Chemical reaction endergonic

Every chemical reaction reaches after a time a state of equilibrium in which the forward and back reactions proceed at the same speed. The law of mass action describes the concentrations of the educts (A, B) and products (C, D) in equilibrium. The equilibrium constant K is directly related to the change in free enthalpy G involved in the reaction (see p.l6) under standard conditions (AG° = - R T In K). For any given concentrations, the lower equation applies. At AG < 0, the reaction proceeds spontaneously for as long as it takes for equilibrium to be reached (i.e., until AG = 0). At AG > 0, a spontaneous reaction is no longer possible (endergonic case see p.l6). In biochemistry, AG is usually related to pH 7, and this is indicated by the prime symbol (AG° or AG ). 

AG, is directly associated with the direction in which a particular chemical reaction can proceed. If AG < 0 for a given set of conditions of a particular reaction, then the reaction will proceed spontaneously in the indicated direction until equilibrium is reached. Conversely, if AG is positive, then energy will be needed to shift the reaction further from its equilibrium condition. See Helmholtz Energy Endergonic Exergonic Enthalpy Entropy Thermodynamics Biochemical Thermodynamics... 

Biological oxidation is usually represented by complex chemical reactions, the staging of which allows the release of energy in portions, which are then consumed by cells for endergonic purposes. 

Exergonic and endergonic reactions are balanced in the cell representing a complex of consecutive, parallel-consecutive and conjugated chemical reactions. Intra-mitochondrial space is divided by a complex system of inner membranes, on which redox enzymes are fixed, forming the so-called ensembles. 

Figure 8 shows the vectors on the thermodynamic compass for typical endergonic reactions. It is found that for chemical reactions all three types can be observed. However, the most common one may be the heating type of which examples are the reduction of metallic oxides and the decomposition of water to hydrogen and oxygen. 

The redox potentials for the electron acceptors that react with HO (Table 15) are such that a pure outer-sphere single-electron transfer (SET) step would be endergonic (the HO /HO redox potential is more positive than the redox potential of the electron acceptor). Hence, the observed net reactions must be driven by coupled chemical reactions, particularly bond formation by the HO to the electrophilic atom of the acceptor molecule that accompanies a singleelectron shift. (The formation of the bond provides a driving force sufficient to make the overall reaction thermoneutral or exergonic 1.0 V per 23.1 kcalmol of bond energy.) The effect of various transition metal complexes on the oxidation potential for HO in MeCN illustrates some of these effects the results are summarized in Table 16. ... 

Ion-radical pairs may undergo back electron transfer [22-24] (A et in Scheme 1) in competition with the desired follow-up reactions, which limits the yields of the final reaction products. In fact, the more endergonic the initial electron-transfer step (A et in Scheme 1) is, the faster is the corresponding (exergonic) back electron transfer, which shifts the ET equilibrium towards the precursor (EDA) complex. However, even in such cases of highly endergonic electron transfer, a net chemical reaction to the final products may still be observed if the rate of the follow-up reaction is competitive with that of the back electron transfer in Scheme 1 [25]. This... 

Q-12 Describe how cells obtain energy to do cellular work during endergonic anabolic chemical reactions. 

Various subclassifications exist according to the exact nature of the chemical step, which may eventually be a succession of elementary steps with formation of intermediate products. As explained earlier for the displacement of endergonic electron transfer steps, the C step occurs because it is continuously pulled to the right by the further chemical reaction of the Y species. Note that Chapter 28 is devoted to this class of mechanisms. 

Chemical fuel, however, is not the only means by which energy can be supplied to operate molecular-level devices. As recalled in the previous section, nature shows that in green plants the energy needed to sustain the machinery of life is supplied by sunlight. Photochemical energy inputs can indeed cause the occurrence of endergonic chemical reactions, which can make a device work without formation of waste products. Currently there is an increasing interest in... 

The inclusion of a work cycle seems to be a central feature of this tentative definition, for work cycles link spontaneous and non-spontaneous (exergonic and endergonic) chemical reactions. The collectively autocatalytic system considered in section II might have been entirely exergonic. If one considers the biosphere as a whole, it is a richly interwoven web of linked exergonic and endergonic reactions building up the enormous chemical complexity of the entire biosphere, the most complex chemical system we know. 

---