Page 160 - Algae Anatomy, Biochemistry, and Biotechnology

Photosynthesis 143

belt that docks to PsaA and PsaB and to other 12 proteic subunits of PSI, termed PsaC to PsaN that
contribute to the coordination of antenna chromophores. On the whole PSI binds approximately 200
chromophore molecules. The cyanobacterial PSI exists as a trimer. One monomer consists of at least
12 different protein subunits, (PsaA, PsaB, PsaC, PsaD, PsaE, PsaF, PsaI, PsaJ, PsaK, PsaL, PsaM,
and PsaX) coordinating more than 100 chromophores.
After primary charge separation initiated by excitation of the chlorophyll a pair P 700 , the electron
passes along the ETC consisting of another chlorophyll a molecule, a phylloquinone, and the Fe 4 S 4
clusters. At the stromal side, the electron is donated by Fe 4 S 4 to ferredoxin and then transferred to
þ
þ
NADP reductase. The reaction cycle is completed byre-reduction of P 700 byplastocyanin (or the inter-
changeable cytochrome c 6 ) at the inner (lumenal) side of the membrane. The electron carried by plas-
tocyanin is provided by PSII by the way of a pool of plastoquinones and the cytochrome b 6 f complex.
Photosynthetic eukaryotes such as Chlorophyta, Rhodophyta, and Glaucophyta have evolved by
primary endosymbiosis involving a eukaryotic host and a prokaryotic endosymbiont. All other algae
groups have evolved by secondary (or higher order) endosymbiosis between a simple eukaryotic alga
and a non-photosynthetic eukaryotic host. Although the basic photosynthetic machinery is conserved
in all these organisms, it should be emphasized that PSI does not necessarily have the same compo-
sition and fine-tuning in all of them. The subunits that have only been found in eukaryotes, that is,
PsaG, PsaH, and PsaN, have actually only been found in plants and in Chlorophyta. Other groups
of algae appear to have a more cyanobacteria-like PSI. PsaM is also peculiar because it has been
found in several groups of algae including green algae, in mosses, and in gymnosperms. Thus, the
PsaM subunit appears to be absent only in angiosperms. With respect to the peripheral antenna
proteins, algae are in fact very divergent. All photosynthetic eukaryotes have Lhcs that belong to
the same class of proteins. However, the Lhca associated with PSI appear to have diverged relatively
early and the stoichiometry and interaction with PSI may well differ significantly between species.
Even the green algae do not possess the same set of four Lhca subunits that is found in plants.
Are all those light harvesting complexes necessary? They substantially increase the light harvest-
ing capacity of both photosystems by increasing the photon collecting surface with an associated
resonance energy transfer to reaction centers, facilitated by specific pigment–pigment interactions.
This process is related to the transition dipole–dipole interactions between the involved donor and
acceptor antenna molecules that can be weakly or strongly coupled depending on the distance
between and relative orientation of these dipoles. The energy migrates along a spreading wave
because the energy of the photon can be found at a given moment in one or the other of the many
resonating antenna molecules. This wave describes merely the spread of the probability of finding
the photon in different chlorophyll antenna molecules. Energy resonance occurs in the chromophores
of the antenna molecules at the lowest electronic excited state available for an electron, because only
this state has a life time (10 28 sec) long enough to allow energy migration (10 212 sec). The radiation-
less process of energy transfer occurs towards pigments with lower excitation energy (longer wave-
length absorption bands). Within the bulk of pigment–protein complexes forming the external and
internal antenna system, the energy transfer is directed to chlorophyll a with an absorption peak at
longest wavelengths. Special chlorophylls (P 680 at PSII and P 700 at PSI) located in the reaction

center cores represent the final step of the photon trip, because once excited (P 680 þ hn ! P 680 ;
þ 2 þ 2
P 700 þ hn ! P 700 ) they become redox active species (P 680 ! P 680 þe ;P 700 ! P 700 þe ), that
is, each donor releases one electron per excitation and activates different ETCs.
For an image gallery of the three-dimensional models of the two photosystems and LHCs in
prokaryotic and eukaryotic algae refer to the websites of Jon Nield and James Barber at the
Imperial College of London (U.K.).

ATP-Synthase
ATP production was probably one of the earliest cellular processes to evolve, and the synthesis of
ATP from two precursor molecules is the most prevalent chemical reaction in the world. The