10
Sep
09

Biology: What is “alive” and what is not?

What is life?

What distinguishes something which is alive from that which is not?

How can we explain all life to a hypothetical being unfamiliar with our categorization paradigm?

These questions have often bothered me, and I decided I would share them with you. We define life (most of us, anyway) through cell theory. The idea that cells are the basic unit of every living thing. Where, though, does one draw the line between “cell” and “organized collection of molecules.” For example, are mitochondria independent cells? They have their own metabolism, DNA, replicate independently of the cells in which they live, and interact with the cell in a variety of ways. They are categorized as a “part of a cell” rather than an independent cell. Chloroplasts are similar in this sense. Viruses are a particularly nuanced problem as some viruses (Mimivirus comes to mind) can be larger than the smallest bacterial cells (Mycoplasma). Mimivirus, for example, only requires ribosomes and energy metabolism of the host cell, providing all the necessary DNA  templates for all proteins required for it to replicate.

These problems arise from the fact that biology rarely fits into neat little categories. Strict logical definitions of “life” or “cell” or “organism” don’t fit neatly. Is a biofilm a single organism or a collection of organisms? What about Myxococcus xanthus swarms?

Ultimately, I do not have an answer to this question. In fact, I think I have more questions than I started out with.

“What is life?”

“What is a cell?”

“Is a bacterial spore a cell?”

“What is the threshold level of cooperation to be considered multicellular?”

“How can we go about organizing a system which removes this ambiguity?”

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2 Responses to “Biology: What is “alive” and what is not?”


  1. 1 Pliny-the-in-Between
    September 10, 2009 at 11:29 am

    This question is likely to be the hardest we ever debate. On earth, one could argue that anything that fairly reliably replicates and stores a fairly stable package of nucleic acids from one generation to the next (regardless of mechanism) is alive or some variation on that theme. But is that even required or would it hold elsewhere? Don’t know. But it does seem from experimentation that nucleic acids aren’t that hard to create in the conditions common in the universe.

  2. 2 jaredcormier
    September 10, 2009 at 11:40 am

    Well, the biggest problem we have to describing life is the ability to imagine what life “might” be. Could artificially created bacterial cells which do not reproduce or replicate be considered “living” and even then, what about bacterial endospores? Are they still alive? They aren’t reproducing, they may, in fact, never reproduce. At which point does a “living endospore” become a “nonliving endospore.”

    These are the thoughts that keep me up at night…


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