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Metabolic pathways of a Thiodictyon endosymbiont featuring aerobic respiration (orange) and photosynthesis (purple).
Typical metabolic pathways of heterotrophs. Various catabolic processes supply reducing agents (electron donors, such as CoQ10, NADH and FADH2) for the electron transport chain.

Cellular respiration is the overall process of catabolism and oxidative phosphorylation, extracting energy from substrates with the help of an oxidizing agent (terminal electron acceptor) from an external source. It is generally a series of reduction-oxidation reactions coupled with the formation of an electrochemical gradient, which ultimately serves ATP synthase by providing or maintaining a proton motive force, maximizing the amount of adenosine triphosphate (ATP) produced for a unit of substrate.[1]

Cellular respiration is divided into two categories:

  • Aerobic respiration – when molecular oxygen (O2) is used as the terminal electron acceptor.
  • Anaerobic respiration – when ions or molecules other than O2 are used as the terminal electron acceptor.[1]

Anaerobic respiration is not to be confused with fermentation – which is an anaerobic process, but unlike anaerobic respiration, it does not involve an externally sourced electron acceptor; fermentation is not a form of respiration. What sets fermentation apart from cellular respiration, aerobic or otherwise, is that the former produces its own terminal electron acceptor and generally produces ATP mostly by substrate-level phosphorylation, while the latter is characterized by the involvement of ATP synthase and an electron transport chain.[2]

For eukaryotic cells, the most common oxidizing agent in respiration is O2, and the most common substrates (nutrients) include sugars, amino acids and fatty acids. Metabolism of nutrients may provide either energy for regenerating reducing agents (electron donors), like coenzymes such as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD); or metabolic intermediates to be used as electron donors directly, such as succinate. The chemical energy stored in ATP (the bond of its third phosphate group to the rest of the molecule can be broken, allowing more stable products to form, thereby releasing 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.

Aerobic respiration

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Outline of aerobic respiration in eukaryotes, showing the connection between glycolysis in the host's cytoplasm, the citric acid cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane.

Aerobic respiration uses O2 as the terminal electron acceptor, coupling its reduction into two H2O molecules with the pumping of protons in the electron transport chain. This happens within cytochrome c oxidase (Complex IV). This pathway may also involve alternative oxidase, providing an alternative pathway that is resistant to cyanide poisoning.[3] Other forms of aerobic respiration may use plastid terminal oxidase, which is involved in chlororespiration.[4]

The potential of NADH and FADH2 is converted to more ATP through an electron transport chain with O2 and protons (hydrogen ions) as the terminal electron acceptors. Most of the ATP produced by aerobic respiration is made by oxidative phosphorylation. The energy released is used to create an electrochemical gradient by pumping protons across a membrane. This potential is then used to drive ATP synthase, producing ATP from ADP and phosphate.

Biology textbooks often state that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from oxidative phosphorylation).[5] 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.[5]

Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism.[citation needed] However, some anaerobic organisms, such as methanogens, are able to continue with anaerobic respiration, yielding more ATP by using inorganic molecules other than O2 as final electron acceptors in the electron transport chain. They share the initial pathway of glycolysis. The post-glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells.[6]

Although plants are net consumers of carbon dioxide, plant respiration accounts for about half of the CO2 generated annually by terrestrial ecosystems.[7][8]: 87 

Glycolysis

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Main article: Glycolysis

Glycolysis is a metabolic pathway that takes place in the cytosol of cells in all living organisms[9][better source needed] and occurs regardless of O2's presence or absence. 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, but 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 four ADP by substrate-level phosphorylation to make four ATP, and two NADH are also produced during the pay-off phase. The overall reaction can be expressed this way:[10]

Glucose + 2 NAD+ + 2 Pi + 2 ADP → 2 pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O + energy

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-bisphosphate by the help of phosphofructokinase. Fructose 1,6-biphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate.[8]: 88–90 

Oxidative decarboxylation of pyruvate

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Main article: Pyruvate decarboxylation

