cellular respiration introduction essay

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Role of mitochondria

Tricarboxylic acid cycle, oxidative phosphorylation.

cellular respiration

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• Open Oregon Educational Resources - MHCC Biology 112: Biology for Health Professions - An Overview of Cellular Respiration
• Thompson Rivers University - Human Biology - Cellular Respiration
• Milne Library - Inanimate Life - Cellular Respiration
• BCCampus Publishing - Human Biology – Excerpts for BBIO 053 - Cellular Respiration
• Khan Academy - Cellular respiration introduction
• Biology LibreTexts - Cellular Respiration
• Roger Williams University Open Publishing - Cellular Respiration
• The University of Hawaiʻi Pressbooks - Biology - Regulation of Cellular Respiration
• cellular respiration - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

cellular respiration , the process by which organisms combine oxygen with foodstuff molecules , diverting the chemical energy in these substances into life-sustaining activities and discarding, as waste products, carbon dioxide and water. Organisms that do not depend on oxygen degrade foodstuffs in a process called fermentation . (For longer treatments of various aspects of cellular respiration, see tricarboxylic acid cycle and metabolism .)

One objective of the degradation of foodstuffs is to convert the energy contained in chemical bonds into the energy-rich compound adenosine triphosphate (ATP), which captures the chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes. In eukaryotic cells (that is, any cells or organisms that possess a clearly defined nucleus and membrane-bound organelles) the enzymes that catalyze the individual steps involved in respiration and energy conservation are located in highly organized rod-shaped compartments called mitochondria . In microorganisms the enzymes occur as components of the cell membrane . A liver cell has about 1,000 mitochondria; large egg cells of some vertebrates have up to 200,000.

Main metabolic processes

Biologists differ somewhat with respect to the names, descriptions, and the number of stages of cellular respiration . The overall process, however, can be distilled into three main metabolic stages or steps: glycolysis , the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation (respiratory-chain phosphorylation).

Glycolysis (which is also known as the glycolytic pathway or the Embden-Meyerhof-Parnas pathway) is a sequence of 10 chemical reactions taking place in most cells that breaks down a glucose molecule into two pyruvate (pyruvic acid) molecules. Energy released during the breakdown of glucose and other organic fuel molecules from carbohydrates , fats , and proteins during glycolysis is captured and stored in ATP. In addition, the compound nicotinamide adenine dinucleotide (NAD + ) is converted to NADH during this step ( see below ). Pyruvate molecules produced during glycolysis then enter the mitochondria, where they are each converted into a compound known as acetyl coenzyme A, which then enters the TCA cycle. (Some sources consider the conversion of pyruvate into acetyl coenzyme A as a distinct step, called pyruvate oxidation or the transition reaction, in the process of cellular respiration.)

The TCA cycle (which is also known as the Krebs, or citric acid , cycle) plays a central role in the breakdown, or catabolism , of organic fuel molecules. The cycle is made up of eight steps catalyzed by eight different enzymes that produce energy at several different stages. Most of the energy obtained from the TCA cycle, however, is captured by the compounds NAD + and flavin adenine dinucleotide (FAD) and converted later to ATP. The products of a single turn of the TCA cycle consist of three NAD + molecules, which are reduced (through the process of adding hydrogen , H + ) to the same number of NADH molecules, and one FAD molecule, which is similarly reduced to a single FADH 2 molecule. These molecules go on to fuel the third stage of cellular respiration, whereas carbon dioxide, which is also produced by the TCA cycle, is released as a waste product.

In the oxidative phosphorylation stage, each pair of hydrogen atoms removed from NADH and FADH 2 provides a pair of electrons that—through the action of a series of iron -containing hemoproteins, the cytochromes —eventually reduces one atom of oxygen to form water . In 1951 it was discovered that the transfer of one pair of electrons to oxygen results in the formation of three molecules of ATP .

