Now, I'm not trying to be difficult I promise, but if an apple that is in the fruit bowl (we'll assume recently picked) is considered to be alive... then why is an animal considered to be dead, when many of its cells are still functioning after we declare it 'dead'?
Our organs can survive for a fairly prolonged time in the event of the whole body death. Limbs can survive as long as 6 hours. Bone and ligament, much longer. Hair and fingernails continue to grow after death. The brain can survive for about 10 minutes under optimum conditions without major damage. There is at least one case of a cat being completely revived after one hour of death.*
Now, I freely admit that we're moving beyond scope a bit. We're talking about brain death and clinical death here and the prior conversations have been about living things. But I think that this serves to emphasize a point that I made earlier. Biology is squishy. It is very difficult to objectively and completely define some parts of biological systems, because there are always exceptions and, thanks to 3 billion years of evolution, life is very, very tough. To paraphrase Neil Stevenson, "We come from a long line of stupendous bad-asses... because every living thing in the history of the Earth that wasn't a stupendous bad-ass died quickly."
OK, let's get back to where we were... when considering a multi-celled organism, can we say that it is alive if all of its cells are alive as Arthur suggests? Yes, of course, the cells in the apple are alive, but is that enough to be considered alive? If it is, then why is our animal dead, when the majority of its cells are alive and will continue to be so for quite a while?
The direction I'm taking here, is that an apple fallen from the tree is not that different from a limb that has been severed from the body. That part, whatever it is, can survive for a time, but it is no longer connected to the super-structure that makes the entire thing alive. A living thing can reproduce itself in its entirety, a broken off portion probably cannot (let's not get into Planaria right now). A living thing can intake material and energy, which is then used for maintenance, movement, response to the environment. A broken off part cannot (again, in general, plant cuttings may work fine**).
BTW: In case you are wondering, I'm totally off my planned material at this point and thinking 'outloud'.
Can we say (should we say) that a multicellular organism is no more than it's component parts? Or is a multicellular organism something like what we were previously talking about... is there something that makes it more than the parts. Is there an emergent property that says we shouldn't treat single-celled and multi-celled organisms in the way (with respect to being defined as 'alive')?
But research seems to indicate that there is little difference between single-celled organisms and multi-celled organisms at some level of evolution. This report basically describes the change from single-cell to multi-celled due to predation. (for for the Springerlink link, I thought I had the full article downloaded, but I've lost it). So, again, life is squishy. There's not a dividing line between single-cell and multi-celled, so it will be (probably) futile to discuss a difference between life and non-life from that angle (and thus we see an example of real science in which we take a shot and it didn't quite work how we intended).
Or do we go back to the multi-celled structure having specialized cells and all the parts can't function unless they are connected (however tenuously) to the other parts. The whole organism can do all the functions of life, but pieces cannot. The cells in my reproductive system, while vitally important to the whole and the species, just can't do their job without the lowly small intestine.
I very well may be obsessing about this too much and am being silly. I don't think so, but what do I know. I think this is very important thing to consider. Not because we'll change the definition of life and biologists will stop studying prions or something silly like that. I don't want biologists and computer scientists to get into turf wars over who gets to study some digital organisms and not others.
I do think, that at some point, probably in the near future, some scientists will go for it. They will endeavor to create life in a large, complex simulation. Maybe it will be a giant Uery-Miller set-up with clary substrates all over and pyrite chunks for catalyzing, put a wave machine in to create vesicles on the clay. Will it work? We won't know until someone tries.
But, if we aren't sure what life is, how will we know if they succeeded?
OK, I'll go away now, I'm just blathering. The plan is to talk a little bit more about this concept of what is live, then get into some abiogensis research and see some of the really cool stuff that is being done to examine the questions of what is life and how did it get here?
* Hossmann KA et al., KA; Sato, K (1970). "Recovery of Neuronal Function after Prolonged Cerebral Ischemia". Science (American Association for the Advancement of Science) 168 (3929): 375–6. doi:10.1126/science.168.3929.375. PMID 4908037.
Hossmann KA et al., KA; Schmidt-Kastner, R; Grosse Ophoff, B (1987). "Recovery of integrative central nervous function after one hour global cerebro-circulatory arrest in normothermic cat". Journal of the Neurological Sciences (Elsevier) 77 (2–3): 305–20.
** Which brings up the whole issue of stem cells.