Compare and contrast photosynthesis conducted by green photosynthetic bacteria (photosystem I), purple bacteria (photosystem II), and cyanobacteria ("Z pathway").

Question

What will be an ideal response?

Answer 1

In all cases, there is light absorption by a pigment that gets excited by a photon of light. This separates electrons from a molecule coupled to an ETS that is homologous to those of respiratory ETS.

In photosystem I, absorbed energy allows the transfer of e- from P840 to ferredoxin through a quinone. The e- then gets transferred to Ferredoxin-NAD+ reductase, which reduces NADP+ to NADPH. Bacteriophyll P840 absorbs light over a variety of wavelengths. PSI receives electrons associated with H from H2S (hydrogen sulfide) HS- (hydrogen bisulfide) or H2, or even from Fe2+.

Purple bacteria capture light not used by other phototrophs. The peak wavelength absorbed by bacteriochlorophyll P870 lies so far into the infrared (800-1,100 nm) that the photon energy is insufficient to reduce NAD(P) to NAD(P)H. In purple bacteria, Bchl P870 donates an energized electron to a quinone (Q). Two of these donated electrons complete the conversion of quinone to quinol (QH2). Electrons flow from QH2 to cytochrome bc, then are coupled to pumping of protons across the membrane in cytochrome bc. The proton potential drives synthesis of ATP. The cytochrome bc complex transfers the electrons back to P870, where they can be re-excited by photon energy (this is refered to a cyclic phototrophy). Since the reduction potential is too small to reduce NADP+ to NADPH, photosystem II requires reverse electron flow. In reverse electron flow, an electron donor reduces an ETS with an unfavorable reduction potential, requiring input of energy. Purple bacteria obtain this energy by spending ATP to increase the proton potential

The "Z" pathway of photolysis found in cyanobacteria and chloroplasts combines key features of both PS I and PS II to produce oxygen. In the PSII reaction center, the photoexcitation of chlorophyll P680 yields enough energy to split H2O. Both chlorophylls absorb photons of shorter wavelengths (higher energy) than the P840 and P870. Energy absorption boosts the electrons into a higher energy state. This causes the chlorophyll molecule to strip an electron from water, generating molecular oxygen. When this process happens twice, the molecular oxygens can combine to make O2.

The electrons from PS II do not cycle back to the PS II reaction center, as they do in purple bacteria. Instead, they are transferred to PS I by a protein called plastocyanin. The energy of the electron transferred by plastocyanin is augmented through absorption of a second photon by the chlorophyll of PS I. A second photon excites P700, transferring e- to ferredoxin and then to NADPH. Subsequent electron flow through ferredoxin can now generate NADH or NADPH. The proton gradient drives ATP synthesis. The energy (ATP) yields are high enough to generate NADPH and fix CO2 into biomass.

The components of photosystems I and II (PS I and PS II) run anaerobically, producing sulfur or oxidized organic by-products, but not O2. The Z pathway ultimately generates O2.