Oxidative phosphorylation is the major mechanism by which the large amounts of energy in foodstuffs are conserved and made available to the cell . The series of steps by which electrons flow to oxygen permits a gradual lowering of the energy of the electrons. This part of the oxidative phosphorylation stage is sometimes called the electron transport chain . Some descriptions of cellular respiration that focus on the importance of the electron transport chain have changed the name of the oxidative phosphorylation stage to the electron transport chain.

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Module 6: Metabolic Pathways

Summary: cellular respiration, learning outcomes.

• Describe the process of glycolysis and identify its reactants and products
• Describe the process of the citric acid cycle (Krebs cycle) and identify its reactants and products
• Describe the overall outcome of the citric acid cycle and oxidative phosphorylation in terms of the products of each
• Describe the location of the citric acid cycle and oxidative phosphorylation in the cell

Cellular respiration is a process that all living things use to convert glucose into energy. Autotrophs (like plants) produce glucose during photosynthesis. Heterotrophs (like humans) ingest other living things to obtain glucose. While the process can seem complex, this page takes you through the key elements of each part of cellular respiration.

Let’s Review

Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis is an anaerobic process, while the other two pathways are aerobic. In order to move from glycolysis to the citric acid cycle, pyruvate molecules (the output of glycolysis) must be oxidized in a process called pyruvate oxidation.

Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half. The first half is known as the “energy requiring” steps. This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. In the second half, the “energy releasing: steps, 4 molecules of ATP and 2 NADH are released. Glycolysis has a net gain  of    2 ATP molecules and 2 NADH.

Some cells (e.g., mature mammalian red blood cells) cannot undergo aerobic respiration, so glycolysis is their only source of ATP. However, most cells undergo pyruvate oxidation and continue to the other pathways of cellular respiration.

Pyruvate Oxidation

In eukaryotes, pyruvate oxidation takes place in the mitochondria. Pyruvate oxidation can only happen if oxygen is available. In this process, the pyruvate created by glycolysis is oxidized. In this oxidation process, a carboxyl group is removed from pyruvate, creating acetyl groups, which compound with coenzyme A (CoA) to form acetyl CoA. This process also releases CO 2 .

Citric Acid Cycle

The citric acid cycle (also known as the Krebs cycle) is the second pathway in cellular respiration, and it also takes place in the mitochondria. The rate of the cycle is controlled by ATP concentration. When there is more ATP available, the rate slows down; when there is less ATP the rate increases. This pathway is a closed loop: the final step produces the compound needed for the first step.

The citric acid cycle is considered an aerobic pathway because the NADH and FADH 2 it produces act as temporary electron storage compounds, transferring their electrons to the next pathway (electron transport chain), which uses atmospheric oxygen. Each turn of the citric acid cycle provides a net gain of CO 2 , 1 GTP or ATP, and 3 NADH and 1 FADH 2 .

Electron Transport Chain

Most ATP from glucose is generated in the electron transport chain. It is the only part of cellular respiration that directly consumes oxygen; however, in some prokaryotes, this is an anaerobic pathway. In eukaryotes, this pathway takes place in the inner mitochondrial membrane. In prokaryotes it occurs in the plasma membrane.

The electron transport chain is made up of 4 proteins along the membrane and a proton pump. A cofactor shuttles electrons between proteins I–III. If NAD is depleted, skip I: FADH 2 starts on II. In chemiosmosis, a proton pump takes hydrogens from inside mitochondria to the outside; this spins the “motor” and the phosphate groups attach to that. The movement changes from ADP to ATP, creating 90% of ATP obtained from aerobic glucose catabolism.

Let’s Practice

Now that you’ve reviewed cellular respiration, this practice activity will help you see how well you know cellular respiration:

• Cellular Respiration. Authored by : Shelli Carter and Lumen Learning. Provided by : Lumen Learning. License : CC BY: Attribution
• Understanding Cellular Respiration. Provided by : Lumen Learning. Located at : https://www.oppia.org/explore/LG5n93fp89oh . License : CC BY-SA: Attribution-ShareAlike

8.3 Cellular Respiration

Learning objectives.

