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(Redirected from Terminal electron acceptor) An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are. I use 'xos4 Terminus' for my terminal and some other stuff. Whenever I start an electron app (eg: discord, slack, spotify) its seems like it deletes terminus from my font list. So if I open a terminal afterwards the font is wrong. I don't know what font it is then, some default, but it's not xos4 Terminus. Terminus will run as a portable app on Windows, if you create a datafolder in the same location where Terminus.exelives. A terminal for a more modern age. Contribute to Eugeny/terminus development by creating an account on GitHub.

(Redirected from Sulfate-reducing bacteria)
Desulfovibrio vulgaris is the best-studied sulfate-reducing microorganism species; the bar in the upper right is 0.5 micrometre long.

Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42–) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S).[1][2] Therefore, these sulfidogenic microorganisms 'breathe' sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.

Most sulfate-reducing microorganisms can also reduce some other oxidized inorganic sulfurcompounds, such as sulfite (SO32–), dithionite (S2O42–), thiosulfate (S2O32–), trithionate (S3O62–), tetrathionate (S4O62−), elemental sulfur (S8), and polysulfides (Sn2−). Depending on the context, 'sulfate-reducing microorganisms' can be used in a broader sense (including all species that can reduce any of these sulfur compounds) or in a narrower sense (including only species that reduce sulfate, and excluding strict thiosulfate and sulfur reducers, for example).

Sulfate-reducing microorganisms can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microbes, having contributed to the sulfur cycle soon after life emerged on Earth.[3]

Many organisms reduce small amounts of sulfates in order to synthesize sulfur-containing cell components; this is known as assimilatory sulfate reduction. By contrast, the sulfate-reducing microorganisms considered here reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste; this is known as dissimilatory sulfate reduction.[4] They use sulfate as the terminal electron acceptor of their electron transport chain.[5] Most of them are anaerobes; however, there are examples of sulfate-reducing microorganisms that are tolerant of oxygen, and some of them can even perform aerobic respiration.[6] No growth is observed when oxygen is used as the electron acceptor.[7]In addition, there are sulfate-reducing microorganisms that can also reduce other electron acceptors, such as fumarate, nitrate (NO3−), nitrite (NO2−), ferriciron [Fe(III)], and dimethyl sulfoxide (DMSO).[1][8]

In terms of electron donor, this group contains both organotrophs and lithotrophs. The organotrophs oxidizeorganic compounds, such as carbohydrates, organic acids (e.g., formate, lactate, acetate, propionate, and butyrate), alcohols (methanol and ethanol), aliphatic hydrocarbons (including methane), and aromatic hydrocarbons (benzene, toluene, ethylbenzene, and xylene).[9] The lithotrophs oxidize molecular hydrogen (H2), for which they compete with methanogens and acetogens in anaerobic conditions.[9] Some sulfate-reducing microorganisms can directly utilize metallic iron [Fe(0)] (zerovalent iron, or ZVI) as electron donor, oxidizing it to ferrous iron [Fe(II)].[10]

Ecological importance and markers[edit]

Sulfate occurs widely in seawater, sediment, and water rich in decaying organic material.[5] Sulfate is also found in more extreme environments such as hydrothermal vents, acid-mine drainage sites, oil fields, and the deep subsurface,[11] including the world's oldest isolated ground water.[12][13] Sulfate-reducing microorganisms are common in anaerobic environments where they aid in the degradation of organic materials.[14] In these anaerobic environments, fermenting bacteria extract energy from large organic molecules; the resulting smaller compounds such as organic acids and alcohols are further oxidized by acetogens and methanogens and the competing sulfate-reducing microorganisms.[5]

Sludge from a pond; the black color is due to metal sulfides that result from the action of sulfate-reducing microorganisms.

The toxic hydrogen sulfide is a waste product of sulfate-reducing microorganisms; its rotten egg odor is often a marker for the presence of sulfate-reducing microorganisms in nature.[14] Sulfate-reducing microorganisms are responsible for the sulfurous odors of salt marshes and mud flats. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides. These metal sulfides, such as ferrous sulfide (FeS), are insoluble and often black or brown, leading to the dark color of sludge.[2]

During the Permian–Triassic extinction event (250 million years ago) a severe anoxic event seems to have occurred where these forms of bacteria became the dominant force in oceanic ecosystems, producing copious amounts of hydrogen sulfide.[15]

Sulfate-reducing bacteria also generate neurotoxic methylmercury as a byproduct of their metabolism, through methylation of inorganic mercury present in their surroundings. They are known to be the dominant source of this bioaccumulative form of mercury in aquatic systems.[16]

Uses[edit]

Some sulfate-reducing microorganisms can reduce hydrocarbons, and they have been used to clean up contaminated soils. Their use has also been proposed for other kinds of contaminations.[3]

Sulfate-reducing microorganisms are considered as a possible way to deal with acid mine waters that are produced by other microorganisms.[17]

Problems caused by sulfate-reducing microorganisms[edit]

In engineering, sulfate-reducing microorganisms can create problems when metal structures are exposed to sulfate-containing water: Interaction of water and metal creates a layer of molecular hydrogen on the metal surface; sulfate-reducing microorganisms then oxidize the hydrogen while creating hydrogen sulfide, which contributes to corrosion.

