From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Sat Dec 04 1999 - 12:36:00 MST
Curt had responded privately to me on this topic, I responded
directly to him but I realize the list might have a general
interest (from the perspective of where genetic engineering
might take us and why I view the current AgBio efforts as
the mere doodlings of children). I also am keenly interested
in whether anyone sees any flaws in this.
-----
On Sat, 4 Dec 1999 CurtAdams@aol.com wrote:
> > I had (originally) commented on a better approach to producing ATP
> > from solar power...
>
>
> This is all true, but switching to a different process for solar energy
> capture doesn't change any of that. In and of itself, replacing
> choloroplast 35% efficiency sunlight -> glucose with 20-30% sunlight
> -> electricity which then has to be made into organic matter will
> make things *worse*.
No, you have to go look at the biochemical steps involved.
There are two fundamental things you need to drive most biochemistry:
(a) An energy source (normally ATP)
(b) Reducing capacity (normally NADH or NADPH)
In plants, the chloroplasts have protein complexes embedded in the
thylakoid membranes that use photons absorbed by photosystems (porphyrin
based chlorophyll molecules) to generate ATP and NADPH. Photosystem II
absorbs a photon and splits water to produce H+ ions. Photosystem I
absorbs photons and adds energy to the electron transport that
eventually combines NADP+ and H+ to produce NADPH. The PS-II generated
H+ ions run through an ATP Synthase (with CF0 + CF1 subunits) to
produce ATP.
So the bottom line is that photons get used to produce NADPH and ATP.
Those then get run through the Calvin Cycle to produce Glucose.
The aggregate of all of this is I believe the 35% you mention.
Now, one can harvest photons in a multi-layer thin film solar panel
at an efficiency of ~30% now, and perhaps 90% in the future (based
on communications to me from Geoffrey Landis from NASA's Glenn Research
Center [one of the experts on solar cells]). I believe those electrons
can be used to split water at something close to 100% efficiency
(if someone knows something that contradicts this please let me know).
As splitting water generates some heat it clearly isn't 100% efficient.
(I suspect the trick would be supplying the electrons at the exact
energy level required to split water and minimizing resistive losses
by using metal electrodes.)
Once you have generated the H+ ions you can route them as necessary
to produce ATP or NADPH. So the process should be approximately as
efficient as it is in plants. Since you can use metal conductors
in these "artificial thylakoids" (as opposed to the lipid bilayer and
proteins that are used in the thylakoid membrane for electron
transport), I suspect the system should be more efficient
rather than less. You have to think of plants as machines
that split water and then use the byproducts (NADPH and ATP)
to attach hydrogens to CO2 molecules until you end up with glucose.
>
> You mention losses in constructing and maintaining the plant.
> Well, obviously a nanotech mechanism would have contruction and
> maintenance costs - and self-replication per se doesn't help, because
> plants already do that.
The current system is inefficient precisely because the plants
are engineered to compete against other plants and to make copies
of themselves (those things result in the stems, trunks, and
usually leaves, that we do not use) and the seeds or fruit (that we
do use). Now, if you were engineering a food source, you would engineer
a layer of cells that would absorb all incident sunlight, manufacture
ATP and converted it directly into protein, carbohydrates and lipids
in the proper amounts with appropriate "flavors" added just to keep
the humans happy.
The bottom line is -- Why make potato *plants* if all you want is
mashed potatoes? We harvest the potato, then plow the remnants
of the plant back into the ground where bacteria break it down
and either release the CO2 back into the atmosphere or sequester
it (so it eventually becomes coal or oil). Thats where the
system starts to get inefficient.
> I don't see how any estimate of costs will be anything other than
> wildly speculative.
I'll take that as a complement. Seriously though, unless there
are huge inefficiencies in the electrolysis of water, this approach
should work quite well. Even if there are those efficiencies you
can do away with the solar cells & water electrolysis and use a
completely biotech approach. My reason for implementing things
as I suggest is that it allows you to eliminate the chloroplast
and design solar cells with multiple layers that are highly tuned to
absorbing photons of different energies and turning them into free
electrons. The photosystems I mention are tuned to absorb photons
at 680nm and 700nm (in the red region), so that the energy in
lower energy photons is lost entirely and the excess energy
in higher energy photons tends to produce heat.
Ask yourself this: Why do leaves look green?
Answer: Because the leaves are absorbing all the other wavelengths.
But if leaves were fully efficient they should look black.
In nature we have a wealth of materials to choose from.
Cyanobacteria have phycoerythrin that absorbs best around 550 nm,
phycocyanin that absorbs from 620 to 640 nm and chlorophyll b
absorbs best at 660 nm. Purple phototrophic bacteria have
bacteriochlrophyll a that absorbs best at 850 nm. There are
other molecules that are optimal from 720-780nm and 1020 nm.
So even without resorting to solar cells I can reengineer cells
to be much better at absorbing the different wavelengths in light.
My reason for going to the solar cell/electrolysis approach is that
in theory it allows you to construct a robust one-time infrastructure
(the energy harvesting apparatus) that can be use to feed the energy
into the cells that become food. In contrast, the plants have to
regenerate the energy harvesting apparatus (chloroplasts) every time
they grow up from a seed. Think of it along the lines of the
efficiency of an orchard vs. the efficiency of a wheat field.
> So there's no compelling reason
> to expect vast improvements, although since we have improved efficiences
> in the past and new mechanisms remain under research it's reasonable
> to expect continued moderate improvements.
>
The improvements in the past have been made through selective breeding
of what nature has provided us with, not through conscious reengineering
of the entire system out of the most robust materials available with
careful attention being paid to reducing the need to regenerate things
from scratch every time.
Robert
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