8.7Photochemical Reaction and Electron Transport
In the photosynthetic reaction center, a photochemical reaction occurs in which certain chlorophyll a receives excitation energy and releases electrons (similar to the process seen in photoelectric reactions), thus generating chlorophyll a+. This drives electron transport, creating a concentration gradient of H+ (as seen in mitochondrial electron transport reactions) based on which ATP is synthesized and NADP+ is reduced to produce NADPH. The pigments and electron carriers involved in these reactions are buried in the thylakoid membrane as three types of protein complex, and the mobile electron carriers that connect these complexes are plastoquinone and plastocyanin (Fig. 8-5).
Although not described in detail here, these two photochemical complexes are akin to photoelectric semiconductors, and are designed so that electron transport steadily proceeds without the recombination of electrons and positive charges separated in photochemical reactions. The cytochrome b6f complex, on the other hand, transports H+, and has a structure and functionality similar to those of the mitochondrial cytochrome bc1 complex. The basic skeleton of plastoquinone is the same as that of mitochondrial ubiquinone, and the cytochrome b6f complex also transports H+ using the similar quinone cycle. Plastocyanin is a small, copper-binding protein, and is one of the major reasons why plants require copper in particular. When copper is deficient, some algae instead use c-type cytochrome, which bears a close resemblance to mitochondrial cytochrome c. In this way, respiration and photosynthesis use very similar electron transport mechanisms, and the systems are therefore believed to have evolved from the same energy metabolic system.
When photosynthetic electron transport is considered in terms of oxidation-reduction potential, the chlorophyll a+ (E˚’ = +1.1 - 1.2 V) generated in photosystem II deprives electrons from stable water molecules using its high oxidizing power, thereby emitting oxygen. On the other hand, the highly reducing electrons (E˚’ = -1.4 V) produced in photosystem I reduce NADP+ to produce NADPH. In this way, a series of electron transport from water molecules to NADPH is driven by light energy through the tandem connection of the two photosystems. The reason for these systems being connected in tandem is that the two reactions cannot take place simultaneously with the energy of visible light. Indeed, photosynthetic bacteria which have only one photosystem cannot break down water molecules. Besides the electron transport pathway in which the two photosystems are connected in tandem, a cyclic electron transport pathway also exists, in which electrons flow from photosystem I to near the cytochrome b6f complex. This pathway transports only H+ for ATP synthesis.
One of these two pathways is used in accordance with the environment and the needs at hand. Also in chloroplasts, F-ATP synthase located in the thylakoid membrane works in the coupling of H+ transport and ATP synthesis. This enzyme is essentially the same as those found in mitochondria and eubacteria (Fig. 8-4).
Photosynthetic light reaction can therefore be summarized as follows:
H2O + 2NADP+ + Light energy → O2 + 2NADPH + 2H+ (+ nATP)
The number of ATP molecules synthesized here is n, because it changes depending on the combination of the two electron transport pathways mentioned above. The value of n is 3 (NADPH:ATP = 2:3) in the general photosynthetic dark reactions discussed later (Equation 8-5), and is 4 or 5 in C4 photosynthesis (discussed in 8.9), which requires more ATP molecules.