38 Environmental Biotechnology
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 electrons arrive at a photosystem,
from which point they enter an electron transport chain linked to the reduc-
tion of NAD+and the synthesis of ATP. There are many similarities between
electron transport in respiration and in photosynthesis in that they are both mem-
brane bound and may be coupled to phosphorylation and thus synthesis of ATP,
according to Mitchell’s chemiosmotic theory, by employing a similar strategy of
a proton gradient described earlier for respiration. In eukaryotic higher organ-
isms, 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 bac-
terial 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. 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 noncyclic
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 noncyclic pathway is from the water molecule, through photosystem 2,
along a series of cytochromes to photosystem 1 and thence to ferredoxin and
finally NADP+, which also collects a hydrogen atom to complete the reduction