Bioenergetics of Light Reaction

The light reaction has a definite, quantifiable photon cost. Releasing a single electron from a photosystem's reaction centre requires two quanta of light in total during non-cyclic transport: one quantum drives the transport of that electron from water up to PS I, and a second quantum drives its further transport from PS I onward to NADP+. Because reducing one molecule of NADP+ to NADPH+H+ requires two electrons, producing the two molecules of NADPH+H+ that accompany one full turn of non-cyclic electron transport requires four electrons in total - and since each of those four electrons costs two quanta, the whole process costs 8 quanta of light. This same figure, 8 quanta, is also exactly what is needed to evolve one molecule of oxygen, since releasing one O2 likewise requires transporting four electrons through the chain. These photon and electron costs connect directly to the differing energy demands of the dark reaction pathways covered later in the chapter: C3 plants spend 3 ATP and 2 NADPH+H+ to evolve each molecule of oxygen (equivalently, 18 ATP and 12 NADPH+H+ to evolve six oxygen molecules, i.e. to make one hexose sugar), while C4 plants spend mo …