Hydrogen sulfide from sulfate-reducing microorganisms also plays a role in the biogenic sulfide corrosion of concrete. It also occurs in sour crude oil.[3]

Some sulfate-reducing microorganisms play a role in the anaerobic oxidation of methane:[3]

CH4 + SO42− → HCO3− + HS− + H2O

An important fraction of the methane formed by methanogens below the seabed is oxidized by sulfate-reducing microorganisms in the transition zone separating the methanogenesis from the sulfate reduction activity in the sediments. This process is also considered a major sink for sulfate in marine sediments.

In hydraulic fracturing, fluids are used to frack shaleformations to recover methane (shale gas) and hydrocarbons. Biocide compounds are often added to water to inhibit the microbial activity of sulfate-reducing microorganisms, in order to but not limited to, avoid anaerobic methane oxidation and the generation of hydrogen sulfide, ultimately resulting in minimizing potential production loss.

Biochemistry[edit]

Before sulfate can be used as an electron acceptor, it must be activated. This is done by the enzyme ATP-sulfurylase, which uses ATP and sulfate to create adenosine 5'-phosphosulfate (APS). APS is subsequently reduced to sulfite and AMP. Sulfite is then further reduced to sulfide, while AMP is turned into ADP using another molecule of ATP. The overall process, thus, involves an investment of two molecules of the energy carrier ATP, which must to be regained from the reduction.[1]

Overview of the three key enzymatic steps of the dissimilatory sulfate reduction pathway. Enzymes: sat and atps respectively stand for sulfate adenylyltransferase and ATP sulfurylase (EC 2.7.7.4); apr and aps are both used to adenosine-5'-phosphosulfate reductase (EC 1.8.4.8); and dsr is the dissimilatory (bi)sulfite reductase (EC 1.8.99.5);

The enzyme dissimilatory (bi)sulfite reductase, dsrAB (EC 1.8.99.5), that catalyzes the last step of dissimilatory sulfate reduction, is the functional gene most used as a molecular marker to detect the presence of sulfate-reducing microorganisms.[18]

Phylogeny[edit]

The sulfate-reducing microorganisms have been treated as a phenotypic group, together with the other sulfur-reducing bacteria, for identification purposes. They are found in several different phylogenetic lines.[19] As of 2009, 60 genera containing 220 species of sulfate-reducing bacteria are known.[3]

Among the Deltaproteobacteria the orders of sulfate-reducing bacteria include Desulfobacterales, Desulfovibrionales, and Syntrophobacterales. This accounts for the largest group of sulfate-reducing bacteria, about 23 genera.[1]

The second largest group of sulfate-reducing bacteria is found among the Firmicutes, including the genera Desulfotomaculum, Desulfosporomusa, and Desulfosporosinus.

In the Nitrospirae division we find sulfate-reducing Thermodesulfovibrio species.

Two more groups that include thermophilic sulfate-reducing bacteria are given their own phyla, the Thermodesulfobacteria and Thermodesulfobium.

There are also three known genera of sulfate-reducing archaea: Archaeoglobus, Thermocladium and Caldivirga. They are found in hydrothermal vents, oil deposits, and hot springs.

In July 2019, a scientific study of Kidd Mine in Canada discovered sulfate-reducing microorganisms living 7,900 feet (2,400 m) below the surface. The sulfate reducers discovered in Kidd Mine are lithotrophs, obtaining their energy by oxidizing minerals such as pyrite rather than organic compounds.[20][21][22] Kidd Mine is also the site of the oldest known water on Earth.[23]

See also[edit]

References[edit]

