# Thermodynamics

[ "Vet1", "article:topic" ]

### Cells need energy to

 At the cellular level At the molecular level move ion transport, reversible bond formation grow synthesis and polymerization maintain homeostasis heat production, entropy reduction

### Laws of Thermodynamics

 0) All closed systems move towards equilibrium. 1) Energy (work, heat, etc.) of a closed system is conserved. 2) Entropy of a closed system increases. 3) It is impossible to reach absolute zero (0K)
• However, the second law poses a problem: How can cells reproduce to make more cells by using disordered raw materials, and thus create order? Cells are not closed systems, they interact with their environment.

ΔScell + ΔSenvironment = ΔSsystem

### Biochemical Energy

In order to apply thermodynamics to biological reactions, we need a measure of energy that applies to living conditions.
•  Consider the reaction X↔Y:
• States X and Y have internal energy (E), pressure (P), volume (V), temperature (T), and entropy (S).
• G(Gibb's free energy)=E+PV-TS
• ΔG represents changes in internal energy and entropy for any reaction at constant temperature and pressure
• For reaction X↔Y, G depends on concentrations of reactants and products as well as state variables E, P, V, T, S:

ΔG=ΔGo+RTln([Y]/[X])

• at standard conditions: 1M substrate, 1M product, 25oC, atmospheric pressure: ΔG=ΔGo
• If X→Y is spontaneous, G decreases and the reaction is considered to be exergonic:ΔG=G(products)-G(reactants)<0
• [chart] when G is at a minimum, assume the reaction is at equilibrium and ΔG=0.
• Let K=[Y]/[X], and assume that the reaction is at equilibrium (ΔG=0):

ΔG=ΔGo+RTln([Y]/[X])
0=ΔGo+RTlnKeq
ΔGo=-RTlnKeq

• At pH 7, ΔGo=-RTlnKeq where [H+]=10-7M. This makes a difference when H+ is a reactant or a product.

### Coupled Reactions

• For two reactions: X↔Y    ΔG1
A↔B    ΔG2
• If they are coupled (occur together), then X+A↔Y+B and:

ΔG=ΔG1+ΔG2
ΔGo=ΔGo1+ΔGo2

• The coupled reaction proceeds (is spontaneous) if ΔG<0.
• This means that conditions leading to exergonic (ΔG<0) reactions can be used to power endergonic (ΔG>0) reactions:
 Exergonic reactions: light absorption redox disequilibrium relaxation bond breaking/degradation Endergonic reactions: synthesis polymerization homeostasis
• Example:

PEP + H2O → pyruvate + Pi    ΔG=-78 kJ/mol (exergonic)
ADP + Pi → ATP + H2O           ΔG=55 kJ/mol (endergonic)

• combine the two reactions, and add their ΔG values:

PEP + ADP → pyruvate + ATP  ΔG=-23 kJ/mol (exergonic)