From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Sat Dec 04 1999 - 19:01:19 MST
On Sat, 4 Dec 1999 EvMick@aol.com wrote:
> > I gave perhaps the most highly condensed description of photosynthesis
> > on record...
>
> > Photosystem II
> > absorbs a photon and splits water to produce H+ ions.
>
> Do I understand this correctly?
>
> What I think is being said here is that plants split water into it's
> constituite components.....Hydrogen and Oxygen.
Yep, thats correct. Its highly interesting that planetary evolution
goes from "reducing" atmospheres where the gases are primarily CH4
(methane) and NH4 (ammonia) [reduced in a very gross sense
essentially means "with hydrogen"], to "oxidized" atmospheres
(mostly CO2 or O2). This, I believe is primarily due to
the photodissociation of H atoms which "leak" away into space.
(Its more difficult for planetary gravity to hold onto lighter atoms/molecules
like H+ or H2.) Only if you get an influx of new H (from comets mostly as
H2O) or perhaps the release of methane (CH4) sequestered in methane
hydrates in ocean floors can you go back to a reducing atmosphere.
>
> Duh.....of course that's what plants do. That's why there's oxygen in the
> air.
>
yep. Its a little more complicated though, you have to do an
interesting examination of the reactions to determine whether
the O2 is coming from the H2O or the CO2. In the case of "typical"
plant photosynthesis the O2 is coming from water. The Calvin
cycle then runs as:
6CO2 + 12NADPH + 18ATP --> C6H12O6 (sugar) + 12NADP+ + 18(ADP + Pi)
as I discussed previously, the photons are used split water
2H2O -> 2H+ + 2OH- (bacteria) or 2H2O -> 4H+ + O2 (plants)
The real trick in chloroplast thylokoid membranes (or the mitochondria)
is to force the H+ to go to one side of the membrane while the (OH-/e-)
go to the other side. That results in a voltage gradient.
When the H+ pass throught the ATP synthase (to rejoin with the OH-/e-)
they generate ATP. A separate process in the membrane allows the
H+'s get added to NADP+ to produce NADPH.
Vocab: NADP is Nicotinamide Adenosine Dinucleotide Phosphate and
ADP is Adenosine Diphosphate and ATP is Adenosine Triphospahte.
In simple terms, ATP is the energy carrier (the juice for the
reactions) and NADP is the hydrogen carrier (serving to shuttle
hydrogens between molecules so that oxidized molecules may be
"reduced" (i.e. ultimately converting CO2 + 2H2 into CH4 + O2).
CO2 may be viewed as the most oxidized form of carbon, while
CH4 may be viewed as the most reduced form of carbon.
Glucose (C6H12O6) falls in between.
Ultimately our planet exists in an "oxidized" environment, and
"life" depends on finding (coal, methane gas) or producing
(glucose, alcohol, hydrogen, gasoline) "reduced" fuels that we can
oxidize (burn) with the release of energy as a byproduct.
We are fortunate that there is a huge resource base of
plants and bacteria that make it their business to use
the free energy from the sun (or to a much lesser extent hot
springs) to convert available oxidized resources (CO2) into the
reduced forms that we find so useful.
> Howsomeever....what would prevent some genegineer from tinkering and
> producing a plant from which hydrogen gas could be harvested?
>
No problemo, you take a couple of species of bacteria like
Butyrivibro fibrosolvens or Clostridum locheadii (don't ask me where
they get these names) and feed them cellulose (sawdust, leaves, etc.)
and two of the byproducts are H2 and CO2. You can feed species like
Syntrophomonas or Syntrophobacter fatty acids (leftover cornoil
or margerine) and get H2 as well. [You can probably isolate
these if you have a compost pile in your back yard.] Now, commonly,
the H2 gets quickly absorbed by methanogenic bacteria (many
species) in a reaction like: 4H2 + CO2 --> CH4 + 2H2O (with extra
energy being generated in the process). That is why our
atmosphere has a small amount of methane but very little free
hydrogen (plus the fact that it easily reacts with the free oxygen).
> I evision some type of "fruit'.....(in the sense that corn, maize or beans is
> a fruit).....that might be harvested and consequently used as fuel feedstock
> in a Hydrogen Economy.
>
> REALLY HOT beans?
>
I'd say the beans make some really "hot" after effects if
you apply a match to the results (igniting the H2 or CH4).
Though if you want a more odiferous result, you want
to throw in some sulfur reducing bacteria (Proteus,
Campylobacter, Pseudomonas, Salmonella, Desulfuromonas)
because that will result in the production of H2S
(which our chemistry buffs will recognize as the smell
of rotten eggs...)
Seriously though, you could harvest the gas (H2 or Methane)
directly. You simply surround the field with a few inches
of concrete, pump in a few inches of water, throw in the right
(potentially engineered) bacteria, cover the tank with plastic
and pump out the gas. This is already done in places like
India where they ferment excess plant material (or feces)
into a low quality gas source. What is needed is a much more
efficient system and/or a low cost method of separating the
gases (CO2/N2/O2/H2/CH4). I think the latter problem gets
solved by nanotech (or even near nanotech when you can engineer
materials with highly accurate pore sizes that can be used
for pressure separation of the gases).
Robert
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