569
Photosynthesis in Nature: A New Look
Carbohydrate C x (H 2 O) y is formed (Eq. 3.1), where x and y are whole numbers that differ depending on the specific carbohydrate that is being produced.
The release of O 2 in photosynthesis is the fundamental photoinduced conversion
reaction, which under the present hypothesis is supposed to involve H 2 O 2 either
via disproportionation or upon biological processes (Eq. 3.2) (Komissarov 2003;
Buick 2008; Moffett and Zafiriou 1990; Liang et al. 2006).
The release of O 2 from H 2 O 2 instead of H 2 O can be understood from several
mechanistic approaches: (i) Mechanism for oxygen release from H 2 O 2 instead of
H 2 O; (ii) Effective oxidation of H 2 O 2 instead of H 2 O in releasing photosynthetic
O 2 (iii) generation of H 2 O 2 from DOM and POM; (iv) photoinduced generation
of H 2 O 2 from ultrapure H 2 O; (v) Endogenous H 2 O 2 in the photosynthetic cell and
effects of exogenous H 2 O 2 ; (vi) H 2 O 2 formation in water, lipid and protein environments in the presence of Chlorophyll; and (vii) Occurrence of H 2 O 2 and its
effects on photosynthesis.
3.1 Mechanism for Oxygen Release from H 2 O 2 Instead of H 2 O
Experimental studies show that various cyanobacteria may release O 2 from the
decomposition of H 2 O 2 during photosynthesis (Komissarov 1994, 1995, 2003;
Velthuys and Kok 1978; Mano et al. 1987; Renger 1987; Anan’ev and Klimov
1988; Bader and Schmid 1988, 1989; Schroeder 1989; Schröder and Åkerlund
1990; Miyake and Asada 1992; Bader 1994). Based on the mechanism given by
Komissarov (1994, 1995, 2003 ) and the mechanism of H 2 O 2 photodecomposition
by earlier studies (Christensen et al. 1982; Bielski et al. 1985), the release of O 2
from H 2 O 2 can be generalized as follows (Eqs. 3.3–3.6 or 3.7–3.10):
or
(3.3)
H 2 O 2 → H
+ + HO 2
−
(3.4)
HO 2
− + h → HO 2
(3.5)
HO 2 → H
+ + O 2
−
(3.6)
O 2
− + h → O 2
(3.7)
H 2 O 2 + HO
• → HO 2
• + H 2 O k = 3. 0 × 10
7 M
−1 s
−1
(3.8)
HO 2
• + HO 2
• → H 2 O 2 + O 2 k = 8. 3 × 10
5 M
−1 s
−1
(3.9)
HO 2
• + O 2
•− → HO 2
− + O 2 k = 9. 7 × 10
7 M
−1 s
−1
(3.10)
HO 2
• ↔ O 2
•− + H
+ k = 1. 58 × 10
−5 M
−1 s
−1
Photosynthesis in Nature: A New Look
Carbohydrate C x (H 2 O) y is formed (Eq. 3.1), where x and y are whole numbers that differ depending on the specific carbohydrate that is being produced.
The release of O 2 in photosynthesis is the fundamental photoinduced conversion
reaction, which under the present hypothesis is supposed to involve H 2 O 2 either
via disproportionation or upon biological processes (Eq. 3.2) (Komissarov 2003;
Buick 2008; Moffett and Zafiriou 1990; Liang et al. 2006).
The release of O 2 from H 2 O 2 instead of H 2 O can be understood from several
mechanistic approaches: (i) Mechanism for oxygen release from H 2 O 2 instead of
H 2 O; (ii) Effective oxidation of H 2 O 2 instead of H 2 O in releasing photosynthetic
O 2 (iii) generation of H 2 O 2 from DOM and POM; (iv) photoinduced generation
of H 2 O 2 from ultrapure H 2 O; (v) Endogenous H 2 O 2 in the photosynthetic cell and
effects of exogenous H 2 O 2 ; (vi) H 2 O 2 formation in water, lipid and protein environments in the presence of Chlorophyll; and (vii) Occurrence of H 2 O 2 and its
effects on photosynthesis.
3.1 Mechanism for Oxygen Release from H 2 O 2 Instead of H 2 O
Experimental studies show that various cyanobacteria may release O 2 from the
decomposition of H 2 O 2 during photosynthesis (Komissarov 1994, 1995, 2003;
Velthuys and Kok 1978; Mano et al. 1987; Renger 1987; Anan’ev and Klimov
1988; Bader and Schmid 1988, 1989; Schroeder 1989; Schröder and Åkerlund
1990; Miyake and Asada 1992; Bader 1994). Based on the mechanism given by
Komissarov (1994, 1995, 2003 ) and the mechanism of H 2 O 2 photodecomposition
by earlier studies (Christensen et al. 1982; Bielski et al. 1985), the release of O 2
from H 2 O 2 can be generalized as follows (Eqs. 3.3–3.6 or 3.7–3.10):
or
(3.3)
H 2 O 2 → H
+ + HO 2
−
(3.4)
HO 2
− + h → HO 2
(3.5)
HO 2 → H
+ + O 2
−
(3.6)
O 2
− + h → O 2
(3.7)
H 2 O 2 + HO
• → HO 2
• + H 2 O k = 3. 0 × 10
7 M
−1 s
−1
(3.8)
HO 2
• + HO 2
• → H 2 O 2 + O 2 k = 8. 3 × 10
5 M
−1 s
−1
(3.9)
HO 2
• + O 2
•− → HO 2
− + O 2 k = 9. 7 × 10
7 M
−1 s
−1
(3.10)
HO 2
• ↔ O 2
•− + H
+ k = 1. 58 × 10
−5 M
−1 s
−1