By the end of this section, you will be able to:

• Compare and contrast the electron transport system location and function in a prokaryotic cell and a eukaryotic cell
• Compare and contrast the differences between substrate-level and oxidative phosphorylation
• Explain the relationship between chemiosmosis and proton motive force
• Describe the function and location of ATP synthase in a prokaryotic versus eukaryotic cell
• Compare and contrast aerobic and anaerobic respiration

We have just discussed two pathways in glucose catabolism—glycolysis and the Krebs cycle—that generate ATP by substrate-level phosphorylation. Most ATP, however, is generated during a separate process called oxidative phosphorylation , which occurs during cellular respiration. Cellular respiration begins when electrons are transferred from NADH and FADH 2 —made in glycolysis, the transition reaction, and the Krebs cycle—through a series of chemical reactions to a final inorganic electron acceptor (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration). These electron transfers take place on the inner part of the cell membrane of prokaryotic cells or in specialized protein complexes in the inner membrane of the mitochondria of eukaryotic cells. The energy of the electrons is harvested to generate an electrochemical gradient across the membrane, which is used to make ATP by oxidative phosphorylation.

Electron Transport System

The electron transport system (ETS) is the last component involved in the process of cellular respiration; it comprises a series of membrane-associated protein complexes and associated mobile accessory electron carriers ( Figure 8.15 ). Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons from NADH and FADH 2 are passed rapidly from one ETS electron carrier to the next. These carriers can pass electrons along in the ETS because of their redox potential . For a protein or chemical to accept electrons, it must have a more positive redox potential than the electron donor. Therefore, electrons move from electron carriers with more negative redox potential to those with more positive redox potential. The four major classes of electron carriers involved in both eukaryotic and prokaryotic electron transport systems are the cytochrome s, flavoprotein s, iron-sulfur protein s, and the quinone s.

In aerobic respiration , the final electron acceptor (i.e., the one having the most positive redox potential) at the end of the ETS is an oxygen molecule (O 2 ) that becomes reduced to water (H 2 O) by the final ETS carrier. This electron carrier, cytochrome oxidase , differs between bacterial types and can be used to differentiate closely related bacteria for diagnoses. For example, the gram-negative opportunist Pseudomonas aeruginosa and the gram-negative cholera-causing Vibrio cholerae use cytochrome c oxidase, which can be detected by the oxidase test, whereas other gram-negative Enterobacteriaceae, like E. coli , are negative for this test because they produce different cytochrome oxidase types.

There are many circumstances under which aerobic respiration is not possible, including any one or more of the following:

• The cell lacks genes encoding an appropriate cytochrome oxidase for transferring electrons to oxygen at the end of the electron transport system.
• The cell lacks genes encoding enzymes to minimize the severely damaging effects of dangerous oxygen radicals produced during aerobic respiration, such as hydrogen peroxide (H 2 O 2 ) or superoxide ( O 2 – ) . ( O 2 – ) .
• The cell lacks a sufficient amount of oxygen to carry out aerobic respiration.

One possible alternative to aerobic respiration is anaerobic respiration , using an inorganic molecule other than oxygen as a final electron acceptor. There are many types of anaerobic respiration found in bacteria and archaea. Denitrifiers are important soil bacteria that use nitrate ( NO 3 – ) ( NO 3 – ) and nitrite ( NO 2 – ) ( NO 2 – ) as final electron acceptors, producing nitrogen gas (N 2 ). Many aerobically respiring bacteria, including E. coli , switch to using nitrate as a final electron acceptor and producing nitrite when oxygen levels have been depleted.

Microbes using anaerobic respiration commonly have an intact Krebs cycle, so these organisms can access the energy of the NADH and FADH 2 molecules formed. However, anaerobic respirers use altered ETS carriers encoded by their genomes, including distinct complexes for electron transfer to their final electron acceptors. Smaller electrochemical gradients are generated from these electron transfer systems, so less ATP is formed through anaerobic respiration.

