Kinetics and Energetics of Photosynthetic Micro-Organisms in Photobioreactors
213
Without entering into the details of the biochemical mechanisms, these
processes can be illustrated by the classical Z-scheme (Fig. 1) enabling one to
summarize the overall reaction by one stoichiometric equation:
4hv + HzO + NADP + + ADP + Pi
1
~NADPH, H + + ATP + H20 + ~0 2
(129)
This representation is, however, too simplistic to be used in a biochemically
structured model for growth, as the P/2e- ratio remains equal to 1 which is
obviously not the case when anabolic processes are considered. From Eqs. (25)
and (26), the P/2e- ratio equals 3.568/2.874 = 1.24 for active biomass synthesis,
and 3.3/1.92 = 1.73 for exopolysaccharide synthesis. In other words, NADPH,
H + produced by non-cyclic electron flow is directed to the Calvin cycle, which
fixes CO2, and further to other biosynthesis pathways which always require
more than 1 ATP per NADPH, H + produced. Thus the previous stoichiometry
of ATP synthesis associated with electron flow leaves a shortfall ranging between 24% and 73% whether active biomass or exopolysaccharide synthesis are
considered, which is evidence of a cyclic electron flow exclusively devoted to
ATP synthesis.
Although the idea of cyclic electron flow has been accepted for some time,
the pathway is, surprisingly, still not well understood [10]. A mechanistic
scheme cannot be used as a basic tool for a mathematical model; only a phenomenological treatment can give insight into the coupling of the two processes of
ATP and NADPH, H + synthesis.
6.1.2.2 Phenomenological Treatment of the Light Energy Transducing Process
The analysis proposed here consists of splitting the overall process into two
coupled reactions, one responsible for ATP synthesis:
3hv + ADP + Pi ~ ATP + H20
(130)
and one responsible for water photolysis:
hv + H20 + NADP +
1
~O2 + NADPH, H +
(131)
The sum of these two reactions restores the global reaction (Eq. 129). The
choice of 3hv for ATP synthesis and one hv for water hydrolysis should be
regarded as more or less arbitrary and does not correspond to any wellestablished mechanism. However, this stoichiometry can be justified considering
that two light quanta at PSI and one light quantum at PS II are necessary to
create an H + gradient over the thylakoid membrane while one light quantum
only at PS II is necessary to initiate water splitting. Formally, this separation
between the functioning capabilities of the two photosystems corresponds to the
previous stoichiometric analysis distinguishing between ATP synthesis rate
JATP (Eq. 29) and cofactor reduction rate Jcov (Eq. 28).
213
Without entering into the details of the biochemical mechanisms, these
processes can be illustrated by the classical Z-scheme (Fig. 1) enabling one to
summarize the overall reaction by one stoichiometric equation:
4hv + HzO + NADP + + ADP + Pi
1
~NADPH, H + + ATP + H20 + ~0 2
(129)
This representation is, however, too simplistic to be used in a biochemically
structured model for growth, as the P/2e- ratio remains equal to 1 which is
obviously not the case when anabolic processes are considered. From Eqs. (25)
and (26), the P/2e- ratio equals 3.568/2.874 = 1.24 for active biomass synthesis,
and 3.3/1.92 = 1.73 for exopolysaccharide synthesis. In other words, NADPH,
H + produced by non-cyclic electron flow is directed to the Calvin cycle, which
fixes CO2, and further to other biosynthesis pathways which always require
more than 1 ATP per NADPH, H + produced. Thus the previous stoichiometry
of ATP synthesis associated with electron flow leaves a shortfall ranging between 24% and 73% whether active biomass or exopolysaccharide synthesis are
considered, which is evidence of a cyclic electron flow exclusively devoted to
ATP synthesis.
Although the idea of cyclic electron flow has been accepted for some time,
the pathway is, surprisingly, still not well understood [10]. A mechanistic
scheme cannot be used as a basic tool for a mathematical model; only a phenomenological treatment can give insight into the coupling of the two processes of
ATP and NADPH, H + synthesis.
6.1.2.2 Phenomenological Treatment of the Light Energy Transducing Process
The analysis proposed here consists of splitting the overall process into two
coupled reactions, one responsible for ATP synthesis:
3hv + ADP + Pi ~ ATP + H20
(130)
and one responsible for water photolysis:
hv + H20 + NADP +
1
~O2 + NADPH, H +
(131)
The sum of these two reactions restores the global reaction (Eq. 129). The
choice of 3hv for ATP synthesis and one hv for water hydrolysis should be
regarded as more or less arbitrary and does not correspond to any wellestablished mechanism. However, this stoichiometry can be justified considering
that two light quanta at PSI and one light quantum at PS II are necessary to
create an H + gradient over the thylakoid membrane while one light quantum
only at PS II is necessary to initiate water splitting. Formally, this separation
between the functioning capabilities of the two photosystems corresponds to the
previous stoichiometric analysis distinguishing between ATP synthesis rate
JATP (Eq. 29) and cofactor reduction rate Jcov (Eq. 28).