Question

Why does a large entropy increase in CO2 vs. organic matter make life possible?

Having trouble conceptually understanding this... when does CO2 have a large entropy increase?

Answer 1

Yes spontaneous processess will have a positve sign entropy ( increase in entropy) and at the same time decrease in gibbs free energy (a negative sign) also according to the following equation.

ΔG = ΔH - TΔS. ΔG = Gibbs Free Energy; ΔH = enthalpy change; ΔS = entropy change

Not only CO₂ case. Most of the biological processess are proceeding through decrease in entropy (protein synthesis) and for a normal chemist it appears to be non-spontaneous, but nature adopts a special features which will drive. I have another example with enzyme as you are asked me "Having trouble conceptually understanding this...". I am going to explain the concept with the help of enzymes.

Here's an example from the gluconeogenesis/glycolysis pathway (see below). It's the reaction catalyzed by aldolase where a six-carbon molecule (fructose) is cleaved to produce two three-carbon molecules. The reaction shown here is the one in the glycolysis pathway that breaks down glucose.

The standard Gibbs free energy change for this reaction is ... ΔG'°_reaction = +28 kJ mol^-1

In a chemistry course you might learn that this reaction is NOT spontaneous because the standard Gibbs free energy change is positive. In other words, you need to supply energy—as indicated by the plus sign in the standard Gibbs free energy change—in order to make the reaction go from left to right. The reaction will be "spontaneous" in the opposite direction where ΔG'°_reaction = -28 kJ mol^-1.

The concept of "spontaneous" and "not spontaneous" based on the standard Gibbs free energy change make no sense in a biochemical context. The aldolase reaction, for example is part of the gluconeogenesis pathway where the two three-carbon molecles are joined to produce fructose-1,6-bisphosphate. This eventually leads to the production of glucose.

The adolase reaction is also part of the glycolysis pathway that runs in the opposite direction (as shown above). Cells can easily switch from making glucose to degrading it. How can this happen if the free energy change is +28 kJ mol^-1.

It isn't. The actual Gibbs free energy change inside the cell is very different than the standard Gibbs free energy change. This reaction rapidly reaches equilibrium inside the cell. Under those conditions the rates of the forward and reverse reactions are equal and ΔG = 0.

In the case of the aldolase reaction, the concentrations of the reactants and products at equilibrium will not be equal as the standard Gibbs free energy change requires. Instead, the concentration of fructose-1,6-bisphosphate will be much higher than the concentrations of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

In biochemical terms we say that this is a "near-equilibrium" reaction. Most metabolic reactions are near-equilibrium reaction with ΔG = 0 (or close to it).

This is an important concept in biochemistry. You can't understand pathways and flux if you don't know that most of the reactions are near-equilibrium reactions where ΔG = 0.Sup>1 You also can't understand regulated reactions, where ΔG is not zero, unless you've grasped the fundamentals. (Regulated reactions are called "metabolically irreversible reactions.")

Also as you know, the *real* free energy change ∆G can be calculated from the *standard* free energy change ∆G˚ using the equation ∆G = ∆G˚ + RT ln [products]/[reactants], thus clearly demonstrating the dependence of the real ∆G on the concentration of reactants and products.

"Life creates" (more correctly: Gibb's free energy produces) solid structures which have lower entropy but it also emits gases (such as O2 can CO2) that have high entropy which approximately cancel the effect (according to my summing of the specific entropy of the products and reactants).

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