Biological processes coupled reactions

Redox reactions are ubiquitous and occur in numerous biological processes. In mitochondrial respiration, ATP production is coupled to electron transport. Electrons are transferred from complex I, II, III, and IV, through a series of redox reactions that create a proton gradient outside of the mitochondrial membrane allowing for the production of ATP. Redox reactions are also extremely important in metabolism. CYPs and FMOs rely on redox reactions for their catalytic function (see Chapter 10). 

This phenomenon shows that decrease or absorption of entropy in subsystem II may be compensated by a larger entropy production in subsystem I. This is possible only if subsystems I and n are coupled by some suitable coupling mechanisms leading to dS dS I <7.S II>0. With thermodynamic coupling a process in subsystem n may progress in a direction contrary to that determined by its own thermodynamic force. Some biological reactions represent coupled reactions for which the total entropy production is positive. 

There exist a large number of phenomenological laws for example, Fick s law relates to the flow of a substance and its concentration gradient, and the mass action law explores the reaction rate and chemical concentrations or affinities. When two or more of these phenomena occur simultaneously in a system, they may couple and induce new effects, such as facilitated and active transport in biological systems. In active transport, a substrate can flow against the direction imposed by its thermodynamic force. Without the coupling, such uphill transport would be in violation of the second law of thermodynamics. Therefore, dissipation due to either diffusion or chemical reaction can be negative only if these two processes couple and produce a positive total entropy production. 

Simultaneous heat and mass transfer plays an important role in various physical, chemical, and biological processes hence, a vast amount of published research is available in the literature. Heat and mass transfer occurs in absorption, distillation extraction, drying, melting and crystallization, evaporation, and condensation. Mass flow due to the temperature gradient is known as the thermal diffusion or Soret effect. Heat flow due to the isothermal chemical potential gradient is known as the diffusion thermoeffect or the Dufour effect. The Dufour effect is characterized by the heat of transport, which represents the heat flow due to the diffusion of component / under isothermal conditions. Soret effect and Dufour effect represent the coupled phenomena between the vectorial flows of heat and mass. Since many chemical reactions within a biological cell produce or consume heat, local temperature gradients may contribute in the transport of materials across biomembranes. 

Besides the transport, the most important processes in biological systems are those related to chemical reactions of metabolism. One of the typical aspects of such reactions is the requirement regarding the apparent stoichiometiy of two partially coupled reactions, and the study of the efficiency of such reactions as limited by the constraints of the second law of thermodynamics. 

Finally, the best building blocks identified can be coupled in a modular way to complete photocatalytic reaction cycles, which then should be able to mimic a certain biological process. If these bioinspired photocatalytic systems are performing rmder identical conditions as their native counterparts, a direct comparison of quantitative criteria such as turnover frequencies and the total number of catal5dic cycles is possible and should always be the finad goal to demonstrate the potential usefulness of the biomimetic process. 

Proton-coupled electron transfer (PCET) reactions play a vital role in a wide range of chemical and biological processes. For example, PCET is required for the conversion of energy in photosynthesis [1] and respiration [2], In particular, the coupling between proton motion and electron transfer is involved in the pumping of protons across biological membranes in photosynthetic reaction centers [1] and in the conduction of electrons in cytochrome c [3]. In addition to biological processes, PCET is also important in electrochemical processes [4, 5] and in solid state materials [6]. 

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