In article <1994Jan10.170217.29332 at alchemy.chem.utoronto.ca>, mroussel at alchemy.chem.utoronto.ca (Marc Roussel) writes:
> stuff deleted
> However, I think that we can agree that the first alternative is
>almost certainly right. In outline, photosynthesis does exactly what
>this reaction proposes: Its sole energetic input is light which it
>converts into bond energy of its products; as far as I know, the amount
>of energy provided by absorbed photons is greater than the energy required
>to make glucose from the reactants provided so some of it must
>definitely be converted into heat.
The approach I take on this topic relates to that describe by M. Roussel.
The two separate sets of processes are required to produce the net reduction of
CO2 into a sugar. In the Calvin cycle a phosphate transfer and electron
transfer of energy lead to the formation of glyceraldhyde 3-P, an additional
phosphate transfer is needed to regenerate the starting substrate (RuBP). The
energy utilized in these reactions is derived from the PSI and II photosystems
and a colateral ATP production via H+ flow thru chloroplast membrane ATPases.
The electron flow set up in PS II and I is due to the absorption of photons by
a chlorophyll complex which leads to the loss of an electron--which is regained
by the chlorophyll from the breakup of water. The flow of electrons leads to
the production of NADPH. The endergonic reactions NADP+-->NADPH and ADP-->ATP
are fueled by the excessively exergonic energy available from the electrons
which were excited by the light energy.
This sounds a lot like a lecture--sorry--it is the topic for tomorrows Cell
Biology Class. The bottom line is that more energy flows in than is trapped in
the ATP and NADPH and there is more energy in the ATP's and NADPH than in the
reduced glyceraldhyde 3-P, therefore the net reduction of CO2 must be exergonic
with the excess energy lost as heat.
David DeGroote
Biology
St Cloud State University
St Cloud, MN
