Re glucose

David Alan Walker d.a.walker at sheffield.ac.uk
Wed Jan 26 15:57:18 EST 2005




     Hate to disagree with Dave Hershey but:-
     As someone who acquired  a degree of notoriety with starch pictures
     (and the problems that they pose) see
     <http://www.shef.ac.uk/~daw/starch.ppt> I gladly acknowledge that it
     is great to be able excite students' interest by what is essentially
     a repeat of Sachs' famous experiment. That, however is where the
     present confusion started and has persisted for more than a
     century.. If the product of photosynthesis is said to be starch (and
     starches are said to be (more or less) strings of glucose molecules)
     this prompts the erroneous conclusion that  a chloroplast make
     glucose molecules  and then joins them together to make starch.
     That, in turn, prompts endless difficulties with an overall equation
     and stoichiometry that shouldn't imply that oxygen is derived, in
     part, from CO2.

     No need to discard starch but what is so bad about saying that this
     is  a storage product into which some chloroplasts put
     triosephosphate when they can't export it quickly enough or other
     parts of a plant can't use it fast enough?
     Not all species do this. Conversely how many leaves don't use
     triosephosphate to make sucrose as an export metabolite?

     If you stick to starch how do you explain to a student, who has
     gathered a spinach leaf from a garden on a May morning, that it
     doesn't contain any starch? How do you explain that a snowdrop
     (Galanthus sp) leaf never has any starch? Have they both given up
     photosynthesising?

     Isolate intact fully functional chloroplasts, illuminate them at
     opitmal temperature  and they will utilise CO2 as fast as the parent
     leaf. Deprive them of external orthphosphate and photosynthesis will
     stop. Restore it and they will fix 3 molecules of CO2 for every
     phosphate added.

     Neither starch nor glucose figure in the Benson-Calvin Cycle which
     is uniquely important because of its autocatalytic nature. It's
     products are triosephosphates.  These are exchanged for external
     orthophophate via the phosphate translocator. Cytosolic sucrose
     synthesis from triosephosphate recycles orthophosphate so that this
     can continue.  Naturally, I think that this is important science
     that students ought to know about. If it is really all too
     complicated for them to contemplate let's not take refuge in
     misleading oversimplification.

     David Walker



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