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H2O comes out of PS?

Barry Meatyard B.T.Meatyard at warwick.ac.uk
Mon Mar 13 04:54:46 EST 2000

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
>I bet this is an oversimplification, but it satisfies me and most 17 year

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.


Dr.Barry Meatyard
Science and Plants for Schools
Environmental Sciences Research and Education Unit
Warwick Institute of Education
University of Warwick

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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