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In chemistry or physics, '''abundance''' or '''natural abundance''' refers to the amount of a chemical element isotope existing in nature. The abundance of an isotope on the Earth may vary depending on the place, but remains relatively constant in time (on a short-term scale). In a chemical reaction, the reactant is in abundance when the quantity of a substance is enough (or high) and constant during the reaction. '''Relative abundance''' represents the percentage of the total amount of all isotopes of the element. The relative abundance of each isotope in a sample can be identified using mass spectrometry.  +
'''Acceleration''', '''''a''''', is the change of [[velocity]] over time [m·s<sup>-2</sup>]. '''''a''''' = d'''''v'''''/d''t'' The symbol ''g'' is used for acceleration of free fall. The standard acceleration of free fall is defined as ''g''<sub>n</sub> = 9.80665 [m·s<sup>-2</sup>].  +
'''Acclimation''' is an immediate time scale adaption expressing phenotypic plasticity in response to changes of a single variable under controlled laboratory conditions.  +
'''Acclimatization''' is an immediate time scale adaption expressing phenotypic plasticity in response to changes of habitat conditions and life style where several variables may change simultaneously.  +
The '''accuracy''' of a method is the degree of agreement between an individual test result generated by the method and the true value.  +
[[File:Acetyl coenzyme A 700.png|left|200px|acetyl-CoA]]'''Acetyl-CoA''', C<sub>23</sub>H<sub>38</sub>N<sub>7</sub>O<sub>17</sub>P<sub>3</sub>S, is a central piece in metabolism involved in several biological processes, but its main role is to deliver the acetyl group into the [[TCA cycle]] for its oxidation. It can be synthesized in different pathways: (i) in glycolysis from [[pyruvate]], by pyruvate dehydrogenase, which also forms NADH; (ii) from fatty acids β-oxidation, which releases one acetyl-CoA each round; (iii) in the catabolism of some amino acids such as leucine, lysine, phenylalanine, tyrosine and tryptophan. <br>In the mitochondrial matrix, acetyl-CoA is condensed with [[oxaloacetate]] to form [[citrate]] through the action of [[citrate synthase]] in the [[tricarboxylic acid cycle]]. Acetyl-CoA cannot cross the mitochondrial inner membrane but citrate can be transported out of the mitochondria. In the cytosol, citrate can be converted to acetyl-CoA and be used in the synthesis of fatty acid, cholesterol, ketone bodies, acetylcholine, and other processes.  +
[[File:Aconitase.jpg|right|500px|aconitase]]'''Aconitase''' is a [[TCA cycle]] enzyme that catalyzes the reversible isomerization of [[citrate]] to [[isocitrate]]. Also, an isoform is also present in the cytosol acting as a trans-regulatory factor that controls iron homeostasis at a post-transcriptional level.<br>  +
The '''activity''' (relative activity) is a dimensionless quantity related to the concentration or partial pressure of a dissolved substance. The activity of a dissolved substance B equals the [[concentration]], ''c''<sub>B</sub> [mol·L<sup>-1</sup>], at high dilution divided by the unit concentration, ''c''° = 1 mol·L<sup>-1</sup>: ''a''<sub>B</sub> = ''c''<sub>B</sub>/''c''° This simple relationship applies frequently to substances at high dilutions <10 mmol·L<sup>-1</sup> (<10 mol·m<sup>-3</sup>). In general, the concentration of a [[solute]] has to be corrected for the activity coefficient (concentration basis), ''γ''<sub>B</sub>, ''a''<sub>B</sub> = ''γ''<sub>B</sub>·''c''<sub>B</sub>/''c''° At high dilution, ''γ''<sub>B</sub> = 1. In general, the relative activity is defined by the [[chemical potential]], ''µ''<sub>B</sub> ''a''<sub>B</sub> = exp[(''µ''<sub>B</sub>-''µ''°)/''RT'']  +
'''Acyl-CoA dehydrogenases''' ACADs are localized in the mitochondrial matrix. Several ACADs are distinguished: short-chain (SCAD), medium-chain (MCAD), and long-chain (LCAD). ACAD9 is expressed in human brain. ACADs catalyze the reaction :::: acyl-CoA + FAD → ''trans''-2-enoyl-CoA + FADH<sub>2</sub>  +
