Cellular respiration - wikipedia Cellular respiration This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( September 2014 ) Typical eukaryotic cell Cellular respiration is 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 ), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process, as weak so - called `` high - energy '' bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions. Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and the most common oxidizing agent ( electron acceptor ) is molecular oxygen ( O ). The chemical energy stored in ATP ( its third phosphate group is weakly bonded to the rest of the molecule and is cheaply broken allowing stronger bonds to form, thereby transferring energy for use by the cell ) can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes. Contents 1 Aerobic respiration 1.1 Aerobic respiration summed up 1.2 Glycolysis 1.3 Oxidative decarboxylation of pyruvate 1.4 Citric acid cycle 1.5 Oxidative phosphorylation 2 Efficiency of ATP production 3 Fermentation 4 Anaerobic respiration 5 See also 6 References 7 External links Aerobic respiration Aerobic respiration ( red arrows ) is the main means by which both fungi and animals utilize chemical energy in the form of organic compounds that were previously created through photosynthesis ( green arrow ). Aerobic respiration requires oxygen ( O ) in order to create ATP. Although carbohydrates, fats, and proteins are consumed as reactants, it is the preferred method of pyruvate breakdown in glycolysis and requires that pyruvate enter the mitochondria in order to be fully oxidized by the Krebs cycle. The products of this process are carbon dioxide and water, but the energy transferred is used to break bonds in ADP as the third phosphate group is added to form ATP ( adenosine triphosphate ), by substrate - level phosphorylation, NADH and FADH Simplified reaction : C H O ( s ) + 6 O ( g ) → 6 CO ( g ) + 6 H O ( l ) + heat ΔG = − 2880 kJ per mol of C H O The negative ΔG indicates that the reaction can occur spontaneously. The potential of NADH and FADH is converted to more ATP through an electron transport chain with oxygen as the `` terminal electron acceptor ''. Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. This works by the energy released in the consumption of pyruvate being used to create a chemiosmotic potential by pumping protons across a membrane. This potential is then used to drive ATP synthase and produce ATP from ADP and a phosphate group. Biology textbooks often state that 38 ATP molecules can be made per oxidised glucose molecule during cellular respiration ( 2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system ). However, this maximum yield is never quite reached because of losses due to leaky membranes as well as the cost of moving pyruvate and ADP into the mitochondrial matrix, and current estimates range around 29 to 30 ATP per glucose. Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism ( which yields 2 molecules ATP per 1 molecule glucose ). However some anaerobic organisms, such as methanogens are able to continue with anaerobic respiration, yielding more ATP by using other inorganic molecules ( not oxygen ) as final electron acceptors in the electron transport chain. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post-glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells. Aerobic respiration summed up 1. Glycolysis : -- - 2 ATPs + Glucose → 2 Pyruvic Acid + 4 Hydrogen + 4 ATPs 2. Formation of Acetyl CoA : -- - 2 Pyruvic Acid + 2 CoA → 2 Acetyl CoA + 2 Carbon Dioxide + 2 Hydrogen 3. Krebs Cycle : -- - 2 Acetyl CoA + 3 O2 → 6 Hydrogen + 4 Carbon Dioxide + 2 ATPs 4. Electron Transport System : -- - 12 Hydrogen + 3 O2 → 6 Water + 32 ATPs Overall Reaction : -- - Glucose + 6 O2 → 6 Carbon Dioxide + 6 Water + 36 ATPs Glycolysis Out of the cytoplasm it goes into the Krebs cycle with the acetyl CoA. It then mixes with CO and makes 2 ATP, NADH, and FADH. From there the NADH and FADH go into the NADH reductase, which produces the enzyme. The NADH pulls the enzyme 's electrons to send through the electron transport chain. The electron transport chain pulls H ions through the chain. From the electron transport chain, the released hydrogen ions make ADP for an end result of 32 ATP. O attracts itself to the left over electron to make water. Lastly, ATP leaves through the ATP channel and out of the mitochondria. Main article : Glycolysis Glycolysis is a metabolic pathway that takes place in the cytosol of cells in all living organisms. This pathway can function with or without the presence of oxygen. In humans, aerobic conditions produce pyruvate and anaerobic conditions produce lactate. