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Figure 4.10.1 Ready to make s’mores!

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

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Cellular respiration is the process by which living cells break down glucose molecules and release energy. 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 ATP, 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 aerobic process.

Cellular respiration occurs in the cells of all living things, both autotrophs and heterotrophs. All of them burn glucose to form ATP. The reactions of cellular respiration 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.

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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?

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

Splitting Glucose

The word glycolysis literally means “glucose splitting,” which is exactly what happens in this stage. Enzymes 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.

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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 glycolysis 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

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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).


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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 mitochondrion, 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 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) happens on the inner membrane.


Recall that glycolysis 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 Krebs cycle (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.

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Figure 4.10.6 Reactants and products of the Krebs Cycle.

The Krebs cycle 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 exergonic reactions– 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 endergonic reaction. Using an exothermic reaction to power an endothermic reaction is known as energy coupling.


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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 Electron transport chain 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 electrochemical gradient that drives the synthesis of ATP.

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

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.


You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. 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.


Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.The first stage of cellular respiration, called glycolysis, 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 Krebs cycle, 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 electron transport, 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.
What is the purpose of cellular respiration? Provide a concise summary of the process.State what happens during glycolysis.Describe the structure of a mitochondrion.What molecule is present at both the beginning and end of the Krebs cycle?What happens during the electron transport stage of cellular respiration?How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?Do plants undergo cellular respiration? Why or why not?Explain why the process of cellular respiration described in this section is considered aerobic.Name three energy-carrying molecules involved in cellular respiration.

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 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 . 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 . 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


Glucose (also called dextrose) is a simple sugar with the molecular formula C6H12O6. Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight.


A set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP).


A complex organic chemical that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer.


An organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis).


An organism that cannot produce its own food, relying instead on the intake of nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers.


The metabolic pathway that converts glucose C₆H₁₂O₆, into pyruvate. The free energy released in this process is used to form the high-energy molecules ATP and NADH. Glycolysis is a sequence of ten enzyme-catalyzed reactions.


The aqueous component of the cytoplasm of a cell, within which various organelles and particles are suspended.


The jellylike material that makes up much of a cell inside the cell membrane, and, in eukaryotic cells, surrounds the nucleus. The organelles of eukaryotic cells, such as mitochondria, the endoplasmic reticulum, and (in green plants) chloroplasts, are contained in the cytoplasm.


A double-membrane-bound organelle found in most eukaryotic organisms. Mitochondria convert oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy "currency" of the cell that powers the cell"s metabolic activities.


The space occurring between two or more membranes. In cell biology, it"s most commonly described as the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast.


In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm.


A series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.


A specific type of exothermic reaction which not only releases energy, but also occurs spontaneously.


A series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water.


A gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane.

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The process of producing cellular energy involving oxygen. Cells break down food in the mitochondria in a long, multi-step process that produces roughly 36 ATP. The first step in is glycolysis, the second is the Krebs cycle and the third is the electron transport system.