BIOGEOCHEMICALREACTIONS
Used to harness energy for biosynthesis
Take advantage of chemical “potential” energy
Important consequences for element cycling
• Chemical potential energy implies a reaction yields net energy although may require activation/catalysis.
G = H - T S = Gibbs Free Energy
• = change in enthalpy - T *change in entropy– If negative, reaction will proceed– If positive requires energy input– For most biology can neglect 2nd term
• Many important biogeochemical reactions involve electron transfer (redox reactions)– Donor Donor + and e- (G = pos or neg)
– Acceptor+ and e- Acceptor (G = pos or neg)
D + A+ D+ + ASummed G must be negative for reaction to yield energy
Overall ∆G is negative
DONORD→D+ and e-
ACCEPTORA←A+ and e-
BIOTA
Enzymes (electron transport) are the “teeth” on the gears
production
decomposition
organicinorganic
Fig. x. Weathers et al., Fundamentals of Ecosystem Science
Analogous for most biologically essential elements
CO2
CH2O
e-
CO2
CH2O
e-
EQUILIBRIA
• A + B C + D
• K = [C][D] / [A][B]– Equilibrium constant
G = G0 + rT ln CD/AB– Linked element cycles– Sources/sinks
EQUILIBRIA
• A + B C + D
• K = [C][D] / [A][B]– Equilibrium constant
G = G0 + rT ln CD/AB– Linked element cycles– Sources/sinks
SLOWERAdd C,DRemove A,B
FASTERRemove C,DAdd A,B
• Many important biogeochemical reactions involve electron transfer (redox reactions)
G = -nFE (E is voltage)+ voltage implies spontaneousn is # moles of electrons (equivalents)F is Faraday’s constant
• CH4 + 2 O2 CO2 + 2 H2O + heat
• CH2O + O2 CO2 + H2O + heat
• Both are redox reactions ie something gets oxidized (valence goes up); something gets reduced (valence goes down)
• CH4 + 2 O2 CO2 + 2 H2O + heat
C-4 C+4
O0 2O-2
G = -213 kcal
Two O2 per Carbon
H valence = +1O valence is -2 (when combined)
Redox couples
• C0 H2O C+4 + 4 e-
E=0.47
• O02 + 4 e- 2O-2 E=0.81
= CH2O + O2 CO2 + H2O E = 1.28 v
CH2O is the electron donor O2 is the electron acceptor
Different electron acceptors (not O2)Org Matter is e- donor E=0.47
• NO3- + e- N2
N Val = +5 Val =0E = 0.75
• Fe+3 + e- Fe+2
E=0.77
• SO4-2 + e- HS-
S Val = +6 Val = -2E = -0.22
• CO2 + e- CH4C Val = +4 Val = -4E = -0.24
Other electron donors (not organic matter)
All have + E
• Mn +2 + O2 Mn +4 + H2O
• Fe +2 + O2 Fe +3 + H2O
• NH4+ + O2 NO3- (nitrification)
• H2 H+ e-
Fermentation(No “external” electron acceptor)
• Methanogenesis
CH3COOH CH4 + CO2– (C-3) (C+3) (C-4) (C+4)
• C3H6O3 CH3CH2OH + CO2
C0 C-3 , C-1 and C+4
• Humic acids
N fixation
(reduction)
N2 Org N
(protein)
Anoxic;
Requires
Energy
Rhizobium
Cyanobct
Nitrification
(oxidation)
NH3 NO3 Oxic;
Yields energy
Chemoauto-trophic
Denitrification
(reduction)
NO3 N2 Accepts electrons
Widely distributed
Assimilation
(Same valence)
(reduction)
NH3 Org N
NO3 NH3
Intra-
cellular
Plants, fungi
bacteria
Process Reaction Conditions Who