Thermodynamics
- Page ID
- 1347
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| 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
- Exergonic reactions:
- 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)