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate ( pyruvic acid ), generating energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced, however, two are consumed as part of the preparatory phase. The initial phosphorylation of glucose is required to increase the reactivity ( decrease its stability ) in order for the molecule to be cleaved into two pyruvate molecules by the enzyme aldolase. During the pay - off phase of glycolysis, four phosphate groups are transferred to ADP by substrate - level phosphorylation to make four ATP, and two NADH are produced when the pyruvate are oxidized. The overall reaction can be expressed this way : Glucose + 2 NAD + 2 P + 2 ADP → 2 pyruvate + 2 NADH + 2 ATP + 2 H + 2 H O + heat Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6 - phosphate. Glycogen can be converted into glucose 6 - phosphate as well with the help of glycogen phosphorylase. During energy metabolism, glucose 6 - phosphate becomes fructose 6 - phosphate. An additional ATP is used to phosphorylate fructose 6 - phosphate into fructose 1, 6 - disphosphate by the help of phosphofructokinase. Fructose 1, 6 - diphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate. Glycolysis can be literally translated as `` sugar splitting ''. Oxidative decarboxylation of pyruvate Main article : Pyruvate decarboxylation Pyruvate is oxidized to acetyl - CoA and CO by the pyruvate dehydrogenase complex ( PDC ). The PDC contains multiple copies of three enzymes and is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. In the conversion of pyruvate to acetyl - CoA, one molecule of NADH and one molecule of CO is formed. Citric acid cycle Main article : Citric acid cycle This is also called the Krebs cycle or the tricarboxylic acid cycle. When oxygen is present, acetyl - CoA is produced from the pyruvate molecules created from glycolysis. Once acetyl - CoA is formed, aerobic or anaerobic respiration can occur. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl - CoA is produced, the molecule then enters the citric acid cycle ( Krebs cycle ) inside the mitochondrial matrix, and is oxidized to CO while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl - CoA must be metabolized by the Krebs cycle. Two waste products, H O and CO, are created during this cycle. The citric acid cycle is an 8 - step process involving 18 different enzymes and co-enzymes. During the cycle, acetyl - CoA ( 2 carbons ) + oxaloacetate ( 4 carbons ) yields citrate ( 6 carbons ), which is rearranged to a more reactive form called isocitrate ( 6 carbons ). Isocitrate is modified to become α - ketoglutarate ( 5 carbons ), succinyl - CoA, succinate, fumarate, malate, and, finally, oxaloacetate. The net gain of high - energy compounds from one cycle is 3 NADH, 1 FADH, and 1 GTP ; the GTP may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule ( 2 pyruvate molecules ) is 6 NADH, 2 FADH, and 2 ATP. Oxidative phosphorylation Main articles : Oxidative phosphorylation, Electron transport chain, Electrochemical gradient, and ATP synthase In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It comprises the electron transport chain that establishes a proton gradient ( chemiosmotic potential ) across the boundary of inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. The electrons are finally transferred to exogenous oxygen and, with the addition of two protons, water is formed. Efficiency of ATP production The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide. It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation. Step coenzyme yield ATP yield Source of ATP Glycolysis preparatory phase − 2 Phosphorylation of glucose and fructose 6 - phosphate uses two ATP from the cytoplasm. Glycolysis pay - off phase Substrate - level phosphorylation 2 NADH 3 or 5 Oxidative phosphorylation : Each NADH produces