Examination of the Calvin cycle shows that three ATP
and two NADPH are required to assimilate one molecule
of CO2 into carbohydrate and to regenerate one ribulose-
1,5-bisphosphate (RuBP) to complete the cycle. In whole
chain linear electron transport the absorption of four
photons will reduce one molecule of NADPH while
translocating a maximum of six protons into the thylakoid
lumen; two from water oxidation and four from plastoquinol
oxidation by the cytochrome b6/f complex [3].
Since two NADPH, and therefore the absorption of eight
photons, are required for the assimilation of one CO2 into
carbohydrate, a maximum of 12 protons are transported
into the lumen for each CO2 assimilated. With 4 protons
required for the synthesis of 1 ATP [4–6], the 12 protons
transported would therefore be just sufficient to support
the synthesis of the 3 ATP required to balance 2 NADPH
in the assimilation of one CO2. The eight moles of red
photons, the minimum required to convert 1 mol of CO2
to carbohydrate, represents 1388 kJ energy. One-sixth of a
mole of glucose, that is, a 1C carbohydrate unit, contains
477 kJ energy. Therefore the minimum energy loss in electron transport and synthesis of carbohydrate within
the Calvin cycle, that is, between energy trapping at the
reaction centers and carbohydrate production, is 1 (477/
1388); this amounts to a loss of 24.6% of the energy
contained in the original incident solar radiation
(Figure 2). Therefore, the maximal energy conversion
efficiency (ec) of C3 photosynthesis, before photorespiration
and respiration is 12.6%