> From: henry@spsystems.net (Henry Spencer)   
   > tracer experiments using O-18 found O-18 in the released oxygen when   
   > there was O-18 in the water, but not when there was O-18 in the CO2.   
   > Precisely how that can be reconciled with the stoichiometry of the   
   > net reaction, they didn't explain.   
      
   I posted a query in one of the biochemistry/evolution newsgroups,   
   hoping to get a definitive answer to this puzzle, but never got one, so   
   I've given up and come back here to summarize the minor clues I got.   
   Apparently, in the long chain of reactions, from the actual   
   photosynthesis which uses solar energy to convert ADP + PO4+ to ATP,   
   through the chain of uses for that energy in ATP to pry hydrogens off   
   some hydrogen donor (H2S, H2O, etc. depending on which kind of bacteria   
   we're talking about), it's the hydrogen being pried off the hydrogen   
   donor that releases whatever the rest of it is (S2 or O2 etc.), rather   
   than oxygen from the CO2 elsewhere in the chain giving off O2. The O2   
   from the CO2 apparently all or mostly gets incorporated in the   
   carbohydrate instead of released. But like I said, I couldn't get any   
   definitive answer to satisfy our query.   
      
   > Photosynthesis is complicated, but it seems to work by prying the   
   > hydrogens off water (thus releasing oxygen) and combining them with   
   > CO2. That combination yields carbohydrates,   
      
   Yes, looking only at the original input to the process and the final   
   output, not the middle steps.   
      
   > but it ought to also yield water as a byproduct   
      
   Why? What makes you think that? In particular, if the overall reaction,   
   after prying the H's from the H2O, is H + H + CO2 = HCOOH or something   
   like that, in a biochemical system evolved to be as efficient as   
   possible, why would any of the O's be wasted by re-combining them with   
   those H's to yield water again? I'm not saying it doesn't happen, I'm   
   just asking why you would think it ought to happen??   
      
   Are you perhaps saying that respiration, good old Kreb's citric-acid   
   cycle, would be going on at night when photosynthesis isn't available   
   for making ATP so the creature (plant/algae/cyanobacterium) would have   
   to find another source of ATP to keep alive, so would convert   
   carbohydrates and O2 back into CO2 and H2O, so tracer on the O's in the   
   original CO2 that went into the carbohydrate would yield traced O's in   
   the H2O from Kreb's respiration? But maybe just as all the O's in the   
   H2O went to O2 while the O's from CO2 went to carbohydrate during   
   photosynthesis, now during respiration all the O's from carbohydrate   
   went into CO2 while O's from O2 went into H2O? Just speculating how   
   things could come out the way that is observed based on   
   post-photosynthesis carbohydrate-production and Kreb's cycle being   
   basically inverses, not just in overall stiochemistry (sp?) but in the   
   paths of specific atoms (in this case O's from two sources) through the   
   chain of reactions both forward and backwards.   
      
   > and it's not clear to me why some of that doesn't get mixed in with   
   > the incoming water.  Perhaps such mixing was too minor to be readily   
   > detected due to the choice of experimental conditions.   
      
   If a vast majority of the respiration, or whatever you believe should   
   be a source of water from biochemistry, produces water where the O's   
   come from O2 not from carbohydrates, as I guessed above, then the   
   mixing is irrelevant because it re-creates H2O where the O's come from   
   O2 released when the original H2O gave up its hydrogen, so if O's are   
   not labeled in water on input it won't be labeled in O2 released in the   
   middle and won't be labled in water re-created later and won't be   
   released in O2 the second time around. Still I wish I had been able to   
   find an expert in the other newsgroup who would post a definitive   
   answer to our puzzle, sigh.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)