Pyruvate is oxidized to acetyl-CoA and CO2 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 CO2 is formed.[11]

Citric acid cycle

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Main article: Citric acid cycle

The citric acid cycle 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 CO2 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 low-energy waste products, H2O and CO2, are created during this cycle.[12][13]

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.[14]

The net gain from one cycle is 3 NADH and 1 FADH2 as hydrogen (proton plus electron) carrying compounds and 1 high-energy GTP, which may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH2, and 2 ATP.[12][13][8]: 90–91 

Oxidative phosphorylation

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Main articles: Oxidative phosphorylation, Electron transport chain, Electrochemical gradient, and ATP synthase
Diagram of oxidative phosphorylation

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 the 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.[15]

  1. ^ a b Kelly, David J.; Hughes, Nicky J.; Poole, Robert K. (2001), Mobley, Harry LT; Mendz, George L.; Hazell, Stuart L. (eds.), "Microaerobic Physiology: Aerobic Respiration, Anaerobic Respiration, and Carbon Dioxide Metabolism", Helicobacter pylori: Physiology and Genetics, Washington (DC): ASM Press, ISBN 978-1-55581-213-3, PMID 21290713, retrieved 2025-07-16
  2. ^ Hackmann, Timothy J (2024-07-01). "The vast landscape of carbohydrate fermentation in prokaryotes". FEMS Microbiology Reviews. 48 (4): fuae016. doi:10.1093/femsre/fuae016. ISSN 0168-6445. PMC 11187502.
  3. ^ Moore, Anthony L.; Siedow, James N. (August 1991). "The regulation and nature of the cyanide-resistant alternative oxidase of plant mitochondria". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1059 (2): 121–140. doi:10.1016/S0005-2728(05)80197-5.
  4. ^ Houille-Vernes, Laura; Rappaport, Fabrice; Wollman, Francis-André; Alric, Jean; Johnson, Xenie (2011-12-20). "Plastid terminal oxidase 2 (PTOX2) is the major oxidase involved in chlororespiration in Chlamydomonas". Proceedings of the National Academy of Sciences. 108 (51): 20820–20825. doi:10.1073/pnas.1110518109. ISSN 0027-8424. PMC 3251066.
  5. ^ a b 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.
  6. ^ Buckley, Gabe (2017-01-12). "Krebs Cycle - Definition, Products and Location". Biology Dictionary. Retrieved 2025-01-31.
  7. ^ O'Leary, Brendan M.; Plaxton, William C. (2016). "Plant Respiration". eLS. pp. 1–11. doi:10.1002/9780470015902.a0001301.pub3. ISBN 9780470016176.
  8. ^ a b c Mannion, A. M. (12 January 2006). Carbon and Its Domestication. Springer. ISBN 978-1-4020-3956-0.
  9. ^ Reece, Jane; Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Jackson, Robert (2010). Campbell Biology Ninth Edition. Pearson Education, Inc. p. 168.
  10. ^ Chaudhry, Raheel; Varacallo, Matthew A. (2025), "Biochemistry, Glycolysis", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 29493928, retrieved 2025-01-31
  11. ^ Sapkota, Anupama (2024-10-17). "Krebs Cycle: Steps, Enzymes, Products & Diagram". microbenotes.com. Retrieved 2025-02-01.
  12. ^ a b R. Caspi (2012-11-14). "Pathway: TCA cycle III (animals)". MetaCyc Metabolic Pathway Database. Retrieved 2022-06-20.
  13. ^ a b R. Caspi (2011-12-19). "Pathway: TCA cycle I (prokaryotic)". MetaCyc Metabolic Pathway Database. Retrieved 2022-06-20.
  14. ^ Haddad, Aida; Mohiuddin, Shamim S. (2025), "Biochemistry, Citric Acid Cycle", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 31082116, retrieved 2025-02-01
  15. ^ Deshpande, Ojas A.; Mohiuddin, Shamim S. (2025), "Biochemistry, Oxidative Phosphorylation", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 31985985, retrieved 2025-02-01
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