Re: Simple cellular structures are more complex than people make it sound.


"Nic" <harrisondalen@xxxxxxxxxxx> wrote in message news:1173756960.956996.156320@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On 12 Mar, 22:28, "Perplexed in Peoria" <jimmene...@xxxxxxxxxxxxx>
wrote:

Almost all of the molecules in the autocatalytic membrane are molecules that
were synthesized BY the membrane. There are only a few kinds of reactions
that the membrane CAN catalyze, and a few more reactions that can take place
somewhat spontaneously. The catalyzed reactions are not specific for the
exact molecular species involved, but they are specific for the reaction
type. They have to be. The reactions involved are simple ones - adding
CO2 and [H2] to R-CH=NH to give R-CHNH2-COOH, for example. The catalysts
don't catalyze other reactions enough so that any more than a handful of
different 'head group' configurations exist in any significant quantity.
All reactions take place at the 'head', so all carbon skeletons are
simple chains. All reductions take place at or near the 'head' because
that is where the mineral catalysts are. When a carbon at the base of the
head group of hydrophilic carbons becomes reduced to -CH2-, it is reclassified
as a tail carbon, and the molecule sinks a bit deeper into the membrane.
All of those -CH2- carbons in the tail of the molecule are unreactive - they
don't participate in the reaction and they don't distinguish molecules so
as to provide specificity.

All of the chemistry takes place at the heads, and the heads just are not
that big or that complicated. There simply are not hundreds of thousands
of variants to choose among. Maybe a dozen different kinds of heads to be
considered. Maybe less. Throw in the variation in tail length and we have
a few hundred different species to be considered, at most. Maybe a dozen
reaction types to be considered and another dozen or so side reactions to
be considered. For example, if CO is the inorganic carbon source, the
reactions will not be specific enough to exclude the occasional incorporation
of HCN instead. Or a reaction adding water across a double bond will
occasionally add H2S or NH3 instead. It is not clear to me that this is
always going to be a bad thing. In many cases, it just doesn't matter.
in other cases, those side products actually have desirable functional
properties. This kind of thing is what permits the autocatalytic system
to evolve.

I'm not following this too well, so please ignore if spurious. The
picture I'm getting is one of a slick over a rock, where the rock may
have metals which can act as catalysts. The growing ends of the
chains are head-down, and those are the hydrophilic ends.

Yes, that is the picture for the very beginning. Initially, the only catalyst
is the rock. But later, the autocatalytic organism begins doing its own
catalyzing by developing head groups that chelate metal ions. That was the
era in which I see porphyrins being synthesized.

I can see
your point that there is a kind of selection going on in that the
heads that push their way to the front are the ones that continue to
be hydrophilic.

No, that is not my point. There is no natural selection between individual
lipid molecules. Lipid molecules do not individually reproduce themselves.
There is selection between organisms - between slicks. Some slicks have a
different composition than others, and some blends of lipids grow faster than
others, or grow in a wider variety of environments, or are otherwise more 'fit'.

What I don't see is where any chemical potential is
supposed to come from. What is driving this system?

Several possibilities. Since autotrophic growth of membrane is mostly a
matter of converting oxidized carbon (CO2, CO, or HCN) to reduced carbon
(-CH2-), that means that to drive the process you need a source of reducing
power. In effect, energetic hydrogen equivalents. I will denote a 'high-power'
form of hydrogen as [H2].

- Wachtershauser suggested the reaction
H2S + S--(solid sulfide) -> [H2] + S2--(solid pyrite)
- Considering that metalic iron is falling from the skies, I like this:
H2S + (Fe,Ni,Co)(metal) -> [H2] + (Fe,Ni,Co)S(solid sulfide)
- In the recent paper from Harvard on sphalerite, the energy source is photons
and the hydrogen source is unspecified. Water or H2S would do.
- Or, again with a photo-assist
2Fe++ 2H2O -> 2Fe+++ + 2OH- + [H2]
- Or, if you have a local concentration of HCN to work with, you can convert
some of the carbons to -CH2-, and the rest to CO2 with NH3 as a waste
product. (Balancing the equations is left as an exercise ;-) It is
called disproportionation and can provide some net energy. I like the
disproportionation idea because you can have net lipid production
(reduction) on one side of the membrane and net lipid destruction
(oxidation) on the other side, thereby generating a cross-membrane
potential which can evolve to an electron transport chain. Overall
net production, though.

Different energy sources were in use in different eras of the "lipid world".
I imagine that several of the items on the list above were used at one
time or another. The "lipid world" lasted for a long time, and in the late
lipid world, when they were getting around to inventing RNA, the origin as an
oil slick on a rock was only a distant memory.


.

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