8. REACTIONS OF INORGANIC SUBSTANCES
357
sions, they cannot oxidize mineral sulfur compounds aerobically in
the presence of oxygen. He also demonstrated that, in contrast to plants,
the photosynthetic bacteria cannot eliminate as 0 2 the oxidizing constituent produced from the photochemical reaction, and that they must
replace this mechanism by the oxidation of an exogenous electron donor.
This difference between the photosynthesis of green plants and that of
bacteria is shown schematically in Fig. 1.
(CH 2 0)
4
Biosynthesis ® Γ
co 2
FIG. 1. Comparative scheme of photosynthesis in green plants and in photolithotrophic bacteria according to van Niel's theory.
If photosynthesis is considered from the thermodynamic point of
view, it may be seen that in green plants the energy required for the
assimilation of carbon dioxide
C0 2 + H 2 0 -> (CH 2 0) + 0 2
(-Δ^° 29 8θ = -115 kcal.)
can be provided in theory by 3 quanta of red light per molecule of C0 2 .
The experimental attempts to establish the amount of energy which is
actually necessary led to discordant results and are still the subject of
controversy. However, the most commonly accepted average value is
about 9 quanta of light energy per molecule of fixed C0 2 (18).
In photo-lithotrophic bacteria, the oxidation of the exogenous electron donor constitutes an appreciable source of free energy. The question arises whether it may be utilized by the organism for biosynthetic
activities. Following the work of Buder (19) and Blum (20), this question was first answered in the affirmative and the green and purple sulfur
bacteria were considered to be both photosynthetic and chemo-lithotrophic. According to this interpretation, the energy balance sheet of
the fixation of C0 2 by the sulfur bacteria is established in the presence
of sulfide as follows:
C0 2 + 2H 2 S -* (CH 2 0) + 2S + H 2 0
(-ΔΓ 298 ο = -15 kcal.)
Chlorophyll
—
* D+H 2 0+energy
Electron
donor
357
sions, they cannot oxidize mineral sulfur compounds aerobically in
the presence of oxygen. He also demonstrated that, in contrast to plants,
the photosynthetic bacteria cannot eliminate as 0 2 the oxidizing constituent produced from the photochemical reaction, and that they must
replace this mechanism by the oxidation of an exogenous electron donor.
This difference between the photosynthesis of green plants and that of
bacteria is shown schematically in Fig. 1.
(CH 2 0)
4
Biosynthesis ® Γ
co 2
FIG. 1. Comparative scheme of photosynthesis in green plants and in photolithotrophic bacteria according to van Niel's theory.
If photosynthesis is considered from the thermodynamic point of
view, it may be seen that in green plants the energy required for the
assimilation of carbon dioxide
C0 2 + H 2 0 -> (CH 2 0) + 0 2
(-Δ^° 29 8θ = -115 kcal.)
can be provided in theory by 3 quanta of red light per molecule of C0 2 .
The experimental attempts to establish the amount of energy which is
actually necessary led to discordant results and are still the subject of
controversy. However, the most commonly accepted average value is
about 9 quanta of light energy per molecule of fixed C0 2 (18).
In photo-lithotrophic bacteria, the oxidation of the exogenous electron donor constitutes an appreciable source of free energy. The question arises whether it may be utilized by the organism for biosynthetic
activities. Following the work of Buder (19) and Blum (20), this question was first answered in the affirmative and the green and purple sulfur
bacteria were considered to be both photosynthetic and chemo-lithotrophic. According to this interpretation, the energy balance sheet of
the fixation of C0 2 by the sulfur bacteria is established in the presence
of sulfide as follows:
C0 2 + 2H 2 S -* (CH 2 0) + 2S + H 2 0
(-ΔΓ 298 ο = -15 kcal.)
Chlorophyll
—
* D+H 2 0+energy
Electron
donor