net 1.5 ATP ( instead of usual 2.5 ) due to NADH transport over the mitochondrial membrane Oxidative decarboxylation of pyruvate 2 NADH 5 Oxidative phosphorylation Krebs cycle Substrate - level phosphorylation 6 NADH 15 Oxidative phosphorylation 2 FADH Oxidative phosphorylation Total yield 30 or 32 ATP From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes. Although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized because of losses such as the cost of moving pyruvate ( from glycolysis ), phosphate, and ADP ( substrates for ATP synthesis ) into the mitochondria. All are actively transported using carriers that utilize the stored energy in the proton electrochemical gradient. Pyruvate is taken up by a specific, low Km transporter to bring it into the mitochondrial matrix for oxidation by the pyruvate dehydrogenase complex. The phosphate carrier ( PiC ) mediates the electroneutral exchange ( antiport ) of phosphate ( H PO ; P ) for OH or symport of phosphate and protons ( H ) across the inner membrane, and the driving force for moving phosphate ions into the mitochondria is the proton motive force. The ATP - ADP translocase ( also called adenine nucleotide translocase, ANT ) is an antiporter and exchanges ADP and ATP across the inner membrane. The driving force is due to the ATP ( − 4 ) having a more negative charge than the ADP ( − 3 ), and thus it dissipates some of the electrical component of the proton electrochemical gradient. The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H are needed to make 1 ATP. Obviously this reduces the theoretical efficiency of the whole process and the likely maximum is closer to 28 -- 30 ATP molecules. In practice the efficiency may be even lower because the inner membrane of the mitochondria is slightly leaky to protons. Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in brown fat thermogenesis of newborn and hibernating mammals. Stoichiometry of aerobic respiration and most known fermentation types in eucaryotic cell. Numbers in circles indicate counts of carbon atoms in molecules, C6 is glucose C H O, C1 carbon dioxide CO. Mitochondrial outer membrane is omitted. According to some of newer sources the ATP yield during aerobic respiration is not 36 -- 38, but only about 30 -- 32 ATP molecules / 1 molecule of glucose, because : ATP : NADH + H and ATP : FADH ratios during the oxidative phosphorylation appear to be not 3 and 2, but 2.5 and 1.5 respectively. Unlike in the substrate - level phosphorylation, the stoichiometry here is difficult to establish. ATP synthase produces 1 ATP / 3 H. However the exchange of matrix ATP for cytosolic ADP and Pi ( antiport with OH or symport with H ) mediated by ATP -- ADP translocase and phosphate carrier consumes 1 H / 1 ATP as a result of regeneration of the transmembrane potential changed during this transfer, so the net ratio is 1 ATP : 4 H. The mitochondrial electron transport chain proton pump transfers across the inner membrane 10 H / 1 NADH + H ( 4 + 2 + 4 ) or 6 H / 1 FADH ( 2 + 4 ). So the final stoichiometry is 1 NADH + H : 10 H : 10 / 4 ATP = 1 NADH + H : 2.5 ATP 1 FADH : 6 H : 6 / 4 ATP = 1 FADH : 1.5 ATP ATP : NADH + H coming from glycolysis ratio during the oxidative phosphorylation is 1.5, as for FADH, if hydrogen atoms ( 2H + 2e ) are transferred from cytosolic NADH + H to mitochondrial FAD by the glycerol phosphate shuttle located in the inner mitochondrial membrane. 2.5 in case of malate - aspartate shuttle transferring hydrogen atoms from cytosolic NADH + H to mitochondrial NAD So finally we have, per molecule of glucose Substrate - level phosphorylation : 2 ATP from glycolysis + 2 ATP ( directly GTP ) from Krebs cycle Oxidative phosphorylation 2 NADH + H from glycolysis : 2 × 1.5 ATP ( if glycerol phosphate shuttle transfers hydrogen atoms ) or 2 × 2.5 ATP ( malate - aspartate shuttle ) 2 NADH + H from the oxidative decarboxylation of pyruvate and 6 from Krebs cycle : 8 × 2.5 ATP 2 FADH from the Krebs cycle : 2 × 1.5 ATP Altogether this gives 4 + 3 ( or 5 ) + 20 + 3 = 30 ( or 32 ) ATP per molecule of glucose The total ATP yield in ethanol or lactic acid fermentation is only 2 molecules coming from glycolysis, because pyruvate is not transferred to the mitochondrion and finally oxidized