oxygen production

Archie Portis arportis at uiuc.edu
Fri Dec 6 11:11:34 EST 1996


The simple answer is that oxygen is required (taken up) to produce the
superoxide (or hydrogen peroxide).  If you balance all the reactions, there
is no net oxygen production.  In contrast, the simple result of
photosynthesis is:  CO2 +H2O ---> CH2O + O2 and the ratio of carbon dioxide
uptake to oxygen production is one.

Detailed answer is to balance all of the following:

PSII:   2 H2O --> 4H+ + 4e + O2

Superoxide production:  O2 + e --> O2-

Superoxide dismutase:  O2- + O2- + 2H+ -->  H2O2 + O2

Catalase:  2 H2O2 --> H2O + O2

You will see that the energy captured by "splitting" water is lost if the
electrons and protons react back with oxygen to reform water via these
reactions.




Rod Savidge wrote:
>
> Can somebody please clarify the following for me.
>
> It is my understanding that enzymically catalyzed photolysis of water to
> generate oxygen, protons and electrons occurs at PSII (otherwise known as
> the Hill reaction), that oxgygen generation can be demonstrated from
> isolated chloroplasts in a cell-free system, and that the PSII core acts as
> the catalyst in vitro.
>
> It is also my information that photosynthetic tissue is rich in
> superoxide dismutase activity (which produces oxygen and hydrogen
> peroxide from superoxide anion) and also in catalase activity, which serves
> to produce oxygen by dismutation of hydrogen peroxide.
>
> Assuming the correctness of the assumption that catalase is
> compartmentalized exclusively to peroxisomes (glyoxisomes) whereas PSII is
> localized to chloroplasts, but not knowing exactly where superoxide
> dismutase is, what is the experimental evidence which permits us to teach
> that PSII - as opposed to catalase  or SOD - is the primary source of
> oxygen production during photosynthesis?   Are there any published
> estimates of the relative proportions and absolute amounts of oxygen
> generated by each of the three catalytic systems during light and dark
> conditions?





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