The growth medium for Rhodospirillum rubrum, a phototrophic non-sulfur bacterium, contains succinate as the sole source of electrons and carbon. The organisms metabolize succinate via the oxidative tricarboxylic acid cycle and produce intermediates which are assimilated (dashed arrows in the figure).
Succinate carbon atoms which serve as electron donors get oxidized in the cycle to CO₂. Succinate is a C-4 dicarboxylic acid which can release 14 electrons maximally.
Mark with a red dot the positions where decarboxylation reactions release CO₂.
Mark with a blue dot the positions where electrons are released. Use a dot for every pair of electrons released.
The listing of the structural formulas of the intermediates might aid in identifying the number of electrons available in the compound.