stages to cellular respiration: glycolysis, the Krebs cycle, and the electron transport chain. Interpret Visuals Which stage(s) of cellular respiration occur in the mitochondrion?

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Oxygen and Energy Oxygen is required at the very end of the electron transport chain. Any time a cell's demand for energy increases, its use of oxygen increases, too. As you know, the word respiration is often used as a synonym for breathing. This is why we have used the term cellular respiration to refer to energy-releasing pathways within the cell. The double meaning of respiration points out a crucial connection between cells and organisms: Most of the energy-releasing pathways within cells require oxygen, and that is the reason we need to breathe, to respire.

Pathways of cellular respiration that require oxygen are said to be aerobic (“in air”). The Krebs cycle and electron transport chain are both aerobic processes. Even though the Krebs cycle does not directly require oxygen, it is classified as an aerobic process because it cannot run without the oxygen-requiring electron transport chain. Glycolysis, however, does not directly require oxygen, nor does it rely on an oxygen-requiring process to run. Glycolysis is therefore said to be anaerobic (“without air”). Even though glycolysis is anaerobic, it is considered part of cellular respiration because its final products are key reactants for the aerobic stages.

Recall that mitochondria are structures in the cell that convert chemical energy stored in food to usable energy for the cell. Glycolysis actually occurs in the cytoplasm of a cell, but the Krebs cycle and electron transport chain, which generate the majority of ATP during cellular respiration, take place inside the mitochondria. If oxygen is not present, another anaerobic pathway, known as fermentation, makes it possible for the cell to keep glycolysis running, generating ATP to power cellular activity. You will learn more about fermentation later in this chapter.

In Your Notebook Make a flowchart that shows the different steps of cellular respiration.

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