Question

Reverse electron transport makes unique ecological niches available to micro-organisms. Explain this statement.

Answer 1

Generally, electrons are transfered between the two photosystems, where PSI produces NADPH and PS II generates ATP. ATP is generated by Cyclic electron transport of PS I. NADPH is generated by non-cyclic electron transport that involves both the photosystems. Reverse Electron Flow is a mechanism in microbial metabolism. Chemolithotrophs use an electron donor with a higher redox potential than NAD+ must use energy to reduce NAD+ . One such chemolithotroph is PurpleSulfur bacteria. They are involved in anoxygenic phototrophy i.e. they don't generate oxygen. They have a single photosystem with bacteriochlorophyll that uses cyclic phosphorylation to generate ATP. For a photoautotrophic purple bacteria reducing power in the form of NAD is required. Thus, autotrophic purple bacteria engagaes in a process called reverse elctron flow that uses energy from the proton motive force to drive up the electrons. H2S or elemental Sulfur is used as an electron donor to replenish the electrons diverted to NAD. Purple bacteria often exist as photoheterotrophs in presence of organic compounds eliminating the need for reverse electron flow. Hence, reverse electron flow makes uniques ecological niche to the microorganisms.    .