H2O comes out of PS?

Jon Greenberg jongreen at bluemarble.net
Mon Mar 13 21:46:53 EST 2000


I do disagree on one point. I can't speak about exam questions or "dogma" in UK
education, but glucose as a Calvin cycle product is not an oversimplification--
It is just not true. Photosynthesis produces very litle glucose; the main
product is 3-phosphoglyceraldehyde.

Jon Greenberg

Barry Meatyard wrote:

> On 13/3/00 John Hewitson wrote:
> >
> >I teach that the formula is actually 6CO2 + 12H2O --> C6H12O6 + 6H2O + 6O2
> >
> >Light is used to split H2O releasing O2 and producing ATP (from ADP + Pi)
> >and reduced NADPH2 - this takes place on the thlakoids of the chloroplast.
> >
> >The NADPH2 is used in the Calvin cycle to reduce CO2 and one of the
> >byproducts of one (?) of the stages of the Calvin cycle of reactions is
> >H2O.  The H in this water will have come via the NADPH2 and the O in this
> >water has come from CO2.  This takes place in the stroma of the
> >chloroplast.
> >
> >I bet this is an oversimplification, but it satisfies me and most 17 year
> >olds.
>
> Yes it almost certainly is! Although I too used to teach this when I taught
> the same programmes as John in UK schools. However the pentose phosphate
> pathway / Calvin cycle don't operate in isolation and there will inevitably
> be drains from other metabolic processes, and also potential inputs of
> metabolic intermediates from them (via anapleurotic routes?). Thus the
> concept of a 'balanced equation' is probably flawed from the outset. The
> same applies to respiration and the number of ATP molecules generated by
> one turn of the Kreb's cycle. However for the purposes of answering public
> exam questions - which inevitably will require dogma (certainly in the UK!)
> John's equation is the best, but it should be seen as the starting point
> for discussion.
>
> Barry
>
> Dr.Barry Meatyard
> Science and Plants for Schools
> Environmental Sciences Research and Education Unit
> Warwick Institute of Education
> University of Warwick
> Coventry
> CV4 7AL
> UK
>
> Email: barry.meatyard at warwick.ac.uk
> Tel: 44 (0) 2476 524228
> Fax: 44 (0) 2476 523237
>
> SAPS is at http://www-saps.plantsci.cam.ac.uk
>
> ---



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