Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 542

PO 2 −

H 2 C − O −

C = O
HCOH
HCOH
H 2 C

ADP

ATP

CO 2

H 2 C − OH

COO -

C = O
HC

PO 2 − − O −

HCOH

Ribulose-
1,5-biphosphate

− OH

PO 2 −
3-phosphoglycerate

H 2 C − O −

HC OH

−

PO 2 −
Ribulose-5-phosphate

H 2 C − O −

ATP
ADP

CO 2
Fixation

2 −

O=C − O − PO 3

Reduction

Regeneration

2 −

HCOH
H 2 C

O − PO 3

O

PO 2 −
1,3-biphosphoglycerate

O

− −

OH OH

HO

HO

NADH

Glucose-6-phosphate

HC=O

HCOH
H 2 C

NAD +

PO 2 −
Glyceraldehyde-3-phosphate

− O −

FIGURE 10.36 Schematic representation of the Calvin e Benson cycle, illustrating its three phases: CO 2 fixation,
reduction of fixed CO 2 , and regeneration of the CO 2 acceptor.

NADP þ , leaving behind molecular oxygen. Simultaneously, ADP is phosphorylated to ATP.
The light-phase reaction of photosynthesis is

light O

NADP þ þ H þ þ H O þ H PO

4 þ

ADP

2 þ

NADPH þ ATP (10.55)

! 2 3

In the second phase, the energy-rich products of the first phase, NADPH and ATP, are
used as the sources of energy to reduce the CO 2 to yield glucose (see Fig. 10.36 ). Simulta-
neously, NADPH is reoxidized to NADP þ , and the ATP is converted into ADP and phos-
phate. This dark phase is described by the following reaction:

/ 1
6

H þ þ NADP þ þ CO

2 þ

NADPH þ ATP glucose þ ADP þ H PO (10.56)

3 4

Both prokaryotic and eukaryotic cells can fix CO 2 by photosynthesis. In prokaryotes (e.g.
cyanobacteria), photosynthesis takes place in stacked membranes, whereas in eukaryotes an
organelle called the chloroplast conducts photosynthesis. Both systems contain chlorophyll to
absorb light. Light absorption by chlorophyll molecules results in an electronic excitation.
The excited chlorophyll molecule returns to the normal state by emitting light quanta in
a process known as fluorescence. The excited chlorophyll donates an electron to a sequence
of enzymes, and ATP is produced as the electrons travel through the chain. This ATP gener-
ation process is called photophosphorylation. Electron carriers in this process are ferredoxin
and several cytochromes.