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Entropy
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Covers the Entropy lecture of BIOL2016/7016. Complete with extra reading from various textbooks and papers.
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Entropy, enthalpy and energy flux – Life from an energeticsperspective
A REDOX equilibrium in the environment (eg. food/oxygen/hydrogen sulphide/ nitrous
oxide) is converted into an electrochemical difference in H + (or Na+) which drives ATP
synthesis from ADP and Pi, pushing the ATP/ADP ratio orders of magnitude from
equilibrium
Overall: Environmental disequilibrium drives ATP synthesis, which drives growth
What is living?
Life feeds on negative entropy
Life is a far-from equilibrium process, an
unceasing chemical reaction, sustained by
environmental disequilibria
Entropy is a measure of disorder (S = klnW)
- W is the number of ways of arranging particles that gives rise to a particular
observed state of the system
- k is Boltzmann’s constant (1.38 x 10-23 J.K-1) – relates to energy at particle level
to temperature
More heat energy put into a system, the more entropy increases as there are nire
quanta and more ways (W) to distribute them
lnW scales W to a smaller, more manageable number
Entropies are measured in J.K-1.mol-1
Gibbs Free Energy
Negative sign = exothermic, which increases
entropy of surroundings
The more the negative ΔH (change in enthalpy
of system), more positive increase of the
surroundings
Same amount of energy makes more difference at a lower temperature (hence
division by T)
ΔG = ΔH – TΔS
If ΔG is –ve, reaction exergonic
If ΔG is +ve, reaction endergonic
If ΔG is 0: equilibrium, no change
As decrease in enthalpy must offset
decrease in entropy, the enthalpy term
reflects energy that is not free to be used for any other purpose, hence ΔG is what
remains after this term is subtracted
,Standard Gibbs Energy ΔGO
Compare reactions, standardise reaction
conditions (1M for solutes, pure liquid or
pure gas, 1 atm, pH 0, 25oC
For other conditions (eg. pH7, 37oC) - ΔG’
ATP Production
, ΔG of ATP hydrolysis depends on how far [ATP] is
displaced from equilibrium
Bond energy in ATP is not special/”high-energy”
It is the reaction that is significant ADP + Pi
ATP (dynamic eqm.)
- Needs to pushed 10 orders of magnitude away
from equilibrium
- If ATP, ADP and Pi are left at their natural
equilibrium, ATP would have no capacity to do work
- Living (eg. continuous respiration, photosynthesis) pushes ATP/ADP ratio far
from equilibrium
Across life there are 2 ways of generating ATP
- Substrate-level phosphorylation (eg. fermentation)
Embden-Mayerhoff pathway – almost certainly
arose from gluconeogenesis, later than chemiosmosis
- Chemisosmotic coupling
Universal to life as the universal genetic
code itself
Chemisomotic coupling relies on
bioenergetic membranes to produce
ATP/ADP disequilibrium
Range of redox potentials (Eh) for electron
acceptors is much larger than electron donors
- Reflects oxidation of environment over
Earth history
- Ecological challenge now is not to find suitable electron acceptors, but suitable
electron donors (eg. H2)
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