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Term	Abbreviation	Description
Absorption spectrum		An absorption spectrum is similar to an absorbance spectrum of a sample, but plotted as a function of absorption against wavelength.
Abundance		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, g [m·s^-2]	Acceleration, a, is the change of velocity over time [m·s^-2]. a = dv/dt The symbol g is used for acceleration of free fall. The standard acceleration of free fall is defined as g_n = 9.80665 [m·s^-2].
Acclimation		Acclimation is an immediate time scale adaption expressing phenotypic plasticity in response to changes of a single variable under controlled laboratory conditions.
Acclimatization		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.
Accuracy		The accuracy of a method is the degree of agreement between an individual test result generated by the method and the true value.
Acetyl-CoA		Acetyl-CoA, C₂₃H₃₈N₇O₁₇P₃S, 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. 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.
Aconitase	Aco	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.
Activity	a	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_B [mol·L^-1], at high dilution divided by the unit concentration, c° = 1 mol·L^-1: a_B = c_B/c° This simple relationship applies frequently to substances at high dilutions <10 mmol·L^-1 (<10 mol·m^-3). In general, the concentration of a solute has to be corrected for the activity coefficient (concentration basis), γ_B, a_B = γ_B·c_B/c° At high dilution, γ_B = 1. In general, the relative activity is defined by the chemical potential, µ_B a_B = exp[(µ_B-µ°)/RT]
Acyl-CoA dehydrogenase	ACAD	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 + FADH₂
Acyl-CoA oxidase		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 (>C₂₀). Electrons are directly transferred from FADH₂ to O₂ with the formation of H₂O₂.
Acylcarnitine	AC	Acylcarnitines are esters derivative of carnitine and fatty acids, 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		Adaptation is an evolutionary time scale expression of phenotypic plasticity in response to selective pressures prevailing under various habitat conditions.
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Additive effect of convergent electron flow	A_α&β	Additivity A_α&β 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 or CI&II e-input). Further additivity may be observed by convergent electron flow through 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 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&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 is zero). In most cases, however, additivity is incomplete, NS < N+S.
Adenine nucleotide translocase	ANT	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 and bongkrekik acid. The ANT is a component of the phosphorylation system.
Adenine nucleotides	AN	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	ADK	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.
Advancement	d_trξ [MU]	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^-1], e.g., ampere for electric flow or current, I_el = d_elξ/dt [A≡C∙s^-1], watt for thermal or heat flow, I_th = d_thξ/dt [W≡J∙s^-1], and for chemical flow of reaction, I_r = d_rξ/dt, the unit is [mol∙s^-1] (extent of reaction per time). The corresponding motive forces are the partial exergy (Gibbs energy) changes per advancement [J∙MU^-1], expressed in volt for electric force, Δ_elF = ∂G/∂_elξ [V≡J∙C^-1], dimensionless for thermal force, Δ_thF = ∂G/∂_thξ [J∙J^-1], and for chemical force, Δ_rF = ∂G/∂_rξ, the unit is [J∙mol^-1], 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_i, associated with each motive component i (transformant [2]). In a chemical reaction r the motive entity is the stoichiometric amount of reactant, d_rn_i, with stoichiometric number ν_i. The advancement of the chemical reaction, d_rξ [mol], is defined as, d_rξ = d_rn_i·ν_i^-1 The flow of the chemical reaction, I_r [mol·s^-1], is advancement per time, I_r = d_rξ·dt^-1 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],
Advancement per volume	d_trY [MU∙L^-1]	Advancement per volume or volume-specific advancement, d_trY, is related to advancement of a transformation, d_trY = d_trξ∙V^-1 [MU∙L^-1]. Compare d_trY with the amount of substance j per volume, c_j (concentration), related to amount, c_j = n_j∙V^-1 [mol∙V^-1]. Advancement per volume is particularly introduced for chemical reactions, d_rY, and has the dimension of concentration (amount per volume [mol∙L^-1]). In an open system at steady-state, however, the concentration does not change as the reaction advances. Only in closed systems and isolated systems, specific advancement equals the change in concentration divided by the stoichiometric number, d_rY = dc_j/ν_j (closed system) d_rY = d_rc_j/ν_j (general) With a focus on internal transformations (i; specifically: chemical reactions, r), dc_j is replaced by the partial change of concentration, d_rc_j (a transformation variable or process variable). d_rc_j contributes to the total change of concentration, dc_j (a system variable or variable of state). In open systems at steady-state, d_rc_j is compensated by external processes, d_ec_j = -d_rc_j, exerting an effect on the total concentration change of substance j, dc_j = d_rc_j + d_ec_j = 0 (steady state) dc_j = d_rc_j + d_ec_j (general)

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