Photosynthesis is the biological process by which the energy of the sun
is converted into energy-rich compounds that are used to drive cellular
processes. The energy from this remarkable process has provided energy for
essentially all life forms as well as having a dramatic effect on geological
formations over time. The mechanism by which organisms convert light
into chemical energy has been studied for hundreds of years. In the 1770s
Joseph Priestly demonstrated that plants released a substance, we know
now is oxygen, that could support life. In the 1930s Cornelis van Niel
formulated the photosynthetic process in terms of oxidation/reduction
reactions and Robert Hill established that chemical reactions led to oxygen
evolution. Robert Emerson with William Arnold performed the first
modern spectroscopic measurements on photosynthetic organisms that
quantified the amount of oxygen evolution compared to the number of
chlorophyll molecules. This was followed by many years of debate until
the biochemical involvement of ATP and the chemiosomotic hypothesis
were established (Chapter 9). In the late 1950s and early 1960s, Melvin
Calvin identified the basic enzymatic reactions of photosynthetic organ-
isms in what is now termed the Calvin cycle.
The biochemical isolation of the photosynthetic complexes was diffi-
cult as these complexes are located in cell membranes. By the early 1970s,
the use of detergents to solubilize the proteins from the cell membrane
(Chapter 7) led to the isolation of functional complexes. The availability
of these purified complexes provided the chance for detailed biochemical
and spectroscopic studies that have established the molecular mechanism
of light conversion. The initial photochemical process, namely the capture
of the light energy into a stable chemical state, is performed by large
pigment–protein complexes that capture the light and then transfer the
light energy to other complexes to perform electron-transfer reactions. These
pigment–protein complexes make use of bacteriochlorophyll or chlorophyll
cofactors, which are conjugated tetrapyrroles (Figure 20.1). The structures
of these tetrapyrroles from different photosynthetic organisms are similar,
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