On Tue, 6 Apr 2004 16:09:20 +0000 (UTC), irr  wrote:
> 
> wrote in message
> news:c4pfbm$2imq$1{at}darwin.ediacara.org...
>> > The first photosystem to evolve, PSI, uses light to convert carbon
>> > dioxide (CO2) and hydrogen sulfide (H2S) to glucose. This process
>> > releases sulfur as a waste product.
>>
>> In that case, it's obvious that all the oxygen in the resultant glucose
>> came from the CO2, since there isn't any oxygen in H2S in the first
>> place. See an analagous question later...
> 
> I don't want to monopolize the thread in multiple posts, so briefly this 
> FAQ appears to be 10+ years outdated on some crucial pieces of information.
> This ascending ladder of PS I -> PS II -> oxygen-producing photosynthesis 
> is not accepted, and the post is incorrect on which organisms use which 
> type of photosystem (PS I: green sulfur, Gram +; PS II: purple bacteria, 
> green nonsulfur; cyanobacteria, algae, and plants have both types). A good 
> recent reference on this is R.E. Blankenship, Molecular Mechanisms in
> Photosynthesis (2002 or 3, I think).

I agree with you that Chris Colby's "Introduction to Evolutionary Biology"
essay is out of date. I think it's important to emphasize that the primary
goal of photosynthesis is to produce ATP and reducing equivalents. (Some 
species use light to make only ATP.) CO2 fixation and carbohydrate synthesis
are secondary pathways that use ATP and reducing equivalents but they're
not really part of photosynthesis.

The accepted evolutionary path is to photosystems with type I reaction 
centers (e.g. PSI) and type II reaction centers (e.g. PSII) from a common
ancestral photosystem. Later on some bacteria acquired both types of
photosystem, probably by horizontal gene transfer. These bacteria then
evolved a linear coupled system where electrons were passed sequentially
from PSII to PSI. The evolution of an oxygen-evolving complex probaly came 
later when bacteria could make use of water as a source of electrons.



Larry Moran
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