OK, last post about this, I kinda promise. I was thinking about this and reading Luisi's book and he asked another interesting question... one that reflects part of the discussion yesterday. If you survive all the way to the end, then you will be rewarded with my favorite (and only) Halloween joke... just to show I can do stuff besides ask pointless question.
Let's put aside the question about the difference between an apple and its tree and whether a dead thing has live cells and get to the fundamentals.
Take the nucleus out of an oocyte, as in the cloning experiments, is the nucleus living? And is the cell, without a nucleus, alive?
Now we get even more fundamental. The cell itself. Can it be alive without some parts? If so, which parts? Venter and his colleagues made a cell with scratch assembled DNA. There have also been various attempts to make a minimum cell by removing pieces until the cell no longer functions. I don't think this tells us very much about what it takes to be alive though. Even a minimal cell has all the functions that we normally think of a required for life.
On the other hand, we know that to continue living, a cell must have correct DNA. Venter's team missed a single nucleotide and the entire organism died. It must have been a critical function for life.
I think that a lesson we could take from this example is that life has to have instructions. It there has to be some underlying component that can tell a living thing how to do all the things that it needs to do. Again, we often think of a living thing as reproducing, metabolizing, responding, moving, and growing and developing. So the cell has to have the instructions to do all those things.
But leads me to a very unsatisfying definition of life. "The ability to do all the things that living things have to do." A better circular argument hasn't been seen, I think. But, I think we're on a good track. What is the purpose of life? If you go with the selfish gene concept, then the purpose of life is to spam the environment with as many copies of yourself as you can. What do all the copies have in common? The genes, the genetic information to create copies of itself and the ability to keep itself alive to make those copies.
Could we define life as “the existence of genetic information (enough to operate and reproduce the organism) AND the ability to maintain and/or propagate that information”?
That one sentence, after all of ten seconds reflection, really does a nice job of summing up the functions of life and still allows room for the existence of forms of life other than organic systems (e.g. computer based life or non-organic based life).
Since the horse, as a unit, cannot maintain or propagate the genetic information it contains, it is no longer alive. Same with the apple (which is still problematic to me). For a while, the dead organisms can use internal resources to maintain, but not propagate the genetic information. But it cannot continue the process for longer than the cells have resources.
A bacterium, on the other hand, does have genetic information and can maintain and propagate that information.
The cell without a nucleus is an interesting question, even with this definition. But a little thinking about my definition might reveal a new concept (one that human scientists don’t seem to like dealing with).
Alive may not be an all-or-nothing state. It may even be reversible in some situations. Perhaps the cell, without a nucleus, is dead. But by putting a new nucleus in, then the cell can become alive again.
A vampire bat returned to its cave. It was covered in blood.
The other bats crowded around it. "Where'd you get the blood?" "Look at all the blood." "Comon, don't hold out, where's the blood from?"
Finally, tired of the incessant whining of the other bats, he said, "Fine. Follow me."
All other bats followed him out of the cave. The flew across the field. They flew over the river. Finally, they flew into the forest and landed on a tree.
The other bats were so excited. "Are we there yet?" they cried.
"Almost. You see that big tree right over there, the really tall one?"
"Oh yes," they all replied.