Check Your Understanding

• Do both aerobic respiration and anaerobic respiration use an electron transport chain?

Chemiosmosis, Proton Motive Force, and Oxidative Phosphorylation

In each transfer of an electron through the ETS, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions (H + ) across a membrane. In prokaryotic cells, H + is pumped to the outside of the cytoplasmic membrane (called the periplasmic space in gram-negative and gram-positive bacteria), and in eukaryotic cells, they are pumped from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space. There is an uneven distribution of H + across the membrane that establishes an electrochemical gradient because H + ions are positively charged (electrical) and there is a higher concentration (chemical) on one side of the membrane. This electrochemical gradient formed by the accumulation of H + (also known as a proton) on one side of the membrane compared with the other is referred to as the proton motive force (PMF). Because the ions involved are H + , a pH gradient is also established, with the side of the membrane having the higher concentration of H + being more acidic. Beyond the use of the PMF to make ATP, as discussed in this chapter, the PMF can also be used to drive other energetically unfavorable processes, including nutrient transport and flagella rotation for motility.

The potential energy of this electrochemical gradient generated by the ETS causes the H + to diffuse across a membrane (the plasma membrane in prokaryotic cells and the inner membrane in mitochondria in eukaryotic cells). This flow of hydrogen ions across the membrane, called chemiosmosis , must occur through a channel in the membrane via a membrane-bound enzyme complex called ATP synthase ( Figure 8.15 ). The tendency for movement in this way is much like water accumulated on one side of a dam, moving through the dam when opened. ATP synthase (like a combination of the intake and generator of a hydroelectric dam) is a complex protein that acts as a tiny generator, turning by the force of the H + diffusing through the enzyme, down their electrochemical gradient from where there are many mutually repelling H + to where there are fewer H + . In prokaryotic cells, H + flows from the outside of the cytoplasmic membrane into the cytoplasm, whereas in eukaryotic mitochondria, H + flows from the intermembrane space to the mitochondrial matrix. The turning of the parts of this molecular machine regenerates ATP from ADP and inorganic phosphate (P i ) by oxidative phosphorylation , a second mechanism for making ATP that harvests the potential energy stored within an electrochemical gradient.

The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport system complexes can pump through the membrane varies between different species of organisms. In aerobic respiration in mitochondria, the passage of electrons from one molecule of NADH generates enough proton motive force to make three ATP molecules by oxidative phosphorylation, whereas the passage of electrons from one molecule of FADH 2 generates enough proton motive force to make only two ATP molecules. Thus, the 10 NADH molecules made per glucose during glycolysis, the transition reaction, and the Krebs cycle carry enough energy to make 30 ATP molecules, whereas the two FADH 2 molecules made per glucose during these processes provide enough energy to make four ATP molecules. Overall, the theoretical maximum yield of ATP made during the complete aerobic respiration of glucose is 38 molecules, with four being made by substrate-level phosphorylation and 34 being made by oxidative phosphorylation ( Figure 8.16 ). In reality, the total ATP yield is usually less, ranging from one to 34 ATP molecules, depending on whether the cell is using aerobic respiration or anaerobic respiration; in eukaryotic cells, some energy is expended to transport intermediates from the cytoplasm into the mitochondria, affecting ATP yield.

Figure 8.16 summarizes the theoretical maximum yields of ATP from various processes during the complete aerobic respiration of one glucose molecule.

• What are the functions of the proton motive force?

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Photosynthesis and Cellular Respiration Essay

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Photosynthesis is one of the primary sources of energy for living organisms. The fossilized photosynthetic fuels account for almost 90% of the energy in the world (Johnson, 2016). Cellular respiration is a process that takes place in the living organism and converts nutrients into energy. This essay will examine photosynthesis and cellular respiration separately and identify similarities, differences, and interconnectedness between two processes. Two processes are similar in that they both deals with energy, but they are different because one process involves catabolic reactions and another anabolic one.

