Chris Colby Feb2593 03:20PM more bad design and the origin of chloroplasts I got an email

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Chris Colby Feb-25-93 03:20PM more bad design and the origin of chloroplasts Organization: animal -- coelomate -- deuterostome From: colby@bu-bio.bu.edu (Chris Colby) Message-ID: <110888@bu.edu> Newsgroups: talk.origins I got an email message awhile back correcting a mistake in a post of mine concerning bad design. In the post it mentions endosymbiosis as the origin of chloroplasts and says that chloroplasts came from cyanobacterial ancestors. This isn't the whole story. Chloroplasts are, incidentally, the organelles (found in plants and some lineages of protists) where photosynthesis occurs. The chloroplasts of red algae (a multicelluar protist) came from cyanobacterial ancestors. The chloroplasts of green algae (and plants, since they evolved from green algae) came from a group of organisms closely related to cyanobacteria. The bacteria _Prochloron_ is a living member of this group called the Prochloro- phytes. The chloroplasts of brown algae came from a group of bacteria as yet undiscovered or possibly extinct. Red algae chloroplasts and cyanobacteria have chlorophyll a and phycobilins as light harvesting molecules. Green algae (and plant) chloroplasts and Prochlorophytes have chlorophyll a and b and lack phycobilin. Brown algae chloroplasts have chlorophyll a and c. Hopefully someone will stick some of these in blender some day and sequence their rbcl gene and find out what they are related to. (Prochlorophytes may have arisen multiple times from cyanobacteria. At least, thats what two different Nature papers from 1992 claim.) Anyway, that's the scoop on chloroplasts. I just gave a lecture on the evolution of metabolism to the bio lab I teach. I was really surprised so much is known (not my field, I just did some reading on it for the hell of it.) I found out a nice little tidbit about photosynthesis. Let me give a little background first. Photosynthesis is (loosely) the conversion of carbon dioxide into sugar (oxygen is a waste product in some forms of photosynthesis). It is a process driven by light energy. There are two steps in photosynthesis -- the light reactions and the dark reactions. The light reactions harness light energy and the the dark reactions use that energy to make sugar. Some organisms use photosystem I to harness light energy, some use photosystem I and photosystem II. (Photosystem II is the one that produces oxygen). *TANGENT* [Net photosynthetic rxn: 6 CO2 + 12 H2X -> C6H12O6 (glucose) + 6 X2 (or 12 X) + 6 H20 where X can be sulfur or oxygen In organisms where X is oxygen, the net photosynthetic reaction is the exact reverse of aerobic respiration. If X is sulfur, the reaction is the opposite of some forms of anaerobic respiration.] *END TANGENT* Photosystem I (PS I) evolved first and is used by green sulfur bacteria and purple bacteria. Cyanobacteria (and thus algae and plants) use both photosystem I and II. The Calvin cycle, which fixes carbon in purple bacteria and cyanobacteria, evolved before photosystem II and hence in the absense of oxygen. Today, organisms that use the Calvin cycle to fix carbon lose 1/4 to 1/2 of the carbon they initially fix because oxygen competes with the forward reaction of one of the steps in the cycle (the step catalysed by RuBP carboxlyase)! This is a great little tidbit of bio-trivia and something that makes perfect sense in light of evolution; but something that would seem to be hard to explain if plants were designed by an intelligent designer. This obstacle has been overcome by one group of organisms that initially fix carbon by a newer process, then pass it to the Calvin cycle in cells in the interior of the organism away from high oxygen levels. These are called C4 plants -- they are a group of grasses. They are in the angiosperm division and the monocot subdivision of plants. I leave you with a cladogram summarizing one hypothesis about the evolution of metabolism: green sulfur bacteria purple bacteria cyanobacteria light rxn(s)= (PSI) (PSI) (PSI and PSII) dark rxns= (reverse TCA) (Calvin) (Calvin) | \ / | \ / other eubacteria | | | \ / \ | \ / | | eubacterial ancestor The above cladogram gives three lineages (I left out a bunch) of bacteria and the light and dark rxns taking place in them. The reverse TCA cycle fixes carbon by reversing the order of the TCA or Krebs cycle from aerobic respiration. Instead of giving off energy and CO2, energy is added and CO2 is taken in. Aerobic respiration did not evolve until after PSII arrived so it looks like ancestral aerobes just modified an already present metabolic cycle (the reverse TCA cycle) to fit their needs (not consciously, of course -- I'm using bio-slang here.) PS I evolved after the "other"/green-purple-cyano split. The Calvin cycle evolved after the green/purple-cyano split and PS II evolved along the cyano line (probably as a duplication and differentiation of PS I). I should point out that new evidence may change the above inference. I read in a biology of microorganisms book that the Calvin cycle may be present in some archaebacteria -- that would move it's origination down to before the root of the above tree. Not everything is known about the evolution of metabolism, but at least as biologists we can construct hypotheses and test them by surveying the distribution of traits across taxa. I would hate to be a creationist and have to look at all the diversity out there (and how it is distributed) and lamely conclude that we can't tell anything about how it got here. Chris Colby --- email: colby@bu-bio.bu.edu ---

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