Cellular respiration: Difference between revisions
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'''Cellular respiration''' describes some boring stuff no one cares about. |
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== Error 404 == |
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Fuel molecules commonly used by cells in respiration include [[glucose]], [[amino acids]] and [[fatty acids]], and a common [[oxidizing agent]] ([[electron acceptor]]) is molecular [[oxygen]] (O<sub>2</sub>). There are organisms, however, that can respire using other [[organic compound|organic molecules]] as electron acceptors instead of oxygen. Organisms that use oxygen as a final electron acceptor in respiration are described as [[aerobic]], while those that do not are referred to as [[Anaerobic organism|anaerobic]]. |
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Please restart your cmputer. |
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The energy released in respiration is used to synthesize molecules that act as a chemical storage of this energy. One of the most widely used compounds in a cell is [[adenosine triphosphate]] (ATP) and its stored chemical energy can be used for many processes requiring energy, including [[biosynthesis]], [[locomotion]] or transportation of molecules across [[cell membrane]]s. Because of its ubiquitous nature, ATP is also known as the "universal energy currency", since the amount of it in a cell indicates how much energy is available for energy-consuming processes. |
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==Aerobic respiration== |
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''Aerobic respiration'' requires [[oxygen]] in order to generate energy ([[Adenosine triphosphate|ATP]]). It is the preferred method of [[pyruvate]] breakdown from [[glycolysis]] and requires that pyruvate enter the [[mitochondrion]] to be fully oxidized by the [[citric acid cycle|Krebs cycle]]. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by [[substrate-level phosphorylation]], [[NADH]] and [[FADH2|FADH<sub>2</sub>]]. |
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'''Simplified Reaction:''' C<sub>6</sub>H<sub>12</sub>O<sub>6 (aq)</sub> + 6O<sub>2 (g)</sub> → 6CO<sub>2 (g)</sub> + 6H<sub>2</sub>O<sub> (l)</sub> ΔH<sub>c</sub> -2880 kJ |
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The reducing potential of NADH and FADH<sub>2</sub> 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 [[proton]]s across a membrane. This potential is then used to drive ATP synthase and produce ATP from [[adenosine diphosphate|ADP]]. Biology textbooks often state that between 36-38 ATP molecules can be made per oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 32-34 from the electron transport system).{{Fact|date=January 2008}} Generally, 38 ATP molecules are formed from aerobic respiration.{{Fact|date=January 2008}} However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondrial matrix.{{Fact|date=January 2008}} |
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Aerobic metabolism is 19 times more efficient than anaerobic metabolism (which yields 2 mol ATP per 1 mol glucose). 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 [[eukaryote|eukaryotic cell]]s, and in the [[cytoplasm]] in [[prokaryote|prokaryotic cell]]s. |
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===Glycolysis=== |
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{{main|Glycolysis}} |
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Glycolysis is a [[metabolic pathway]] that is found in the cytoplasm of cells in all living organisms and does not require [[oxygen]]. The process converts one molecule of [[glucose]] into two molecules of [[pyruvate]], and makes energy in the form of two net molecules of [[Adenosine triphosphate|ATP]]. Four molecules of ATP per glucose are actually produced; however, two are consumed for the [[Glycolysis#Preparatory phase|preparatory phase]]. The initial [[phosphorylation]] of glucose is required to destabilize the molecule for cleavage into two [[triose]] [[sugar]]s. During the [[Glycolysis#Pay-off phase|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 triose sugars are oxidized. Glycolysis takes place in the [[cytoplasm]] of the [[cell (biology)|cell]]. The overall reaction can be expressed this way: |
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:Glucose + 2 NAD<sup>+</sup> + 2 P<sub>i</sub> + 2 ADP → 2 [[pyruvate]] + 2 NADH + 2 ATP + 2 H<sub>2</sub>O |
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===Oxidative decarboxylation of pyruvate=== |
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{{main|Pyruvate decarboxylation}} |
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The pyruvate is oxidized to acetyl-CoA and CO<sub>2</sub> by the [[Pyruvate dehydrogenase complex]], a cluster of enzymes—multiple copies of each of three enzymes—located in the [[mitochondrial matrix|mitochondria]] of eukaryotic cells and in the cytosol of prokaryotes. In the process one molecule of NADH is formed per pyruvate oxidized. |
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===Citric Acid cycle=== |
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{{main|Citric acid cycle}} |
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This is also called the ''Krebs cycle'' or the ''tricarboxylic acid cycle''. When oxygen is present, [[acetyl-CoA]] is produced from pyruvate. If oxygen is not present the cell undergoes fermentation of the pyruvate molecule. If [[acetyl-CoA]] is produced the molecule then enters the [[citric acid cycle]] (Krebs cycle) inside the mitochondrial matrix, and gets oxidized to [[CO2]] while at the same time reducing [[Nicotinamide adenine dinucleotide|NAD]] to [[NADH]]. [[NADH]] can be used by the [[electron transport chain]] to create further [[Adenosine triphosphate|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 [[cellular waste product|waste products]], H<sub>2</sub>O and CO<sub>2</sub>, are created during this cycle. |
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===Oxidative phosphorylation=== |
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{{main|Oxidative phosphorylation|Electron transport chain|Electrochemical gradient|ATP synthase}} |
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In eukaryotes, oxidative phosphorylation occurs in the mitochondrial [[crista]]e. It comprises the electron transport chain that establishes a [[proton gradient]] (chemiosmotic potential) across the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. |
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==Theoretical yields== |