to the carbon dioxide ( CO ), but reduced to ethanol or lactic acid in the cytoplasm. Fermentation Main article : Fermentation Without oxygen, pyruvate ( pyruvic acid ) is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD+ so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD+ for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain can not process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD+ regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD+ attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate - level phosphorylation, which does not require oxygen. Fermentation is less efficient at using the energy from glucose : only 2 ATP are produced per glucose, compared to the 38 ATP per glucose nominally produced by aerobic respiration. This is because the waste products of fermentation still contain chemical potential energy that can be released by oxidation. Ethanol, for example, can be burned in an internal combustion engine like gasoline. Glycolytic ATP, however, is created more quickly. For prokaryotes to continue a rapid growth rate when they are shifted from an aerobic environment to an anaerobic environment, they must increase the rate of the glycolytic reactions. For multicellular organisms, during short bursts of strenuous activity, muscle cells use fermentation to supplement the ATP production from the slower aerobic respiration, so fermentation may be used by a cell even before the oxygen levels are depleted, as is the case in sports that do not require athletes to pace themselves, such as sprinting. Anaerobic respiration Main article : Anaerobic respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor ( such as oxygen ) to produce large amounts of energy, to drive the bulk production of ATP. Anaerobic respiration is used by some microorganisms in which neither oxygen ( aerobic respiration ) nor pyruvate derivatives ( fermentation ) is the final electron acceptor. Rather, an inorganic acceptor such as sulfate or nitrate is used. Such organisms are typically found in unusual places such as underwater caves or near hydrothermal vents at the bottom of the ocean. See also Maintenance respiration : maintenance as a functional component of cellular respiration Pasteur point Respirometry : research tool to explore cellular respiration Tetrazolium chloride : cellular respiration indicator Complex 1 : NADH : ubiquinone oxidoreductes References Jump up ^ Bailey, Regina. `` Cellular Respiration ''. Archived from the original on 2012 - 05 - 05. ^ Jump up to : Rich, P.R. ( 2003 ). `` The molecular machinery of Keilin 's respiratory chain ''. Biochemical Society Transactions. 31 ( Pt 6 ) : 1095 -- 1105. doi : 10.1042 / BST0311095. PMID 14641005. Jump up ^ Reece1 Urry2 Cain3 Wasserman4 Minorsky5 Jackson6, Jane1 Lisa2 Michael3 Steven4 Peter5 Robert6 ( 2010 ). Campbell Biology Ninth Edition. Pearson Education, Inc. p. 168. ^ Jump up to : `` Cellular Respiration '' ( PDF ). Archived ( PDF ) from the original on 2017 - 05 - 10. Jump up ^ Porter, R. ; Brand, M. ( 1 September 1995 ). `` Mitochondrial proton conductance and H+ / O ratio are independent of electron transport rate in isolated hepatocytes ''. The Biochemical Journal ( Free full text ). 310 ( Pt 2 ) : 379 -- 382. doi : 10.1042 / bj3100379. ISSN 0264 - 6021. PMC 1135905. PMID 7654171. ^ Jump up to : Stryer, Lubert ( 1995 ). Biochemistry ( fourth ed. ). New York -- Basingstoke : W.H. Freeman and Company. ISBN 978 - 0716720096. External links A detailed description of respiration vs. fermentation Kimball 's online resource for cellular respiration Cellular Respiration and Fermentation at Clermont College Library resources about Cellular respiration Resources in your library Metabolism, catabolism, anabolism General Metabolic pathway Metabolic network Primary nutritional groups Energy metabolism Aerobic respiration Glycolysis → Pyruvate decarboxylation → Citric acid cycle → Oxidative phosphorylation ( electron transport chain + ATP synthase ) Anaerobic respiration Electron acceptors are other than oxygen Fermentation Glycolysis → Substrate - level phosphorylation ABE Ethanol Lactic acid Specific paths Protein metabolism Protein synthesis Catabolism Carbohydrate metabolism ( carbohydrate catabolism and anabolism ) Human