Spontaneous Process also

Second law of thermodynamics the total entropy of a system and its surroundings always increases for a spontaneous process. Also, for a spontaneous process at a given temperature, the change in entropy of the system is greater than the heat divided by the absolute temperature. (19.2)... 

The more negative the value of AG, the more energy or useful work can be obtained from the reaction. Reversible processes yield the maximum output. In irreversible processes, a portion of the useful work or energy is used to help carry out the reaction. The cell voltage or emf also has a sign and direction. Spontaneous processes have a positive emf the reaction, written in a reversible fashion, goes in the forward direction. 

Any further reaction beyond (eq) would not only fail to liberate any further energy, but also would in fact consume energy (we would start to go uphill on the right-hand side of the figure). Any further increment of reaction would be characterized by AG > 0, implying a non-spontaneous process, which is why the reaction... 

Figure 2. Scheme for the addition and loss of actin-ATP or actin-ADP from the barbed and pointed ends of an actin filament. The barbed end is the faster growing and more stable end of an actin filament. While the exchange of actin-ADP with ATP to yield actin-ATP and ADP is shown here as a spontaneous process, the actin regulatory protein profilin greatly accelerates the exchange process. Note also that hydrolysis is thought to occur after (and not coincident with) addition of actin-ATP at either end. 

The middle position that the ATP/ADP couple occupies in the potential ranking explains why it is so useful in cellular metabolism. There are numerous compounds lower on the potential scale to which the transfer of Pj represents a spontaneous process, and yet there are also a few cellular compounds with higher potentials that can in turn phosphorylate ADP to regenerate ATP. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

For all practical purposes, we can state that all spontaneous processes in nature result in an increase in the entropy of the universe. We can also generalize with the statement that any system (even the universe) will tend to run down over time (tend in increase in entropy until total chaos—disorder—is reached). 

Protein cross-links may be also produced in reaction of 4-hydroxynonenal with lysine, histidine, serine, and cysteine residues, primarily via Michael addition (J5, R7, U8). These reactions occur spontaneously, but also may be catalyzed by certain glutatione 5-transferases. The glutathione transferase A4-4, which unlike other alpha-class glutathione transferases, shows high catalytic activity toward lipid peroxidation products such as 4-hydroxynon-2-enal, is the key enzyme for these reactions (B31). Products of protein coupling with aldehydes secondary to lipid peroxidation have a specific fluorescence, which can herald the protein oxidative modification process (CIO). 

The general theory of thermodynamics of nonequilibrium processes also tells us the stability of nonequiUbrium stationary states with respect to spontaneous fluctuations of the internal thermodynamic parameters in the system. It turns out that this stability can also be investigated by analyzing the variations in the entropy production or energy dissipation rates on drawing the system away from its stationary state. 

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