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The yields in the table below are for one glucose molecule being fully oxidized into carbon dioxide. It is assumed that all the [[reduction (chemistry)|reduced]] [[coenzyme]]s are oxidized by the electron transport chain and used for oxidative phosphorylation. |
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{| class="wikitable" |
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!Step |
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!coenzyme yield |
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!ATP yield |
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!Source of ATP |
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|- |
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|style="border-top: solid 3px #aaaaaa;" |Glycolysis preparatory phase |
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|style="border-top: solid 3px #aaaaaa;" | |
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|style="border-top: solid 3px #aaaaaa;" align="center"| -2 |
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|style="border-top: solid 3px #aaaaaa;" |Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm. |
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|- |
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|rowspan="2"|Glycolysis pay-off phase |
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| |
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|align="center"|4 |
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|Substrate-level phosphorylation |
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|- |
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|align="center"|2 NADH |
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|align="center"|4 (6) |
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|Oxidative phosphorylation. Only 2 ATP per NADH since the coenzyme must feed into the electron transport chain from the cytoplasm rather than the mitochondrial matrix. If the [[malate shuttle]] is used to move NADH into the mitochondria this might count as 3 ATP per NADH. |
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|- |
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|Oxidative decarboxylation of pyruvate |
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|align="center"| 2 NADH |
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|align="center"| 6 |
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|Oxidative phosphorylation |
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|- |
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|rowspan="3"|Krebs cycle |
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| |
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| align="center"|2 |
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|Substrate-level phosphorylation |
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|- |
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| align="center"|6 NADH |
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| align="center"|18 |
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| Oxidative phosphorylation |
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|- |
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| align="center"|2 FADH<sub>2</sub> |
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| align="center"|4 |
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| Oxidative phosphorylation |
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|- |
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|style="border-top: solid 3px #aaaaaa;" colspan="2" align="center"|'''Total yield''' |
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|style="border-top: solid 3px #aaaaaa;" align="center"|'''36 (38) ATP''' |
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|style="border-top: solid 3px #aaaaaa;" |From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes. |
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|} |
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Although there is a theoretical yield of 36-38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to 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 utilise the stored energy in the proton [[electrochemical gradient]]. |
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*The '''[[pyruvate carrier]]''' is a [[symporter]] and the driving force for moving pyruvate into the mitochondria is the movement of protons from the [[intermembrane space]] to the matrix. |
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*The '''[[phosphate carrier]]''' is an [[antiporter]] and the driving force for moving phosphate ions into the mitochondria is the movement of hydroxyls ions from the matrix to the intermembrane space. |
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*The '''[[adenine nucleotide carrier]]''' 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. |
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The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H<sup>+</sup> 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.<ref>{{cite journal |author=Rich PR |title=The molecular machinery of Keilin's respiratory chain |journal=Biochem. Soc. Trans. |volume=31 |issue=Pt 6 |pages=1095-105 |year=2003 |pmid=14641005 |url=http://www.biochemsoctrans.org/bst/031/1095/bst0311095.htm}}</ref> In practice the efficiency may be even lower due to the inner membrane of the mitochondria being slightly leaky to protons.<ref>{{cite journal |author=Porter RK, Brand MD |title=Mitochondrial proton conductance and H+/O ratio are independent of electron transport rate in isolated hepatocytes |journal=Biochem. J. |volume=310 ( Pt 2) |issue= |pages=379-82 |year=1995 |pmid=7654171 |url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1135905&blobtype=pdf}}</ref> 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 synthase|ATP synthesis]]. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in a baby's brown fat, for thermogenesis, and hibernating animals. |
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==Anaerobic respiration== |
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{{main|Anaerobic respiration}} |
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Without oxygen, pyruvate is not metabolized by cellular respiration but undergoes a process of [[fermentation (biochemistry)|fermentation]]. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to [[cellular waste product|waste products]] that may be removed from the cell. This serves the purpose of oxidizing the hydrogen carriers so that they can perform glycolysis again and removing the excess pyruvate. 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 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 phosphorylation'', which is phosphorylation that does not involve oxygen. |