The purpose of photosynthesis is to convert atmospheric carbon dioxide into carbohydrates using light energy. The light splits one of the reactants, water in the mesophyll of the leaf into oxygen, electrons, and protons during the light-dependent phase (Johnson, 2016). Then carbon dioxide enters the mesophyll of the leaf through openings, stomata, during the light-independent phase. These two reactions differ in light utilization and molecules production. The first reaction products are oxygen, adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH) that are used as energy storages, while by the end of the second reaction, the carbohydrate is obtained, and molecules mentioned above are used (Flügge et al., 2016). Photosynthesis occurs in the chloroplast with the light-dependent reaction taking place in the thylakoid membrane, and light-independent reaction in the stroma. The energy produced in the light reaction is used to fix carbon dioxide and produce carbohydrates while oxygen is released outside. According to the following equation of the photosynthesis, C → O2 + 2H20 + photons (CH2O)n + electrons + O2 carbon monoxide and water are transferred into carbohydrates under the light with the release of atmospheric oxygen.

The purpose of cellular respiration is to convert nutrients into energy. The reactants of the respiration are glucose circulating in the blood and oxygen obtained from breathing, while the product is ATP. Cellular respiration starts from glycolysis in the mitochondria’s stroma, where the glucose is broken down into pyruvate (Bentley & Connaughton, 2017). Then it continues with the citric acid cycle that generates ATP, NADH, and FADH2. In the final stage, the electron transport chain uses these molecules to generate more ATP. The energy produced is then used for metabolic processes in the organism, while carbon dioxide is released with breathing (BBC Bitesize, n.d.). According to the following equation of the cellular respiration, C → 6H12O6 + 6O2 6CO2 + 6H2O the glucose is broken down into carbon dioxide and water with the presence of oxygen.

There are two main differences between photosynthesis and cellular respiration. The first one is the anabolic process, during which complex compounds are synthesized, while the second one is catabolic, which involves breaking down the compounds (Panawala, 2017). The second crucial difference is that photosynthesis is found only in chloroplasts, while cellular respiration is found in any living cell, making it a universal process. There are also two main similarities between photosynthesis and respiration. The first similarity is that both processes involve the production of ATP (Stauffer et al., 2018). The second similarity is that both processes utilize ATP but for different purposes.

Photosynthesis and cellular respiration are connected in such a way that they allow to perform metabolic functions normally. Moreover, these processes help to regulate the concentration of oxygen and carbon dioxide in the atmosphere. If photosynthesis stopped occurring, the level of oxygen would drop dramatically This would lead to deaths of all living organisms whose lives depend on this molecule. Whereas if cellular respiration stopped happening, living creatures would not be able to generate energy and sustain life.

To conclude, photosynthesis plays a crucial role in maintaining life on Earth. Photosynthesis uses light energy to produce oxygen, while cellular respiration uses oxygen to break down complex molecules and provide energy. These processes are different in their metabolic nature, but similar in terms of energy storage. If photosynthesis did not exist, the life for oxygen-dependent creatures would become extinct. Similarly, in the case of cellular respiration disappearing, living organisms would not be able to produce energy.

BBC Bitesize . (n.d.). Respiration. 2020. Web.

Bentley, M., & Connaughton, V, P. (2017). A simple way for students to visualize cellular respiration: Adapting the board game MousetrapTM to model complexity . CourseSource. 4, 1-6. Web.

Flügge, W., Westhoff, P., & Leister, D. (2016). Recent advances in understanding photosynthesis. F1000 Research, 5, 1-10.

Johnson, M. P. (2016). Photosynthesis. Essays Biochemistry , 60 (3), 255-273.

Panawala, L. (2017). Difference between photosynthesis and respiration. IE PEDIAA. Web.

Stauffer S., Gardner A., Ungu D.A.K., López-Córdoba A., & Heim M. (2018). Cellular respiration. In Labster virtual lab experiments: Basic biology (pp. 43-55). Springer.

