Photolysis is one of the critical step for photosynthesis as an electron source.Do you know any plant that undergoes photosynthesis by passing PSII, photosystem II, (or without it), generating alternative electron source apart from water splitting.
PS1 and PS2 essentially do the same thing but at different optimal light wavelengths, but only PS2 provides electrons for the production of NADPH. So I doubt there is any plant that bypasses PS2 since it is an essential step in the photosynthetic process :-)
The topic is under scrutiny in artificial photosynthesis by using a variety of donors, but as far improving real plants a review can be a rationale, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4054791/ and http://www.nature.com/nmat/journal/v5/n10/full/nmat1734.html
It is possible to by pass Photo system II by subjecting the plants to wave length above 680nm. this result in the production of ATP only. there is no oxidation of water and no reduction of CO2,and the process of photosynthesis is strongly inhibited. in fact, many herbicides kill plants by inhibiting photo system II. some anaerobic microorganisms have Photo system I only.
It does exist the bacterial anoxygenic photosynthesis. They use sulfur as an electron donor, not water. They have a system of light harvesting similar to PSII. The anoxygenic photosynthesis is not known for plants
When the excess amount of NAPH is produced and more ATP is required, PSII is by-passed and cyclic photosynthesis take place where electrons are travelled through PS I to Q. This is a well-known phonemenon in plants.
Moreover, C4 plants were evolved to escape from unwanted effects of oxygen. Therefore PS II is almost inoperational in bundle sheath cells.
Thank you for the comments. @Dr. Sökmen, what about the electron source in this system?
You mean the production of NADPH? The answer is hidden in the types of C4 plants especially those containing NADP_Malic enzyme. Regards
In cyanobacteria and algae there is a host of PS II-less mutants that live a mixotrophic lifestyle: ATP in part comes from some sort of cyclic electron flow around PS I and reducing equivalents come from added organic substrates. There obviously is no oxygen evolution in such strains.
Please have a careful look on the question. It is asked whether there is a plant which can bypass the PSII for doing photosynthesis. I think there should be a source for electron generation and excitation, in the plant the electrons come from the process of water splitting and the driving force for excitation of electron is the quantum energy of the light. In the process of cyclic electron transport there is no extra exited electron which goes towards NADP, therefore the yield of the cyclic electron transport is just production of ATP which help to reduce the pressure of extra light energy. If you are aware of any other source of electron which is not originated from water, It would raise my knowledge in this regards and would be happy to hear about it.
The best
Sasan
Dear Sasan,
I have already answered your question:
It does exist the bacterial anoxygenic photosynthesis. They use sulfur as an electron donor, not water. They have a system of light harvesting similar to PSII.
Do plants undergo photosynthesis without PSII or by passing PSII? - ResearchGate. Available from: https://www.researchgate.net/post/Do_plants_undergo_photosynthesis_without_PSII_or_by_passing_PSII [accessed Feb 8, 2017].
Dear Gelettii,
Your answer seems not cover the question. Sulfur is well known electron source for the photosynthetic bacteria, but question is on the plants. When stoma are fully closed, plants should develop alternative electron sources. In many physiology books, this point is not clear. Dr. Sokmens' point NADP-Malic enzyme might describe such electron source, and CO2 outcome for the calvin cycle. Does it mean "PSII is turned off" at all or just ratio of electrons derived from water was minimised as much as possible in such plants (i.e. C4 or CAM)...
It is well known that photosynthesis early events lead to NADPH and ATP synthesis in proper proportions, what imposes a linear operation of the redox chain with PS2 and PS1 in series. Nevertheless, PS1 may also cyclically generate extra ATP and thus raise somewhat the ATP/NADPH ratio to the level needed by several metabolic processes. But PS1 cannot only turn in that way: the above ATP/NADPH ratio implies that NADPH also is synthesized. Notice that such cyclic operation could also be a mechanism to dissipate excessive absorbed energy. It should then be decoupled from photophosphorylation, because the proton gradient would prevent the ATP hydrolysis needed to regenerate ADP. (PS2 has its proper dissipative mechanism .) It could be possible to feed PS1 with electrons other than those coming from PS2 by reducing plastoquinone with molecules from the general metabolism that would cross the chloroplast envelope. Think of the phenomenon described, several decades ago, by Goedheer then myself, and studied and formalized in a more refined way by Bennoun (chlororespiration). Anyway, all these processes are an energy waste and cannot stand for a long time!
dears
Water actually requires more energy to complete the photosynthetic reaction than hydrogen sulfide. Therefore, where hydrogen sulfide was abundant, you might expect to see more anoxygenic photosynthesis than oxygenic photosynthesis.
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