36 4.10 Cellular Respiration

Created by: CK-12/Adapted by Christine Miller

Image shows a photo of the ingredients for smores sitting on a table. In the background, a campfire is burning.
Figure 4.10.1 Ready to make s’mores!

Bring on the S’mores!

This inviting camp fire can be used for both heat and light. Heat and light are two forms of  that are released when a fuel like wood is burned. The of living things also get energy by “burning.” They “burn” in a process called.

What Is Cellular Respiration?

 is the process by which living cells break down molecules and release . The process is similar to burning, although it doesn’t produce light or intense heat as a campfire does. This is because cellular respiration releases the energy in glucose slowly and in many small steps. It uses the energy released to form molecules of , the energy-carrying molecules that cells use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.  Cellular respiration involves many chemical reactions, but they can all be summed up with this chemical equation:

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

In words, the equation shows that glucose (C6H12O6) and oxygen (O2) react to form carbon dioxide (CO2) and water (H2O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an  process.

Cellular respiration occurs in the of all living things, both and . All of them burn to form . The reactions of can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.

Image shows a diagram of the four stages in cellular respiration: Glycolysis, transition reaction, Kreb's cycle, and the electron transport system.
Figure 4.10.2 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?

 

Cellular Respiration Stage I: Glycolysis

The first stage of cellular respiration is , which happens in the of the .

Splitting Glucose

The word glycolysis literally means “glucose splitting,” which is exactly what happens in this stage.  split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the following diagram.

Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate. This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?

Results of Glycolysis

Energy is needed at the start of to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD  to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.

Transition Reaction

Image shows a diagram of the transition reaction. In this reaction, 2 Pyruvate are converted to two acteyl CoA and 2 Carbon dioxide. In this process, 2 NADH are sent to the ETS carrying high energy electrons. The carbon dioxide leave the cell as metabolic waste and the acetyl CoA enter the Krebs Cycle.
Figure 4.10.4 Transition reaction of 2 pyruvate.

Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.  The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.  The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).

Structure of the Mitochondrion

Image shows a diagram of a mitochondria. Several structures are labelled including cristae, matrix, DNA, intermembrane space, inner membrane, outer membrane, and ATP synthase particles.
Figure 4.10.5 Labelled mitochondrion structure.

Before you read about the last two stages of cellular respiration, you need to know more about the , where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.

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 the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the . The space enclosed by the inner membrane is called the . The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron transport) happens on the inner membrane.

Cellular Respiration Stage II: The Krebs Cycle

Recall that produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the  (also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.  The reactions that occur next are shown in Figure 4.10.6.

Image shows a diagram of the reactants and products of the Krebs Cycle. Two molecules of acetyl CoA are converted to 4 carbon dioxide which are released as cellular waste, 2 ATP which are used in the cell for energy, and 8 NADH and 2 FADH2, both of which travel to the ETS.
Figure 4.10.6 Reactants and products of the Krebs Cycle.

Steps of the Krebs Cycle

The itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.

After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2, another energy-carrying coenzyme. Carbon dioxide is also released as a waste product of these reactions.

The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose.

Results of the Glycolysis, Transition Reaction and Krebs Cycle

After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:

  • 4 ATP (2 from glycolysis, 2 from Krebs Cycle)
  • 12 NADH (2 from glycolysis, 2 from transition reaction, and 8 from Krebs cycle)
  • 2 FADH2 (both from the Krebs cycle)

The events of cellular respiration up to this point are – they are releasing energy that had been stored in the bonds of the glucose molecule.  This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an .  Using an exothermic reaction to power an endothermic reaction is known as .

Cellular Respiration Stage III: Electron Transport Chain

Image shows the reactants and products of the electron transport chain. In this stage, 32 adenosine diphosphate and 32 inorganic phosphates combine to form 32 ATP. In addition, hydrogen and oxygen combine to form 6 molecules of water.
Figure 4.10.7. Reactants and products of the electron transport chain.

 ETC, the final stage in cellular respiration produces 32 ATP.  The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2 is transferred to ATP.  In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2, forming water.  Figure 4.10.8 shows the reactants and products of the ETC.

Transporting Electrons

The is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2, and they move along electron-transport chains on the inner membrane of the mitochondrion. An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This ion transfer creates an  that drives the synthesis of .

 

Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.

Making ATP

As shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the “spent” electrons combine with oxygen to form water.

How Much ATP?

You have seen how the three stages of use the energy in glucose to make . How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2 from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.

4.10 Summary

  • is the process by which living cells break down molecules, release energy, and form molecules of . Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.
  • The first stage of cellular respiration, called , takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.  Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.
  • The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.
  • The second stage of cellular respiration, called the , takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).
  • The third and final stage of cellular respiration, called , takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
  • In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.

4.10 Review Questions

  1. What is the purpose of cellular respiration? Provide a concise summary of the process.
  2. State what happens during glycolysis.
  3. Describe the structure of a mitochondrion.
  4. What molecule is present at both the beginning and end of the Krebs cycle?
  5. What happens during the electron transport stage of cellular respiration?
  6. How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?
  7. Do plants undergo cellular respiration? Why or why not?
  8. Explain why the process of cellular respiration described in this section is considered aerobic.
  9. Name three energy-carrying molecules involved in cellular respiration.
  10. Which stage of aerobic cellular respiration produces the most ATP?

4.10 Explore More

https://www.youtube.com/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo

ATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.

Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.

Attributions

Figure 4.10.1

Smores by Jessica Ruscello on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.10.2

Carbohydrate_Metabolism by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 4.10.3

Glycolysis by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.4

Transition Reaction by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.5

Mitochondrion by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.10.6

Krebs cycle by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.7

Electron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.8

The_Electron_Transport_Chain by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References

CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https://www.youtube.com/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 24.8 Electron Transport Chain [digital image]. In Anatomy & Physiology, Connexions (Section ). OpenStax.  https://openstax.org/books/anatomy-and-physiology/pages/24-2-carbohydrate-metabolism

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 24.9 Carbohydrate Metabolism [digital image]. In Anatomy & Physiology, Connexions (Section 24.2). OpenStax.  https://openstax.org/books/anatomy-and-physiology/pages/24-2-carbohydrate-metabolism

The Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https://www.youtube.com/watch?v=4Eo7JtRA7lg&t=3s

 

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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