Yet more glucose

[David Alan Walker]

“Take a little sunshine. Take some carbon dioxide molecules out of the
air. Take some hydrogen atoms out of water. Shuffle them around inside a
bunch of little green chloroplasts inside green plant cells. What do you
get?
  That's right.  It's GLUCOSE and OXYGEN!”

Back in September, as one of the comments about this sort of stuff, Jim
Perry wrote “I have seen the page proofs of the new chapter on
photosynthesis for the new version of Biology of Plants by Raven, Evert
and Eichhorn. Glucose is no longer the formulaic end product, but rather
C3H6O3”

C3H6O3 may be preferable to  glucose but, (not having seen the proofs of
‘Biology of Plants’ myself, I wonder, yet again, about the overall
equation itself?  If it is written

3CO2 +3H2O → C3H6O3+ 3O2

there will remain the implication that some of the oxygen evolved must
be derived from carbon dioxide.

If additional water is added on the left to avoid this, as in

3CO2 +6H2O → C3H6O3+ 3O2 +3H2O

it poses the question of what the additional water is doing there when
it then reappears on the right.


In reality, if we add together all of the partial reactions of the
Benson-Calvin Cycle we get

3 CO2+ 2H2O + H3PO4 → CH2OH.CO.CH2OPO(OH)2 + 3O2

but if this is too much for students to handle why don’t we all avoid
unnecessary confusion by saying that the principal end-product of
photosynthetic carbon assimilation is sucrose? This is undoubtedly where
much of the triosephosphate ends up as the carbon source for much of
plant metabolism. Of course sucrose isn't made in the chloroplast but
then neither is glucose. At least I know of no firm evidence that free
glucose is either made, or is an important metabolite within, chloroplasts.

Am I right?

Regards

David Walker

See also:-  <http://www.digitalpublisher.co.uk/Oxygraphics/ohwhat.htm >