Glycolysis ⇄ Gluconeogenesis Glycogenolysis ⇄ Glycogenesis Pentose phosphate pathway Fructolysis Galactolysis Glycosylation N - linked O - linked Nonhuman Photosynthesis Anoxygenic photosynthesis Chemosynthesis Carbon fixation Xylose metabolism Radiotrophism Lipid metabolism ( lipolysis, lipogenesis ) Fatty acid metabolism Fatty acid degradation ( Beta oxidation ) Fatty acid synthesis Other Steroid metabolism Sphingolipid metabolism Eicosanoid metabolism Ketosis Reverse cholesterol transport Amino acid Amino acid synthesis Urea cycle Nucleotide metabolism Purine metabolism Nucleotide salvage Pyrimidine metabolism Other Metal metabolism Iron metabolism Ethanol metabolism Metabolism map Carbon Fixation Photo - respiration Pentose Phosphate Pathway Citric Acid Cycle Glyoxylate Cycle Urea Cycle Fatty Acid Synthesis Fatty Acid Elongation Beta Oxidation Peroxisomal Beta Oxidation Glyco - genolysis Glyco - genesis Glyco - lysis Gluconeo - genesis Decarb - oxylation Fermentation Keto - lysis Keto - genesis feeders to Gluconeo - genesis Direct / C4 / CAM Carbon Intake Light Reaction Oxidative Phosphorylation Amino Acid Deamination Citrate Shuttle Lipogenesis Lipolysis Steroidogenesis MVA Pathway MEP Pathway Shikimate Pathway Transcription & Replication Translation Proteolysis Glycosy - lation Sugar Acids Double / Multiple Sugars & Glycans Simple Sugars Inositol - P Amino Sugars & Sialic Acids Nucleotide Sugars Hexose - P Triose - P Glycerol P - glycerates Pentose - P Tetrose - P Propionyl - CoA Succinate Acetyl - CoA Pentose - P P - glycerates Glyoxylate Photosystems Pyruvate Lactate Acetyl - CoA Citrate Oxalo - acetate Malate Succinyl - CoA α - Keto - glutarate Ketone Bodies Respiratory Chain Serine Group Alanine Branched - chain Amino Acids Aspartate Group Homoserine Group & Lysine Glutamate Group & Proline Arginine Creatine & Polyamines Ketogenic & Glucogenic Amino Acids Amino Acids Shikimate Aromatic Amino Acids & Histidine Ascorbate ( Vitamin C ) δ - ALA Bile Pigments Hemes Cobalamins ( Vitamin B ) Various Vitamin B 's Calciferols ( Vitamin D ) Retinoids ( Vitamin A ) Quinones ( Vitamin K ) & Carotenoids ( Vitamin E ) Cofactors Vitamins & Minerals Antioxidants PRPP Nucleotides Nucleic Acids Proteins Glycoproteins & Proteoglycans Chlorophylls MEP MVA Acetyl - CoA Polyketides Terpenoid Backbones Terpenoids & Carotenoids ( Vitamin A ) Cholesterol Bile Acids Glycero - phospholipids Glycerolipids Acyl - CoA Fatty Acids Glyco - sphingolipids Sphingolipids Waxes Polyunsaturated Fatty Acids Neurotransmitters & Thyroid Hormones Steroids Endo - cannabinoids Eicosanoids Major metabolic pathways in metro - style map. Click any text ( name of pathway or metabolites ) to link to the corresponding article. Single lines : pathways common to most lifeforms. Double lines : pathways not in humans ( occurs in e.g. plants, fungi, prokaryotes ). Orange nodes : carbohydrate metabolism. Violet nodes : photosynthesis. Red nodes : cellular respiration. Pink nodes : cell signaling. Blue nodes : amino acid metabolism. Grey nodes : vitamin and cofactor metabolism. Brown nodes : nucleotide and protein metabolism. Green nodes : lipid metabolism. Metabolism portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Cellular_respiration&oldid=846811056 '' Categories : Cellular respiration Metabolism Plant physiology Hidden categories : Wikipedia indefinitely move - protected pages Articles needing additional references from September 2014 All articles needing additional references Talk View source Contents About Wikipedia বাংলা Bân - lâm - gú Беларуская Български Bosanski Brezhoneg Català Čeština Cymraeg Dansk Deutsch Eesti Ελληνικά Español Euskara فارسی Français Galego 한국어 Հայերեն हिन्दी Hrvatski Bahasa Indonesia Íslenska Italiano עברית Қазақша Kreyòl ayisyen Lietuvių Македонски Bahasa Melayu Nederlands Norsk Occitan Polski Português Română Русский සිංහල Simple English Slovenščina کوردی Српски / srpski Srpskohrvatski / српскохрватски Basa Sunda Suomi Svenska தமிழ் ไทย Türkçe Українська Tiếng Việt 粵語 中文 46 more Edit links This page was last edited on 21 June 2018, at 01 : 35 ( UTC ). 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Question: during which step of cellular respiration is the majority of atp formed
Answer: Krebs cycle