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1ESC 590Microbial Growth & Metabolism

Reading Assignment

Soil Microbiology:An exploratory

ApproachChapters 1 & 2

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Biological Oxidations

One of the fundamental properties of living

organisms is their requirement of energy.

Phototrophs, meet their energy requirements

by absorption of a quanta of solar radiation.

Chemotrophs obtain energy needs by

oxidation of preformed organic molecules.

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Biological oxidations reactions are

frequently dehydrogenation reactions.

Biological oxidation reactions almost

always involve two electron transfers.

In chemotrophic energy metabolism the

ultimate energy acceptor of electrons isfrequently oxygen.

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The electrons are generally passed to the

final electron acceptor through intermediate

electrons acceptors In most biological oxidations, the

immediate electron acceptor is one of

several coenzymes.-specialized moleculesthat function specifically as carriers of

electrons.


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The most common coenzymes are NAD, +

NADP+ and FAD

Aerobic energy metabolism involvesstepwise process collectively called

respiration.

Under anaerobic conditions, oxygen is notavailable as electron acceptor, and the

electrons are passed instead to some organic

or inorganic molecule .

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Under anaerobic conditions, oxygen is not

available as electron acceptor, and the

electrons are passed instead to some organic

or inorganic molecule .

All anaerobic processes are called

fermentations and they are usually further

identified in terms of the principal end

product i.e. the reduced form of the organic or

inorganic electron acceptor.

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Organisms with an absolute requirement for

oxygen arestrict or obligate aerobes. Most

higher animals are in this category. Strict or Obligate anaerobes cannot tolerate

the presence of oxygen.

Most are bacteria , including soil Clostridiaand those responsible for denitrification

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Comparision of Fermentation, Aerobic

Respiration, and Anaerobic Respiration.

Energy-Producing Process Growth Conditions

Fermentation Aerobic or anaerobic

Aerobic respiration Aerobic

Anaerobic respiration Anaerobic

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Respiration

Respiration is properly defined as the

oxidation of organic molecules with

molecular oxygen serving as the ultimateelectron acceptor.

The result of respiration is the complete

degradation of organic molecules to theproducts, CO2 and H2O.

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Fermentation Reactions

In fermentation reactions, organic molecules

are oxidized with the electrons being given to

electron acceptors other than O2.

One common fact concerning each of the

classes of fermentation is that the source of

electrons involved is organic compounds.

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Class Electron Donor Electron AcceptorPhotoautotrophic

(Photolithotroph) H2O, H

2S, H

2R CO

2

Respiration

(organothrops) Organic Compounds O

2

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Class Electron Donor Electron Acceptor

Heterofermentative Organic Compounds Same molecule o

fragment of it.

or

Multifermentative Organic Compounds Different organic

compound or CO2

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Class Electron Donor Electron

AcceptorIsofermentative Organic Compounds Another molecule

of substrate

Anaerobic Respiration Organic Compounds Inorganiccompound

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Concept of pe

Just as pH is based on moles L-1, redox

potential can also be expressed in terms of

pe (-log of electron activity) which iscompatible with units of moles per liter.

In this way, electrons can be treated as other

reactants and products so that both can beexpressed by a single equilibrium constant.

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Concept of pe

pe = -log (e-)

It measures the relative tendency of a

solution to accept electrons.

Reducing solutions have low pe and tend to

donate electrons to species placed in thesolution.

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Concept of pe

Oxidizing solutions have high pe and tend

to accept electrons from species placed in

the solution. Large values of pe favor the existence of

electron- poor (i.e) oxidized species just as

large values of pH favor the existence ofproton poor species (i.e bases)

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Concept of pe

In soils the pe range can be divided into 3

parts that corresponds to:

1) oxic soils (pe > + 7 at pH 7)

2) Sub oxic soils (2 < pe < + 7 pe at pH 7)

3) Anoxic soils ( pe < + 2 at pH 7)

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pe + pH

On the oxidized side the redox limit is given

by the reaction

H+ + e- + 1/4 O2(g) = 1/2 H2O Rxn.2K =1/2 H2O /(H

+)(e-)(O2)1/4 =1020.78.

Log K =Log (H+) -log(e-) 1/4 log O2(g)

= 20.78

pe + pH = 20.78 + 1/4 O2(g)

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pe + pH

Thus when O2 is 1 atm,

pe + pH = 20.78.

This represents the most oxidized

equilibrium conditions in natural aqueousenvironments

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pe + pH

When the redox limits of natural aqueous

environments defined by reactions 1 and 2

are plotted we get a graph which is knownas a pE-pH diagram, and shows the domain

of electron and proton activity that has been

observed in soil environment worldwide.

Both pe and pH are needed to specify the

redox status of aqueous systems.

1esc590. ch 1.biol ox

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