Evolution of photosynthesis

Helen has asserted that the evolution of photosynthesis is impossible because of entropy.

Something recently came to light in that area:

http://www.howstuffworks.com/news-item215.htm

(Whole genome analysis of photosynthetic prokaryotes.
J.Raimond et.al. Science 298, 2002, 1616-1620)

"We know that the process evolved in bacteria, probably before 2.5 billion years ago, but the history of photosynthesis's development is very hard to trace," said Blankenship. "There's a bewildering diversity of photosynthetic microorganisms out there that use clearly related, but somewhat different processes. They have some common threads tying them together, but it has never been clear how they relate to each other and how the process of photosynthesis started, how it developed, and how we actually wind up with two photosystems working together in more complex photosynthetic organisms."

In a paper forthcoming in the November 22 issue of the journal Science, Blankenship and colleagues partially unravel this mystery through an analysis of the genomes of five bacteria representing the basic groups of photosynthetic bacteria and the complete range of known photosynthetic processes. The paper is co-authored by ASU doctoral student Jason Raymond, Olga Zhazybayeva and J. Peter Gogarten of the University of Connecticut at Storrs, and Sveta Y. Gerdes of Integrated Genomics in Chicago, Illinois.

http://www.spaceref.ca/news/viewpr.html?pid=9875

It turns out that horizonal gene transfer (a frequent occurance in the primitive sort of bacterial sexual reproduction called "conjugation") contributed greatly to the evolution of photosynthesis in cyanobacteria.

Cyanobacteria are very much like chloroplasts, having the same sort of circular (bacterial) DNA, almost idential thylakoids (stacked membranous sacs holding photosynthetic pigment)

They reproduce on their own, and if they were free-living would certainly have been classified as cyanobacteria. They are endosymbionts, bacteria that now live inside plant cells.

Source for studying photosynthesis:
http://photoscience.la.asu.edu/photosyn/education/learn.html

It turns out that genetic analyses of all the major photosynthetic bacteria indicates that heliobacteria gave rise to cyanobacteria.

Chlorophyll owes it's activity to the structure of its porphyrin ring. Here's the structure:

http://www.chm.bris.ac.uk/motm/chlorophyll/chphyll.gif

It's the alternating single and double bonds that do it. This creates a delocalization of bonding electrons, the "resonance bonding" (at least that's what they called it in 1967, when I took my first organic chem) that permits a flow of electrons. These free electons provide the energy.

That's all that photosynthesis does. The energy splits a water molecule, with the end result of changing a molecule of ADP to ATP, which powers many cellular processes, including the synthesis of sugars.

But that's another process.

How did chlorophyll evolve? Well, it existed essentially as it is today in the Precambrian.

http://www.ucmp.berkeley.edu/bacteria/cyanofr.html

Research has shown that the
The PRC-barrel: a widespread, conserved domain shared by photosynthetic reaction center subunits and proteins of RNA metabolism
Vivek Anantharaman1 and L Aravind1
Genome Biology 2002 3(11):research0061.1-0061.9

The Helicobacterial photosynthesis is rather different from that of cyanobacteria and plants; it is simpler and not as efficient. But as noted above, genetic studies have confirmed that a system like this:

http://bioinfo.weizmann.ac.il:3456/kegg/dblinks/hpy/hpy00860.gif

gave rise to the chlorphylls found in cyanobacteria and plants.

So where did this come from? Well, it is very much like oxidative phosphorylation, which is even more ancient, and is found in all living things:

http://www.biologie.uni-hamburg.de/b-online/e19/19d.htm

So we can see that the porphyrin ring is a sort of generalized energy-transfer molecule that does a lot of different things.

What does entropy do to prevent any of this?

[ June 06, 2003, 11:11 PM: Message edited by: The Galatian ]

 

The evolution of the plant cell by endosymbiosis of a larger cell with photosynthetic bacteria was considered likely because an example of bacteria have done this with a species of amoeba.

But there's better evidence for it than that. There are endosymbiotic cyanobacteria, which function precisely like chloroplasts.

http://treebiol.forest.wisc.edu/forestry415/TreeStructure/leaves/lightReactions/symbiont.htm

[ June 07, 2003, 06:29 PM: Message edited by: The Galatian ]

 

I guess Helen is no longer interested...