(urth) note Re: Short Sun blog

[António Pedro Marques]

Dan'l Danehy-Oakes wrote (27-09-2010 16:58):
> On Mon, Sep 27, 2010 at 4:48 AM, David Stockhoff<dstockhoff at verizon.net>  wrote:
>
>> Copper-based life seems to prove the rule by not existing here.
>
> Hemocyanins (also spelled haemocyanins) are respiratory proteins in
> the form of metalloproteins containing two copper atoms that
> reversibly bind a single oxygen molecule (O2). Oxygenation causes a
> color change between the colorless Cu(I) deoxygenated form and the
> blue Cu(II) oxygenated form. Hemocyanins carry oxygen in the hemolymph
> of most molluscs, and some arthropods, including the horseshoe crab,
> Limulus polyphemus. They are second only to hemoglobin in biological
> popularity of use in oxygen transport.[1] Unlike the hemoglobin in red
> blood cells found in vertebrates, hemocyanins are not bound to blood
> cells but are instead suspended directly in the hemolymph.

The relevant issue here are carbon-based lifeforms. Carbon-based means that 
they're structured around compounds of carbon (and oxygen and hydrogen). As 
logn as they are carbon-based, they are able to interact with each other at 
a molecular level (i.e., digestion, immunitary response, etc). Just what 
kind of molecule carries oxygen to cells isn't that much important, as long 
as it too interacts as an organic molecule. ('Organic' is precisely the name 
Chemistry gives to carbon/hydrogen/oxygen compounds, and the properties they 
exhibit and the ways theyr play together and so on.)

In fact, they way the Universe is, there is just not much room for other 
possibilities. The specific traits of hydrogen and oxygen make it that they 
are more or less irreplaceable, that is, the matter we consider as living 
can't not depend on them. Carbon is not exactly the same, but almost. Carbon 
can be replaced by elements with similiar chemical properties, but the fact 
that they don't behave identically renders them unsuitable for the purpose. 
For instance, arsenic is poisonous precisely because it can bind to 
molecules the same way as carbon, but then changes their behaviour. Silicon 
is another element that could replace carbon, but the compounds it enters in 
don't have the flexibility needed for life.

That's to say, differently-based lifeforms probably would be very different 
from the ones we know, not just somewhat different.