Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 38

from potential damage by oxidation. A rather unusual pigment which functions as
a primary pigment, is bacteriorhodopsin which causes the archaea which express
it, to appear purple. Returning to the principally eukaryotic process, the chloro-
phylls described above which receive the incident light are clustered in highly
organised structures called antennae located on the cell surface. The incident
light excites the energy state of the recipient chlorophyll to a higher energy state.
When the chlorophyll returns to its normal level, it releases electrons which are
transferred to a neighbouring chlorophyll. The transfer is repeated until the elec-
trons arrive at a photosystem, from which point they enter an electron transport
chain linked to the reduction of NAD + and the synthesis of ATP. There are many
similarities between electron transport in respiration and in photosynthesis in
that they are both membrane bound and may be coupled to phosphorylation and
thus synthesis of ATP, according to Mitchell's chemiosmotic theory, by employ-
ing a similar strategy of a proton gradient described earlier for respiration. In
eukaryotic higher organisms, photosynthesis occurs in the chloroplast while in
bacteria it occurs in the cytoplasmic membrane. The precise location in bacteria
is sometimes described as being the mesosome. This has been reported as an
infolding of the bacterial cellular membrane which sometimes appears to be in
association with the bacterial DNA and often is found near nascent cell walls.
Although considerable effort has been invested in determining its function, there
is still disagreement as to whether or not the mesosome is indeed a bacterial
cell structure or is simply an artefact occurring during preparation of samples for
microscopy (Fernandez et al ., 2005). Thus the site of bacterial photosynthesis
remains uncertain beyond it being bound in the cytoplasmic membrane.
Photosystems in eukaryotes and cyanobacteria
There are two types of photosystem which may occur in photosynthetic organisms
indicated in Figure 2.10; photosystem 1 which receives electrons from photo-
system 2 but may also operate independently by cyclic electron transport, and
photosystem 2 which is not present in all such organisms. The pathway for elec-
tron transport has two principal routes. One involves only photosystem 1. In this,
electrons transferred from the antennae to photosystem 1 cause excitation of the
chlorophyll in this system. When the chlorophyll returns to its lower energy state,
the electrons are transferred to ferredoxin, which is one of the iron containing
proteins in the chain of electron carriers. From this point there is a choice of
routes; either the cyclic path by way of a chain of cytochrome molecules starting
with cytochrome b 563 and finally returning to chlorophyll a, or by the non-cyclic
route which is the transfer of electrons to NADP + .
The source of the hydrogen atom required to reduce the NADP + to NADPH
in this system is the water molecule which donates its electrons to photosystem
2 to replace those lost to NADP + . It is the origin of the oxygen released dur-
ing photosynthesis, hence the term oxygenic. Thus the overall flow of electrons
in the non-cyclic pathway is from the water molecule, through photosystem 2,
along a series of cytochromes to photosystem 1 and thence to ferredoxin and