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Summary: (Reprinted from Nature, Vol. 256, No. 5518, pp. 599-600, A ugust 14, 1975)
Oxygen reduction and optimum
production of ATP in photosynthesis
THEaccepted pathway of CO., fixation in plant photosynthesis
requires that the photosynthetic light reactions produce ATP
and reduced pyridine nucleotide (NADPH) in the molar ratio
3:2 (ref. 1). Early studies of photosynthetic phosphorylation
suggested that non-cyclic electron transport could produce only
equimolar amounts of ATP and NADPH, and the source of
the extra ATP was presumed to be cyclic electron flow2. The
view that the non-cyclic system is by itself able to produce
twice as much ATP as NADPH has been expressed3v4,and
removes the need for the in vivo operation of a cyclic electron
flow which can be demonstrated in vitro only in artificial con-
ditions5. An inflexible ATP-NADPH ratio of 2:l for the
products of the light reactions would, however, result in a feed-
back inhibition of electron transport, with ADP concentration
as the limiting factor. One way of achieving flexibility in the
relative production of ATP and NADPH would be for a low
Ferredoxin
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