Page 139 - Color Atlas of Biochemistry

P. 139

130 Metabolism

Photosynthesis: dark reactions quinone pool takes place as described on the
preceding page and shown in B.
The “light reactions” in photosynthesis bring
about two strongly endergonic reactions—the
+
+
reduction of NADP to NADPH+H and ATP B. Redox series
synthesis (see p. 122). The chemical energy It can be seen from the normal potentials E 0
needed for this is produced from radiant en- (see p. 18) of the most important redox sys-
ergy by two photosystems. tems involved in the light reactions why two
excitation processes are needed in order to
+
transfer electrons from H 2 OtoNADP .After
A. Photosystem II 0
excitation in PS II, E rises from around –1 V
The photosynthetic electron transport chain back to positive values in plastocyanin
in plants starts in photosystem II (PS II; see (PC)—i. e., the energy of the electrons has to
p. 128). PS II consists of numerous protein be increased again in PS I. If there is no NADP +
subunits (brown) that contain bound pig- available, photosynthetic electron transport
ments—i. e., dye molecules that are involved can still be used for ATP synthesis. During
in the absorption and transfer of light energy. cyclic photophosphorylation, electrons return
The schematic overview of PS II presented from ferredoxin (Fd) via the plastoquinone
here (1) only shows the important pigments. pool to the b/f complex. This type of electron
These include a special chlorophyll molecule, transport does not produce any NADPH, but
+
the reaction center P 680 ; a neighboring Mg 2+ does lead to the formation of an H gradient
free chlorophyll (pheophytin); and two bound and thus to ATP synthesis.
plastoquinones (Q A and Q B ). A third quinone
(Q P ) is not linked to PS II, but belongs to the
plastoquinone pool. The white arrows indi- C. Calvin cycle
cate the direction of electron flow from water The synthesis of hexoses from CO 2 is only
to Q P . Only about 1% of the chlorophyll mol- showninavery simplified form here;acom-
ecules in PS II are directly involved in photo- plete reaction scheme is given on p. 407. The
chemical excitation (see p. 128). Most of them actual CO 2 fixation—i. e., the incorporation of
are found, along with other pigments, in what CO 2 into an organic compound—is catalyzed
are known as light-harvesting or antenna by ribulose bisphosphate carboxylase/oxygen-
complexes (green). The energy of light quanta ase (“rubisco”). Rubisco, the most abundant
striking these can be passed on to the reaction enzyme on Earth, converts ribulose 1,5-bis-
center, where it can be utilized. phosphate, CO 2 and water into two mole-
In Fig. 2, photosynthetic electron transport cules of 3-phosphoglycerate. These are then
in PS II is separated into the individual steps converted, via 1,3-bisphosphoglycerate and
involved. Light energy from the light-harvest- 3-phosphoglycerate, into glyceraldehyde
ing complexes (a)raises anelectronof the 3-phosphate (glyceral 3-phosphate). In this
chlorophyll in the reaction center to an way,12 glyceraldehyde 3-phosphates are syn-
excited “singlet state.” The excited electron is thesized from six CO 2 . Two molecules of this
immediately passed on to the neighboring intermediate are used by gluconeogenesis re-
pheophytin. This leaves behind an “electron actions to synthesize glucose 6-phosphate
gap” in the reaction center—i. e., a positively (bottom right). From the remaining 10 mole-
charged P 680 radical (b). This gap is now filled cules, six molecules of ribulose 1,5-bisphos-
by an electron removed from an H 2 Omole- phate are regenerated, and the cycle then
cule by the water-splitting enzyme (b). The starts over again. In the Calvin cycle, ATP is
excited electron passes on from the pheophy- required for phosphorylation of 3-phospho-
tin via Q A to Q B ,convertingthe latter into a glycerate and ribulose 5-phosphate.
+
semiquinone radical (c). Q B is then reduced to NADPH+H , the second product of the light
hydroquinone by a second excited electron, reaction, is consumed in the reduction of 1,3-
and is then exchanged for an oxidized qui- bisphosphoglycerate to glyceraldehyde 3-
none (Q P ) from the plastoquinone pool. Fur- phosphate.
ther transport of electrons from the plasto-

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