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The “Natural Trick” for Bioenergetics to Function
Published in Jean-Louis Burgot, Thermodynamics in Bioenergetics, 2019
The problem is all the more complicated with bioenergetics as it often involves reactions occurring in chains, whose steps themselves result from the superimposition of several equilibria. In several cases, an endergonic process may be coupled with an exergonic one with the result that the latter delivers energy to the endergonic process. In such coupled systems, the endergonic process takes place only if the decrease of free energy of the exergonic process is larger than the gain in free energy of the endergonic process. More precisely, the algebraic sum of the free energy changes in the two processes must be negative in sign, corresponding to a net decline of free energy, in order for a coupled reaction to occur.
Basic Chemical Hazards to Human Health and Safety — I
Published in Jack Daugherty, Assessment of Chemical Exposures, 2020
The chemical reactions in tissues undergo degradation in catabolism and resynthesis in anabolism. Exergonic reactions occur during catabolism, and release energy from within the reaction system. Endergonic reactions occur during anabolism, and require external energy. After the reactant and product substances involved in an endergonic reaction absorb energy, they sometimes start an exergonic reaction going. Oxidation sets off endergonic reactions within cells. When an exergonic reaction drives an endergonic reaction, the two are said to be coupled.
X-Nuclei MRI and Energy Metabolism
Published in Guillaume Madelin, X-Nuclei Magnetic Resonance Imaging, 2022
Energy coupling. ATP is a highly unstable molecule that spontaneously dissociates into ADP + Pi in the presence of water, and the free energy released during this process can be quickly lost as heat. To be able to use this released energy from the bonds of ATP, cells use a strategy called energy coupling: the exergonic reaction of ATP hydrolysis is coupled with the endergonic reactions of cellular processes.
Ionic strength effect on the kinetics and mechanism of N-vinyl compound formation in the presence of heterocyclic biological base: empirical and theoretical approaches
Published in Molecular Physics, 2021
Sayyed Mostafa Habibi-Khorassani, Mehdi Shahraki, Sadegh Talaiefar, Fatemeh Ghodsi
Evidently, step4 is the rate-determining-step (RDS) of this domino process and step3 with a low energy barrier cannot be considered as the RDS. This is in agreement with data in section 3.1 and a good discussion in section 5.1 for polar reaction between the high electrophile character of I2 and high nucleophilic character of species N– (step3) that create a process with a low-energy barrier (CDFT study). The activation enthalpy associated via TS3 and TS4 are 5.28 and 34.70 kcal mol−1, respectively, the reactions being exothermic by 7.73 (step3) and with 16.09 kcal mol−1 (step4). Inclusion of entropies to enthalpies raises the activation Gibbs free energy for TS3 and TS4 with 17.38 and 39.04 kcal mol−1, respectively; while the formation of I3 is endergonic by 6.10 kcal mol−1 and the formation of trans-alken 4 (N-vinyl compound, product) is exergonic by −32.40 kcal mol−1. Consequently, the formation of trans-alkene 4 is favourable thermodynamically.
Active Thermochemical Tables: the thermophysical and thermochemical properties of methyl, CH3, and methylene, CH2, corrected for nonrigid rotor and anharmonic oscillator effects
Published in Molecular Physics, 2021
In order to fairly compare the NRRAO and RRHO thermochemistry, each examined reaction has initially the same 298.15 K reaction enthalpy within both models, fixed by the ATcT 298.15 K enthalpies of formation of the reactants and the product, but then the two models use differing thermophysical properties to develop the thermochemistry at other temperatures. Being the reverse of bond dissociation processes, both examined reactions are exothermic throughout the explored temperature range. They both also start as exergonic at the low temperature end, but become endergonic at the high temperature end (meaning that at sufficiently high temperature the reverse reaction, which corresponds to thermal dissociation, becomes spontaneous). Within the RRHO thermochemistry, becomes endergonic at ∼3110 K. Within the NRRAO thermochemistry, this point is pushed upward by more than 100 K, and the reaction switches from being exergonic to becoming endergonic at ∼3235 K. Similarly, the becomes endergonic at ∼2265 K within the RRHO thermochemistry, but at ∼2320 K within the NRRAO thermochemistry. What this means is that NRRAO thermochemistry results in both cases in slightly lower free energies at the high temperature end than their RRHO counterparts. Thus, in both cases at combustion temperatures the NRRAO thermochemistry favours association (as opposed to dissociation) slightly more than the RRHO thermochemistry.