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Home — Essay Samples — Science — Photosynthesis — Photosynthesis and Cellular Respiration

Photosynthesis and Cellular Respiration

• Categories: Photosynthesis

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Words: 573 |

Published: Feb 12, 2019

Words: 573 | Page: 1 | 3 min read

Table of contents

Prompt examples for the "photosynthesis" essays, photosynthesis essay example.

• The Process of Photosynthesis: Breaking It Down Explain the process of photosynthesis in detail, breaking down each step, the key resources involved (light energy, carbon dioxide, water), and the outcomes (glucose and oxygen). How does photosynthesis enable plants to create their own food?
• Photosynthesis vs. Cellular Respiration: Understanding the Differences Compare and contrast photosynthesis and cellular respiration. What are the key distinctions between these two processes? How do plants use these processes differently, and why is it essential for plants to perform photosynthesis during the day and cellular respiration at night?
• The Importance of Photosynthesis for Plant Survival Discuss the critical role of photosynthesis in a plant's survival. How does it provide plants with the necessary energy and nutrients? Explore the potential consequences if a plant were unable to perform photosynthesis.
• Common Misconceptions About Photosynthesis Address common misconceptions or incorrect claims about photosynthesis, such as those mentioned by Mika in the essay. Provide clear explanations to refute these misconceptions and offer accurate information about when photosynthesis and cellular respiration occur in plant cells.
• The Energy Acquisition Strategies of Plants and Animals Compare how plants and animals acquire and utilize energy. Explain the fundamental differences in their energy sources and processes. Why do plants rely on photosynthesis, while animals need to consume other organisms for energy?

Works Cited

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2014). Molecular Biology of the Cell. Garland Science.
• Campbell, N. A., & Reece, J. B. (2017). Biology. Pearson.
• Cox, M. M., & Doudna, J. A. (2017). Principles of Molecular Biology. W. H. Freeman.
• Freeman, S., Quillin, K., Allison, L., Black, M., Taylor, E., & Podgorski, G. (2017). Biological Science. Pearson.
• Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., & Darnell, J. (2016). Molecular Cell Biology. W. H. Freeman.
• National Science Teachers Association. (2016). Photosynthesis and cellular respiration. NSTA.
• Raven, P. H., Evert, R. F., & Eichhorn, S. E. (2017). Biology of Plants. W. H. Freeman.
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2014). Campbell Biology. Pearson.
• Sadava, D. E., Hillis, D. M., Heller, H. C., & Berenbaum, M. R. (2014). Life: The Science of Biology. W. H. Freeman.
• Taiz, L., & Zeiger, E. (2013). Plant Physiology. Sinauer Associates.

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 3.

• Cellular respiration introduction
• Introduction to cellular respiration and redox

Steps of cellular respiration

• Overview of cellular respiration
• Oxidative phosphorylation and the electron transport chain
• Oxidative phosphorylation
• Fermentation and anaerobic respiration
• ATP synthase
• Cellular respiration

Introduction

NAD + ‍   + ‍   2 e − ‍   + ‍   2 H + ‍   → ‍   NADH ‍   + ‍   H + ‍  

FAD ‍   + ‍   2 e − ‍   + ‍   2 H + ‍   → ‍   FADH 2 ‍  

• Glycolysis. In glycolysis, glucose—a six-carbon sugar—undergoes a series of chemical transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, ATP is made, and NAD + ‍   is converted to NADH ‍   .
• Pyruvate oxidation. Each pyruvate from glycolysis goes into the mitochondrial matrix—the innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and NADH ‍   is generated.
• Citric acid cycle. The acetyl CoA made in the last step combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP, NADH ‍   , and FADH 2 ‍   are produced, and carbon dioxide is released.
• Oxidative phosphorylation. The NADH ‍   and FADH 2 ‍   made in other steps deposit their electrons in the electron transport chain, turning back into their "empty" forms ( NAD + ‍   and FAD ‍   ). As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.

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