Thermodynamics
Thermodynamics of Chemical Bonds
Living things require a constant input of energy for their life processes, including the maintenance of order and the powering of cellular processes. The lack of these would result in death for the organism. As such, the concepts of free energy and thermodynamics are extremely important in biochemistry and how life actually works.
The following are the First Two Laws of Thermodynamics:
Energy can be transferred from one system to another, or transformed from one form into another, but not created or destroyed.
As energy flows through a system, the entropy of the system increases.
The energy of a system can be discussed in terms of Gibbs Free Energy. The lower the free energy, the more stable a system is.
Δ G = change in Free Energy
Δ H = change in enthalpy
Δ S = change in entropy
T = temp (in Kelvin)
If the ΔG is negative, that means the reaction is spontaneous. It will release energy for work.
If the ΔG is positive, it requires energy to happen.
Let's start by defining two major terms from that concept.
Enthalpy, in the context of biochemistry, is the bond energy of a system. Essentially, it is a measure of how much heat energy it would take to break or form all of the bonds in the system. When bonds are formed, heat is released, which is favorable enthalpy as it lowers the ΔG.
Entropy, as you likely know, is disorder. In terms of biochemistry, it can be seen as the degrees of freedom of a molecule or system. Take a look at the image to the right: things like identical bonds but different conformations of molecules, or movement of molecules, all add to the disorder and degrees of freedom of a system. The greater the entropy, the lower the ΔG.
Based on the second law of thermodynamics, and the fact that an increase in entropy lowers the ΔG, it might make one wonder how life, as ordered as it is, is possible. Why does it not violate these laws of thermodynamics? A folded protein clearly has less entropy than an unfolded polypeptide chain, so how does this happen?
Life functions and maintains order through coupled reactions. In essence, processes that are favorable and release energy, or that have a negative Δ G are coupled and paired with processes that are unfavorable and require energy, or that have a positive Δ G.
To add to this, when dealing with biology and biochemistry, we do not talk solely in terms of individual molecules, but of the state as a whole. For example, using the protein folding as an example: while the polypeptide chain itself loses entropy during folding, the surrounding water in the cytosol has a great increase in entropy during the folding. In addition, favorable enthalpic changes such as electrostatic interactions and Van der Walls forces between R-groups occur during folding. So, while some aspects of the process are unfavorable, putting them all together makes it a favorable process.