'''Acyl-CoA oxidase''' is considered as a rate-limiting step in peroxysomal ''β''-oxidation, which carries out few ''β''-oxidation cycles, thus shortening very-long-chain fatty acids (>C<sub>20</sub>). Electrons are directly transferred from FADH<sub>2</sub> to O<sub>2</sub> with the formation of H<sub>2</sub>O<sub>2</sub>.  +
'''Acylcarnitines''' are esters derivative of [[carnitine]] and [[fatty acid]]s, involved in the metabolism of fatty acids. Long-chain acylcarnitines such as [[palmitoylcarnitine]] must be transported in this form, conjugated to carnitine, into the mitochondria to deliver fatty acids for fatty acid oxidation and energy production. Medium-chain acylcarnitines such as [[octanoylcarnitine]] are also frequently used for high-resolution respirometry.  +
'''Adaptation''' is an evolutionary time scale expression of phenotypic plasticity in response to selective pressures prevailing under various habitat conditions.  +
'''Add:''' A new graph is added at the bottom of the screen. Select plots for display in the new graph, Ctrl+F6. '''Delete: Delete one of the graphs displayed in DatLab.  +
'''Additivity''' ''A''<sub>''α&β''</sub> describes the principle of substrate control of mitochondrial respiration with [[convergent electron flow]]. The '''additive effect of convergent electron flow''' is a consequence of electron flow converging at the '''[[Q-junction]]''' from respiratory Complexes I and II ([[NS-linked substrate state |NS or CI<small>&</small>II e-input]]). Further additivity may be observed by convergent electron flow through [[Glycerophosphate_dehydrogenase_Complex|glycerophosphate dehydrogenase]] and [[electron-transferring flavoprotein Complex]]. Convergent electron flow corresponds to the operation of the [[TCA cycle]] and mitochondrial substrate supply ''in vivo''. Physiological substrate combinations supporting convergent NS e-input are required for reconstitution of intracellular TCA cycle function. Convergent electron flow simultaneously through Complexes I and II into the [[Q-junction]] supports higher [[OXPHOS capacity]] and [[ET capacity]] than separate electron flow through either CI or CII. The convergent [[NS]] effect may be completely or partially additive, suggesting that conventional bioenergetic protocols with [[Mitochondrial preparations|mt-preparations]] have underestimated cellular OXPHOS-capacities, due to the gating effect through a single branch. Complete additivity is defined as the condition when the sum of separately measured respiratory capacities, N + S, is identical to the capacity measured in the state with combined substrates, NS (CI<small>&</small>II). This condition of complete additivity, NS=N+S, would be obtained if electron channeling through supercomplex CI, CIII and CIV does not interact with the pool of redox intermediates in the pathway from CII to CIII and CIV, and if the capacity of the phosphorylation system does not limit OXPHOS capacity ([[Excess E-P capacity factor |excess ''E-P'' capacity factor]] is zero). In most cases, however, additivity is incomplete, NS < N+S.  +
The '''adenine nucleotide translocator''', ANT, exchanges [[ADP]] for [[ATP]] in an electrogenic antiport across the inner mt-membrane. The ANT is inhibited by [[atractyloside]], [[carboxyatractyloside|carboxyatractyloside]] and [[bongkrekik acid]]. The ANT is a component of the [[phosphorylation system]].  +
'''Adenine nucleotides''', which are also sometimes referred to as adenosines or adenylates, are a group of organic molecules including AMP, [[ADP]] and [[ATP]]. These molecules present the major players of energy storage and transfer.  +
'''Adenylate kinase''', which is also called myokinase, is a phosphotransferase enzyme that is located in the mitochondrial intermembrane space and catalyzes the rephosphorylation of AMP to ADP in the reaction ATP + AMP ↔ ADP + ADP.  +
