Biological complexity:photosynthesis new research (Introduction)

by David Turell , Friday, October 20, 2017, 18:35 (2591 days ago) @ David Turell

More complexity unearthed:

https://www.quantamagazine.org/simple-bacteria-offer-clues-to-the-origins-of-photosynth...

"But in terms of function and structure, the photosystem reaction centers fall into two categories that differ in almost every way. Photosystem I serves mainly to produce the energy carrier NADPH, whereas photosystem II makes ATP and splits water molecules. Their reaction centers use different light-absorbing pigments and soak up different portions of the spectrum. Electrons flow through their reaction centers differently. And the protein sequences for the reaction centers don’t seem to bear any relation to each other.

"Both types of photosystem come together in green plants, algae and cyanobacteria to perform a particularly complex form of photosynthesis — oxygenic photosynthesis — that produces energy (in the form of ATP and carbohydrates) as well as oxygen, a byproduct toxic to many cells. The remaining photosynthetic organisms, all of which are bacteria, use only one type of reaction center or the other.

***

"After carefully taking images of the crystallized reaction centers, the team found that although the reaction center is officially classified as type I, it seemed to be more of a hybrid of the two systems. “It’s less like photosystem I than we thought,” Redding said. Some people might even call it a “type 1.5,” according to Gisriel.

"One reason for that conclusion involves greasy molecules called quinones, which help transfer electrons in photosynthetic reaction centers. Every reaction center studied so far uses bound quinones as intermediates at some point in the electron transfer process. In photosystem I, the quinones on both sides are tightly bound; in photosystem II, they are tightly bound on one side, but loosely bound on the other. But that’s not the case in the heliobacterium reaction center: Redding, Fromme and Gisriel did not find permanently bound quinones among the electron transfer chain’s stepping stones at all. That most likely means its quinones, although still involved in receiving electrons, are mobile and able to diffuse through the membrane. The system might send electrons to them when another, more energetically efficient molecule isn’t available.

***

"When an organism is exposed to too much light, electrons build up in the transfer chain. If oxygen is around, this buildup can lead to a harmfully reactive oxygen state. Adding a firmly bound quinone to the complex not only provides an additional slot to deal with potential traffic jams; the molecule, unlike others used in the transfer chain, also does not pose any risk of producing that deleterious form of oxygen. A similar explanation works for why reaction centers became asymmetric, Gisriel added: Doing so would have added more stepping stones as well, which would have similarly buffered against damage caused by the accumulation of too many electrons. (my bold)

***

"That hypothesis contradicts one of the widely held ideas about the origins of photosynthesis: that species incapable of photosynthesis suddenly obtained the capacity through genes passed laterally from other organisms. According to Cardona, in light of the new discoveries, horizontal gene transfer and gene loss may both have played a role in the diversification of reaction centers, although he suspects that the latter may have been responsible for the earliest events. The finding, he said, might suggest that “the balance skews toward the gene-loss hypothesis” — and toward the idea that photosynthesis was an ancestral characteristic that some groups of bacteria lost over time.

"Not everyone is so sure. Blankenship, for one, is skeptical. “I don’t buy that,” he said. “I don’t see any data here that suggests that oxygenic photosynthesis occurred that much earlier.” To him, the work by Redding, Fromme and their collaborators has not answered these questions; it has only conjectured about what may have happened. To solve that puzzle, scientists will need the reaction center structures of other bacteria, so they can continue evaluating the structural differences and similarities to refine the twisting roots of their evolutionary trees."

Comment: Photosynthesis is vital for multicellular life to evolve. How it arrived is still a puzzle as this article shows. But note my bold. Producing oxygen, unless there are existing protections (antioxidant factors), is dangerous. Brings me to the usual point. It all had to be developed with all parts in place, not stepwise. Obviously an argument for God.