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Anaerobic respiration is less efficient at using the energy from glucose since 2 ATP are produced during anaerobic respiration per glucose, compared to the 30 ATP per glucose produced by aerobic respiration. This is because the [[cellular waste product|waste products]] of anaerobic respiration still contain plenty of energy. [[Ethanol]], for example, can be used in gasoline (petrol) solutions. 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. Thus, during short bursts of strenuous activity, muscle cells use anaerobic respiration to supplement the ATP production from the slower aerobic respiration, so anaerobic respiration 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 [[Sprint (race)|sprint]]ing. |
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==Efficiency of aerobic and anaerobic respiration== |
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'''Aerobic respiration''' |
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During aerobic respiration 38 molecules of ATP are produced for every molecule of glucose that is oxidised. |
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C<sub>6</sub>H<sub>12</sub>O<sub>6 (aq)</sub> + 6O<sub>2 (g)</sub> → 6CO<sub>2 (g)</sub> + 6H<sub>2</sub>O<sub> (l)</sub> + 38 ATP |
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The energy released by the complete oxidation of glucose is 2880KJ per mole. The |
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energy contained in 1 mole of ATP is 30.6KJ. Therefore the energy contained in 38 |
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moles of ATP is 30.6×38=1162.8 kj. Therefore efficiency of transfer of energy in aerobic respiration is=1162.8/2880=40.4%. |
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'''Anaerobic respiration''' |
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(1) Yeast (alcoholic fermentation). During alcoholic fermentation, two molecules of ATP are produced. for every molecule of glucose used. |
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glucose → 2ethanol + 26CO<sub>2 (g)</sub> +2 ATP |
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The total energy released by the conversion of glucose to ethanol is 210kj per mole. |
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The energy contained in 2 molecules of ATP is 2×30.6=61.2kj.Therefore efficiency of |
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transfer of energy during alcoholic fermentation is 61.2/210=29.1%. |
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(2) Muscle (lactate fermentation). During lactate fermentation, 2 molecules of ATP |
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are produced for every molecule of glucose used. |
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glucose → 2 lactate + 2ATP |
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The total energy released by conversion of glucose to lactate is 150kj per mole. |
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Therefore efficiency of transfer of energy in lactic acid fermentation is |
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61.2/150=40.8%. |
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The amount of energy captured as ATP during aerobic respiration is 19 times as much |
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as for anaerobic respiration.From this point of view Aerobic respiration is more |
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efficient than anaerobic respiration.This is because a great deal of energy remains |
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locked within lactate and ethanol. |
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Extract from 'Biological Science' by D.J. Taylor, N.P.O Green and G.W. Stout, Cambridge University Press, ISBN 0 521 639239 |
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==See also== |
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*[[Tetrazolium chloride]]: cellular respiration indicator |
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==References== |
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{{Reflist}} |
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{{metabolism}} |
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==External links== |
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* [http://departments.oxy.edu/biology/bio130/lectures_2000/metabolic_products.htm Chart of Important Metabolic Products] |
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* [http://www.ufp.pt/~pedros/bq/respi.htm A detailed description of respiration vs. fermentation] |
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* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellularRespiration.html Kimball's online resource to cellular respiration] |
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* [http://biology.clc.uc.edu/courses/bio104/cellresp.htm Cellular Respiration and Fermentation] at Clermont College |
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[[Category:Cellular respiration]] |
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[[Category:Metabolism]] |
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[[Category:Plant physiology]] |
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[[cs:Buněčné dýchání]] |
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[[da:Aerob respiration]] |
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[[de:Zellatmung]] |
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[[es:Respiración celular]] |
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[[fr:Respiration cellulaire]] |
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[[it:Respirazione cellulare]] |
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[[he:נשימה תאית]] |
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[[mk:Клеточно дишење]] |
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[[ms:Respirasi sel]] |
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[[nl:Celademhaling]] |
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[[no:Celleånding]] |
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[[pl:Oddychanie komórkowe]] |
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[[pt:Respiração celular]] |
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[[ru:Клеточное дыхание]] |
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[[simple:Cellular respiration]] |
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[[sl:Celično dihanje]] |
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[[fi:Soluhengitys]] |
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Revision as of 12:27, 13 May 2008
Error 404
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