  1. ^ abcdMuyzer, G.; Stams, A. J. (June 2008). 'The ecology and biotechnology of sulphate-reducing bacteria'(PDF). Nature Reviews Microbiology. 6 (6): 441–454. doi:10.1038/nrmicro1892. PMID18461075. S2CID22775967. Archived from the original(PDF) on 2012-04-25.
  2. ^ abErnst-Detlef Schulze; Harold A. Mooney (1993), Biodiversity and ecosystem function, Springer-Verlag, pp. 88–90, ISBN9783540581031
  3. ^ abcdeBarton, Larry L. & Fauque, Guy D. (2009). Biochemistry, Physiology and Biotechnology of Sulfate-Reducing Bacteria. Advances in Applied Microbiology. 68. pp. 41–98. doi:10.1016/s0065-2164(09)01202-7. ISBN9780123748034. PMID19426853.
  4. ^Rückert, Christian (2016). 'Sulfate reduction in microorganisms—recent advances and biotechnological applications'. Current Opinion in Microbiology. 33: 140–146. doi:10.1016/j.mib.2016.07.007. PMID27461928.
  5. ^ abcLarry Barton (ed.) (1995), Sulfate-reducing bacteria, Springer, ISBN9780306448577CS1 maint: extra text: authors list (link)
  6. ^Kasper U. Kjeldsen; Catherine Joulian & Kjeld Ingvorsen (2004). 'Oxygen Tolerance of Sulfate-Reducing Bacteria in Activated Sludge'. Environmental Science and Technology. 38 (7): 2038–2043. Bibcode:2004EnST...38.2038K. doi:10.1021/es034777e. PMID15112804.
  7. ^'Simone Dannenberg; Michael Kroder; Dilling Waltraud & Heribert Cypionka (1992). 'Oxidation of H2, organic compounds and inorganic sulfur compounds coupled to reduction of O2 or nitrate by sulfate-reducing bacteria'. Archives of Microbiology. 158 (2): 93–99. doi:10.1007/BF00245211. S2CID36923153.
  8. ^Plugge, Caroline M.; Zhang, Weiwen; Scholten, Johannes C. M.; Stams, Alfons J. M. (2011). 'Metabolic Flexibility of Sulfate-Reducing Bacteria'. Frontiers in Microbiology. 2: 81. doi:10.3389/fmicb.2011.00081. ISSN1664-302X. PMC3119409. PMID21734907.
  9. ^ abLiamleam, Warounsak; Annachhatre, Ajit P. (2007). 'Electron donors for biological sulfate reduction'. Biotechnology Advances. 25 (5): 452–463. doi:10.1016/j.biotechadv.2007.05.002. PMID17572039.
  10. ^Kato, Souichiro (2016-03-01). 'Microbial extracellular electron transfer and its relevance to iron corrosion'. Microbial Biotechnology. 9 (2): 141–148. doi:10.1111/1751-7915.12340. ISSN1751-7915. PMC4767289. PMID26863985.
  11. ^Muyzer G, Stams AJ (June 2008). 'The ecology and biotechnology of sulphate-reducing bacteria'. Nature Reviews. Microbiology. 6 (6): 441–54. doi:10.1038/nrmicro1892. PMID18461075. S2CID22775967.
  12. ^Lollar, Garnet S.; Warr, Oliver; Telling, Jon; Osburn, Magdalena R.; Lollar, Barbara Sherwood (18 July 2019). ''Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory'. Geomicrobiology Journal. 36 (10): 859–872. doi:10.1080/01490451.2019.1641770. S2CID199636268.
  13. ^'World's Oldest Groundwater Supports Life Through Water-Rock Chemistry'. Deep Carbon Observatory. 29 July 2019. Retrieved 13 September 2019.
  14. ^ abDexter Dyer, Betsey (2003). A Field Guide to Bacteria. Comstock Publishing Associates/Cornell University Press.
  15. ^Peter D. Ward (October 2006), 'Impact from the Deep', Scientific American
  16. ^G.C. Compeau & R. Bartha (August 1985), 'Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment', Applied and Environmental Microbiology, 50 (2): 498–502, doi:10.1128/AEM.50.2.498-502.1985, PMC238649, PMID16346866
  17. ^Ayangbenro, Ayansina S.; Olanrewaju, Oluwaseyi S.; Babalola, Olubukola O. (22 August 2018). 'Sulfate-Reducing Bacteria as an Effective Tool for Sustainable Acid Mine Bioremediation'. Frontiers in Microbiology. 9: 1986. doi:10.3389/fmicb.2018.01986. PMC6113391. PMID30186280.
  18. ^Müller, Albert Leopold; Kjeldsen, Kasper Urup; Rattei, Thomas; Pester, Michael; Loy, Alexander (2014-10-24). 'Phylogenetic and environmental diversity of DsrAB-type dissimilatory (bi)sulfite reductases'. The ISME Journal. 9 (5): 1152–1165. doi:10.1038/ismej.2014.208. ISSN1751-7370. PMC4351914. PMID25343514.
  19. ^Pfennig N.; Biebel H. (1986), 'The dissimilatory sulfate-reducing bacteria', in Starr; et al. (eds.), The Prokaryotes: a handbook on habitats, isolation and identification of bacteria, Springer
  20. ^'Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory, Garnet S. Lollar, Oliver Warr, Jon Telling, Magdalena R. Osburn & Barbara Sherwood Lollar, Received 15 Jan 2019, Accepted 01 Jul 2019, Published online: 18 Jul 2019.
  21. ^World's Oldest Groundwater Supports Life Through Water-Rock Chemistry, July 29, 2019, deepcarbon.net.
  22. ^Strange life-forms found deep in a mine point to vast 'underground Galapagos', By Corey S. Powell, Sept. 7, 2019, nbcnews.com.
  23. ^Oldest Water on Earth Found Deep Within the Canadian Shield, December 14, 2016, Maggie Romuld