In an isomorphic analysis, any form of [[flow]] is the '''advancement''' of a process per unit of time, expressed in a specific [[motive unit]] [MU∙s<sup>-1</sup>], ''e.g.'', ampere for electric flow or current, ''I''<sub>el</sub> = d<sub>el</sub>''ξ''/d''t'' [A≡C∙s<sup>-1</sup>], watt for thermal or heat flow, ''I''<sub>th</sub> = d<sub>th</sub>''ξ''/d''t'' [W≡J∙s<sup>-1</sup>], and for chemical flow of reaction, ''I''<sub>r</sub> = d<sub>r</sub>''ξ''/d''t'', the unit is [mol∙s<sup>-1</sup>] ('''extent of reaction''' per time). The corresponding motive [[force]]s are the partial exergy (Gibbs energy) changes per advancement [J∙MU<sup>-1</sup>], expressed in volt for electric force, Δ<sub>el</sub>''F'' = ∂''G''/∂<sub>el</sub>''ξ'' [V≡J∙C<sup>-1</sup>], dimensionless for thermal force, Δ<sub>th</sub>''F'' = ∂''G''/∂<sub>th</sub>''ξ'' [J∙J<sup>-1</sup>], and for chemical force, Δ<sub>r</sub>''F'' = ∂''G''/∂<sub>r</sub>''ξ'', the unit is [J∙mol<sup>-1</sup>], which deserves a specific acronym [Jol] comparable to volt [V]. For chemical processes of reaction (spontaneous from high-potential substrates to low-potential products) and compartmental diffusion (spontaneous from a high-potential compartment to a low-potential compartment), the advancement is the amount of motive substance that has undergone a compartmental transformation [mol]. The concept was originally introduced by De Donder [1]. Central to the concept of advancement is the [[stoichiometric number]], ''ν''<sub>''i''</sub>, associated with each motive component ''i'' (transformant [2]). In a chemical reaction r the motive entity is the stoichiometric amount of reactant, d<sub>r</sub>''n''<sub>''i''</sub>, with stoichiometric number ''ν''<sub>''i''</sub>. The advancement of the chemical reaction, d<sub>r</sub>''ξ'' [mol], is defined as, d<sub>r</sub>''ξ'' = d<sub>r</sub>''n''<sub>''i''</sub>·''ν''<sub>''i''</sub><sup>-1</sup> The flow of the chemical reaction, ''I''<sub>r</sub> [mol·s<sup>-1</sup>], is advancement per time, ''I''<sub>r</sub> = d<sub>r</sub>''ξ''·d''t''<sup>-1</sup> This concept of advancement is extended to compartmental diffusion and the advancement of charged particles [3], and to any discontinuous transformation in compartmental systems [2], :::: [[File:Advancement.png|100px]]  
'''Advancement per volume''' or volume-specific advancement, d<sub>tr</sub>''Y'', is related to [[advancement]] of a transformation, d<sub>tr</sub>''Y'' = d<sub>tr</sub>''ξ''∙''V''<sup>-1</sup> [MU∙L<sup>-1</sup>]. Compare d<sub>tr</sub>''Y'' with the amount of substance ''j'' per volume, ''c''<sub>''j''</sub> ([[concentration]]), related to [[amount]], ''c''<sub>''j''</sub> = ''n''<sub>''j''</sub>∙''V''<sup>-1</sup> [mol∙''V''<sup>-1</sup>]. Advancement per volume is particularly introduced for chemical reactions, d<sub>r</sub>''Y'', and has the dimension of concentration (amount per volume [mol∙L<sup>-1</sup>]). In an [[open system]] at steady-state, however, the concentration does not change as the reaction advances. Only in [[closed system]]s and [[isolated system]]s, specific advancement equals the change in concentration divided by the stoichiometric number, d<sub>r</sub>''Y'' = d''c''<sub>''j''</sub>/''ν''<sub>''j''</sub> (closed system) d<sub>r</sub>''Y'' = d<sub>r</sub>''c''<sub>''j''</sub>/''ν''<sub>''j''</sub> (general) With a focus on ''internal'' transformations (i; specifically: chemical reactions, r), d''c''<sub>''j''</sub> is replaced by the partial change of concentration, d<sub>r</sub>''c''<sub>''j''</sub> (a transformation variable or process variable). d<sub>r</sub>''c''<sub>''j''</sub> contributes to the total change of concentration, d''c''<sub>''j''</sub> (a system variable or variable of state). In open systems at steady-state, d<sub>r</sub>''c''<sub>''j''</sub> is compensated by ''external processes'', d<sub>e</sub>''c''<sub>''j''</sub> = -d<sub>r</sub>''c''<sub>''j''</sub>, exerting an effect on the total concentration change of substance ''j'', d''c''<sub>''j''</sub> = d<sub>r</sub>''c''<sub>''j''</sub> + d<sub>e</sub>''c''<sub>''j''</sub> = 0 (steady state) d''c''<sub>''j''</sub> = d<sub>r</sub>''c''<sub>''j''</sub> + d<sub>e</sub>''c''<sub>''j''</sub> (general)  +
The '''advantages of preprints''', the excitement and concerns about the role that preprints can play in disseminating research findings in the life sciences are discussed by N Bhalla (2016).  +