3 you are greatly increasing the entropy disorder of a system so the T\u0394S term

3 you are greatly increasing the entropy disorder of

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a large molecule up, such as the breakdown of starch to form maltose units as shown in Figure 8.3, you are greatly increasing the entropy (disorder) of a system so the -TΔS term will be large. This will typically result ina negative ΔG for the reaction.Figure 8.3. Breakdown of starch by amylase. The breaking of the starch bond is favorable (-ΔH) and thedisorder of the system increases by the release of maltose units (+ΔS and -TΔS). Therefore this reaction is favorable (-ΔG).If the formation of products absorbs energy then ΔG is positive and the reaction will not occur spontaneously.For example, building a cell wall, as shown in Figure 8.4 increases the order of a system (a negative ΔS) and the bonds formed require the removal of water, which creates a positive ΔH. This is an unfavorable reaction (ΔG is positive ) and it does not occur spontaneously. But cells have to build cell walls, so how do they do it? To get these reactions to go, you have to provide energy, often in the form of the hydrolysis of ATP or some other phosphate-containing compound. Phosphate bonds have a large amount of energy and the hydrolysis of ATP is a very favorable reaction. The release of energy to the system from breaking apart ATP compensates for the ordering of the cell wall assembly and enables the latter reaction to proceed.Figure 8.4. The synthesis of peptiodglycan. An unfavorable reaction. Synthesis of peptidoglycan by peptidoglycan synthase. This decreases the ordering the peptidoglycan, but is offset by the large amount of phosphate bond hydrolysis that occurs during the process.Now we are going to revisit ΔG from another perspective. The free energy of a reaction can also be expressed using the following equation: ΔG=ΔG° + RTlnKeq. ΔG°is the free energy at standard conditions, R is the universal gas constant, T is the temperature Keq refers to the equilibrium constant of the reaction. You may be wondering what are standard conditions and who decided that. Standard conditions are a conventionthat scientists have come up with to make it possible to compare various reactions. They are defined as a temperature of 25°C where all reactants are at 1 M in concentration at 1 atmosphere of pressure.Now reactions in biological systems rarely, if ever, occur under standard conditions. The second factor, RTlnKeq, takes account of deviations in temperature and concentrations of reactants and when added to ΔG°arrives at ΔG under real conditions. Assuming that the temperature remains constant (often true in biologicalsystems), then the important issue in this equation becomes Keq, which is the ratio of products and reactants. For example in a reaction with two substrates and two products (A + B -> C + D),Keq= [C][D]/[A][B]What should be clear is that ΔG is dependent upon the relative concentrations of reactants and products. If the concentration of products (actually, the arithmetic product of the concentrations of the products) becomes greater than that of the reactants such that Keq > 1, then RTlnKeq is positive and ΔG becomes more positive and therefore more unfavorable than it was under standard conditions. If the concentrations of
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