which is recovered to synthesize ATP by phosphorylation
of ADP. This synthesis is catalyzed by ATP synthases,
insoluble membrane enzymes*. During respiration, electron transfer between carriers takes place in the cytoplasmic
membrane among prokaryotic microorganisms and in the
internal mitochondrial membrane among eukaryotic
microorganisms; the resulting ATP production is obtained
by oxidative phosphorylation. For photosynthesis, electron
transfer occurs in specialized membranes (thylakoids in
cyanobacteria and chloroplasts in photosynthetic eukaryotes
or photosynthetic cytoplasmic membrane in anoxygenic
phototrophic bacteria), and ATP is produced by photophosphorylation. Almost all of enzymes associated with electron
transport are included in the membrane and are insoluble.
The transfer of electrons and protons generates a protonmotive force used as energy source or for the phosphorylation of ADP (Fig. 3.4).
3.3.2 Respirations in Microorganisms
The respiratory metabolism is more diverse in prokaryotic
microorganisms than in eukaryotic microorganisms:
1. The energy source (electron donor) is necessarily organic
in eukaryotes, while it may be organic or inorganic in
prokaryotes (chemoorganotrophs or chemolithotrophs).
2. The terminal electron acceptor is usually dioxygen in
eukaryotes, while in prokaryotes there are a variety of
terminal electron acceptors in addition to dioxygen, such
as nitrate, sulfate, iron, or manganese, thus defining a
variety of anaerobic respirations.
3. The composition of respiratory chain (electron carriers) is
variable in prokaryotes, while it is fairly constant in
eukaryotes.
Thus, four types of respirations are known in
microorganisms. Their characteristics are presented in
Table 3.3.
Chemoorganotrophic microorganisms are generally
heterotrophs, but some heterotrophs can use dihydrogen as
donor of electrons. Chemolithotrophic microorganisms are
mostly autotrophs, but some may use organic compounds as
carbon sources. The microorganisms that use a mineral
energy source and an organic carbon source are called
mixotrophic microorganisms*.
3.3.2.1 Aerobic Respiration in
Chemoorganotrophic Microorganisms
Electrons or reducing equivalents (symbolized e
À or [H])
derived from the oxidation of a compound organic (electron
donor or substrate) are transferred to dioxygen (terminal electron acceptor) by a chain of intermediate carriers (respiratory
ELECTRON FLOW
Reduced
Oxidized
Reduced
Oxidized
Oxidized
Reduced
Reduced
Oxidized
Reduced
Oxidized
Electron
donor
(substrate)
Terminal
electron
acceptor
Carrier 1
Carrier 2
Carrier n
Fig. 3.11 General scheme of a
chain of membrane electron
carriers (Drawing: M.-J. Bodiou)
Table 3.3 Different types of respirations
Respiration in
microorganisms
Electron donors
Final electron acceptors
Microorganisms
Aerobic
chemoorganotrophs
Organic
Dioxygen
Prokaryotic and eukaryotic
Anaerobic
chemoorganotrophs
Organic
Nitrate, sulfate, ferric iron,
fumarate, etc.
Prokaryotic (sometimes
eukaryotic)
Aerobic chemolithotrophs
Inorganic (ammonium, sulfide, ferrous iron, etc.)
Dioxygen
Prokaryotic
Anaerobic
chemolithotrophs
Inorganic (ammonium, sulfide, hydrogen, etc.)
Nitrate, sulfate, etc.
Prokaryotic
3 Structure and Functions of Microorganisms: Production and Use of Material and Energy
37
of ADP. This synthesis is catalyzed by ATP synthases,
insoluble membrane enzymes*. During respiration, electron transfer between carriers takes place in the cytoplasmic
membrane among prokaryotic microorganisms and in the
internal mitochondrial membrane among eukaryotic
microorganisms; the resulting ATP production is obtained
by oxidative phosphorylation. For photosynthesis, electron
transfer occurs in specialized membranes (thylakoids in
cyanobacteria and chloroplasts in photosynthetic eukaryotes
or photosynthetic cytoplasmic membrane in anoxygenic
phototrophic bacteria), and ATP is produced by photophosphorylation. Almost all of enzymes associated with electron
transport are included in the membrane and are insoluble.
The transfer of electrons and protons generates a protonmotive force used as energy source or for the phosphorylation of ADP (Fig. 3.4).
3.3.2 Respirations in Microorganisms
The respiratory metabolism is more diverse in prokaryotic
microorganisms than in eukaryotic microorganisms:
1. The energy source (electron donor) is necessarily organic
in eukaryotes, while it may be organic or inorganic in
prokaryotes (chemoorganotrophs or chemolithotrophs).
2. The terminal electron acceptor is usually dioxygen in
eukaryotes, while in prokaryotes there are a variety of
terminal electron acceptors in addition to dioxygen, such
as nitrate, sulfate, iron, or manganese, thus defining a
variety of anaerobic respirations.
3. The composition of respiratory chain (electron carriers) is
variable in prokaryotes, while it is fairly constant in
eukaryotes.
Thus, four types of respirations are known in
microorganisms. Their characteristics are presented in
Table 3.3.
Chemoorganotrophic microorganisms are generally
heterotrophs, but some heterotrophs can use dihydrogen as
donor of electrons. Chemolithotrophic microorganisms are
mostly autotrophs, but some may use organic compounds as
carbon sources. The microorganisms that use a mineral
energy source and an organic carbon source are called
mixotrophic microorganisms*.
3.3.2.1 Aerobic Respiration in
Chemoorganotrophic Microorganisms
Electrons or reducing equivalents (symbolized e
À or [H])
derived from the oxidation of a compound organic (electron
donor or substrate) are transferred to dioxygen (terminal electron acceptor) by a chain of intermediate carriers (respiratory
ELECTRON FLOW
Reduced
Oxidized
Reduced
Oxidized
Oxidized
Reduced
Reduced
Oxidized
Reduced
Oxidized
Electron
donor
(substrate)
Terminal
electron
acceptor
Carrier 1
Carrier 2
Carrier n
Fig. 3.11 General scheme of a
chain of membrane electron
carriers (Drawing: M.-J. Bodiou)
Table 3.3 Different types of respirations
Respiration in
microorganisms
Electron donors
Final electron acceptors
Microorganisms
Aerobic
chemoorganotrophs
Organic
Dioxygen
Prokaryotic and eukaryotic
Anaerobic
chemoorganotrophs
Organic
Nitrate, sulfate, ferric iron,
fumarate, etc.
Prokaryotic (sometimes
eukaryotic)
Aerobic chemolithotrophs
Inorganic (ammonium, sulfide, ferrous iron, etc.)
Dioxygen
Prokaryotic
Anaerobic
chemolithotrophs
Inorganic (ammonium, sulfide, hydrogen, etc.)
Nitrate, sulfate, etc.
Prokaryotic
3 Structure and Functions of Microorganisms: Production and Use of Material and Energy
37