On Sat, 22 May 2004 22:44:29 +0000 (UTC), 
irr  wrote:
> wrote in message
> news:c8bj99$1ijd$1{at}darwin.ediacara.org...

[snip]

>> I seem to recall reading
>> some detailed description of various kinds of photosynthesis whereby
>> one process uses light to activate a chemical that shoves either an
>> electron or proton across a membrane creating an electrostatic
>> potential, then somewhere else along that membrane that potential is
>> used to drive the ADP + PO4+ -> ATP process, or maybe the corresponding
>> NADPH process, I forget which. But another form of photosynthesis
>> doesn't make any across-membrane-potential at all, rather directly uses
>> solar energy within a single electron-transport chain to make ATP or
>> NADPH. Do I remember at least that part correctly?

[snip]

> Your memory of different types of photosynthesis is indeed correct.
> Chlorophyll-type photosynthesis is able in all cases (that I can think 
> of off hand) to generate NAD(P)H and so can be used by organisms to fix 
> carbon. Protons generated during the process -- during water oxidation, 
> for example -- can also be used to make ATP via ATP synthase.
> Bacteriorhodopsin-type photosynthesis is basically a light-driven proton   
> pump and so only does ATP synthesis (and, strictly speaking, is 
> phototrophy rather than photosynthesis, as carbon isn't being fixed).
  
There are basically two types of photosystem, PSI, and PSII.
  
PSII uses light energy to reduce quinone (to QH2) and QH2 passes electrons
to the cytochrome bc complex. The cytochrome bc complex (also called
Complex III in respiratory electron transport) is responsible for 
transferring protons across the membrane. This proton gradient drives ATP 
synthesis.

Purple bacteria and green filamentous bacteria are examples of species 
that have only PSII. They use light energy to synthesize ATP but not 
NAD(P)H. Most scientists will say that this is *photosynthesis* even
though it has nothing to do with carbon fixation.

PSI uses light energy to reduce ferredoxin. Ferredoxin can be used to 
reduce NADP+ to NADPH. Thus, species with PSII can make NADPH from light 
energy. Ferredoxin can also be used to reduce quinone to QH2, which 
shuffles electrons to the cytochrome bc complex. Species with PSII can 
also make ATP by creating a proton gradient.

Cyanobacteria have both PSI and PSII coupled in series. All eukaryotic 
chloroplasts have evolved from ancient cyanobacteria so they have a very 
similar system. In those species photosynthesis produces both ATP (via
a proton gradient) and NADPH.



Larry Moran
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 5/23/04 5:16:43 PM