External links[edit]

  • 'Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory, Garnet S. Lollar, Oliver Warr, Jon Telling, Magdalena R. Osburn & Barbara Sherwood Lollar, Received 15 Jan 2019, Accepted 01 Jul 2019, Published online: 18 Jul 2019.
  • Deep fracture fluids isolated in the crust since the Precambrian era, G. Holland, B. Sherwood Lollar, L. Li, G. Lacrampe-Couloume, G. F. Slater & C. J. Ballentine, Nature volume 497, pages 357–360 (16 May 2013)
  • Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks, by L. Li, B. A. Wing, T. H. Bui, J. M. McDermott, G. F. Slater, S. Wei, G. Lacrampe-Couloume & B. Sherwood Lollar October 27, 2016. Nature Communications volume 7, Article number: 13252 (2016.)
  • Earth's mysterious 'deep biosphere' may harbor millions of undiscovered species, By Brandon Specktor, Live Science, December 11, 2018, published online at nbcnews.com.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Sulfate-reducing_microorganism&oldid=1019519328'

Learning Outcomes

  • Describe the respiratory chain (electron transport chain) and its role in cellular respiration

You have just read about two pathways in cellular respiration—glycolysis and the citric acid cycle—that generate ATP. However, most of the ATP generated during the aerobic catabolism of glucose is not generated directly from these pathways. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions: the electron transport chain. This causes hydrogen ions to accumulate within the matrix space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the matrix space by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP.

Figure 1. The electron transport chain is 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.

The electron transport chain (Figure 1) is the last component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plants; in animals, it enters the body through the respiratory system. Electron transport is a series of redox reactions that resemble a relay race or bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where the electrons reduce molecular oxygen, producing water. There are four complexes composed of proteins, labeled I through IV in Figure 1, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes. Note, however, that the electron transport chain of prokaryotes may not require oxygen as some live in anaerobic conditions. The common feature of all electron transport chains is the presence of a proton pump to create a proton gradient across a membrane.

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Complex I

To start, two electrons are carried to the first complex aboard NADH. This complex, labeled I, is composed of flavin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin B2, also called riboflavin, is one of several prosthetic groups or co-factors in the electron transport chain. A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic, non-peptide molecules bound to a protein that facilitate its function; prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is a very large protein, containing 45 amino acid chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space, and it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane.

Q and Complex II

Complex II directly receives FADH2, which does not pass through complex I. The compound connecting the first and second complexes to the third is ubiquinone (Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QH2), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I and the electrons derived from FADH2 from complex II, including succinate dehydrogenase. This enzyme and FADH2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.

Complex III

Terminals Electricity

The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe++ (reduced) and Fe+++ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).

Complex IV

The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the two cytochromes, a, and a3) and three copper ions (a pair of CuA and one CuB in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis.

Chemiosmosis

In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.

If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase (Figure 2). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.

Practice Question

Terminus Electronics

Figure 2. ATP synthase is a complex, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi). (Credit: modification of work by Klaus Hoffmeier)

Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight-loss drug. What effect would you expect DNP to have on the change in pH across the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug?

Show Answer
After DNP poisoning, the electron transport chain can no longer form a proton gradient, and ATP synthase can no longer make ATP. DNP is an effective diet drug because it uncouples ATP synthesis; in other words, after taking it, a person obtains less energy out of the food he or she eats. Interestingly, one of the worst side effects of this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed, the energy from electron transport is lost as heat.

Chemiosmosis (Figure 3) is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium, and water is formed.

Practice Question

Figure 3. In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP synthase to form ATP.

Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What effect would cyanide have on ATP synthesis?

Show Answer
After cyanide poisoning, the electron transport chain can no longer pump electrons into the intermembrane space. The pH of the intermembrane space would increase, the pH gradient would decrease, and ATP synthesis would stop.

ATP Yield

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The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are picked up on the inside of mitochondria by either NAD+ or FAD+. As you have learned earlier, these FAD+ molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD+ acts as a carrier. NAD+ is used as the electron transporter in the liver and FAD+ acts in the brain.

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Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways. Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose.

In Summary: Electron Transport Chain

The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them. The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the chain by either NADH or FADH2 complete the chain, as low-energy electrons reduce oxygen molecules and form water. The level of free energy of the electrons drops from about 60 kcal/mol in NADH or 45 kcal/mol in FADH2 to about 0 kcal/mol in water. The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways.

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